Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

The US economic meltdown and slow recovery are being blamed, in part, for the increase in eastern equine encephalitis (EEE) cases in some parts of the US. The premise is that as the economy weakens, people cut down on discretionary spending. One area of discretionary spending is veterinary care, particularly non-emergent care like vaccination. It may be an even greater issue for a rare disease like EEE, as even people who choose to vaccinate their horses may pare down the vaccines they use, by limiting vaccination to core vaccines such as rabies and tetanus.

“The economy is very much a factor” stated veterinarian Steven Halstead of the Michigan Department of Agriculture. Similarly, veterinarian James Connell said "It’s flat out the economy. People can’t afford to feed them, how can they afford to vaccinate them?” Dr. Connell reports a decrease in vaccination rates in his practice of approximately 15% per year over the past 3 years, something that has also been reported by other Michigan vets.

It's always tough to say, with confidence, why disease trends occur. EEE is a sporadic disease in many regions, like Michigan, and there can be no cases some years and clusters other years. So, there's no guarantee that vaccination is the problem. However, it makes sense that it could be involved since decreasing vaccination rates increase the number of susceptible horses.

The only saving grace with regard to decreasing EEE vaccination rates is that it only hurts the horses that are not vaccinated. With some diseases, namely those that are spread horse-to-horse, maintaining a high overall vaccination rate in the horse population is critical to reduce the chance of disease transmission. This is the concept of "herd immunity."  With those diseases, when people stop vaccinating their horses, they also in part increase the risk to other horses in the area. With a disease like EEE that is spread by mosquitoes which cannot be infected by other horses, herd immunity doesn't apply. The percentage of horses in the region that are vaccinated has no impact on the likelihood that any single horse will be exposed.

(click image for source)

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An infant from Florida has died of eastern equine encephalitis (EEE). There have now been three human deaths in Florida this year from EEE, a number that is high compared to most years. Despite being called "equine," this virus can infect people if they are bitten by a mosquito that acquired the virus from an infected bird. Human infections are rare, with only 5-10 cases in the US each year, but they are often devastating, and kill approximately 30% of those who are infected.

It's concerning that there have been three deaths in Florida so far this year, considering we're still not even far into the typical EEE season. It is suspected that the higher number of cases is the result of a greater percentage of infected mosquitoes (not an increase in total number of mosquitoes).

There is a vaccine for horses, and it is recommended for all horses in areas where EEE virus is expected to be active. There is no vaccine for humans, so the focus is on decreasing mosquito populations and reducing mosquito bites. EEE virus cannot be transmitted from horses to people, and horses cannot pass the virus back to mosquitoes, so they play no role in human infections. The main role of horses in terms of human health is as an indicator that EEE virus is active in the area, since equine cases tend to occur earlier and be more common than human cases. If an infected horse is identified in a particular area, it's an indication that the virus is active and people are at increased risk.

(click image for source)

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Eastern equine encephalitis (EEE) season is well underway. EEE is a devastating mosquito-borne disease that can infect (and usually kill) horses and people. It is a regionally important disease that infects many horses in some regions and none elsewhere, mainly based on the types of mosquitoes and birds in the area. The southeastern US is a hotbed, and cases are now being reported in areas such as Florida and Georgia. It is very important that horse owners make themselves aware of infectious disease risks in their own areas, including the risk of EEE. It is equally important for owners to be aware of the disease risks in any areas to which they may take (or send) their horses. Knowing the risks is necessary to develop a logical and effective infection control program.

EEE causes severe neurological disease in horses and people. Horses and people are infected after being bitten by a mosquito that was infected by feeding from an infected bird. Infection is often fatal.

The keys to prevention (for horses and people) are:

1. Avoid mosquito bites by:

• Reducing mosquito breeding grounds (i.e. standing water)
• Staying away from mosquito-infested areas, particularly at dusk or dawn.
• Wearing clothes that cover your arms and legs
• Wearing a DEET-based repellent when outside

2. Vaccination

• EEE vaccination is important in areas where the disease is present. Ideally, vaccination should be performed approximately one month prior to the time of year when cases start to occur, but late is better than never.

More information about EEE can be found in our archives.

Photo: Horse in a full-body fly sheet.  Good in theory but not very helpful in hot weather. (click image for source)

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Florida's first Eastern equine encephalitis (EEE) case of the season has been diagnosed in Marion County. Little information is available, beyond the fact that a horse in the northwestern part of the county was infected. EEE is typically fatal in horses so I suspect the outcome wasn't good.

Like other mosquito-borne diseases, EEE is highly variable between regions, both in how common it is (or if it's even present) and the time of year that cases occur. Timing of infections depends on a few factors, particularly the types and numbers of mosquitoes that are present. Increasing numbers of mosquitoes that feed off both birds and animals is an important aspect of these diseases, since these are the ones that spread the virus from the reservoir (birds) to horses. In some regions, this occurs late in the summer and fall. In other regions, it's earlier (or potentially all year round).

EEE is a rare but potentially devastating disease. It's important for horse owners to understand the EEE status in their region. The incidence of infection has a significant impact on the need for vaccination (considered alongside factors such as risk aversity, cost and many other factors that go into designing a vaccine program). Additionally, the disease pattern needs to be considered. Ideally, vaccines should be given a month or so before the period when exposure is expected to begin. If EEE cases typically are not found until fall, then vaccination in the summer is reasonable. If cases start occurring at this time of year, vaccination in early spring is indicated.

The Florida Department of Health has other general pieces of advice for avoiding mosquito-borne diseases (primarily directed at people, but some of which also apply to horses), including the "5 D's" for prevention:

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A large British equine charitable facility has closed temporarily because of a large influenza outbreak. Approximately 80% of horses at the Bransby Home of Rest for Horses have become infected. In a large facility like this, once you have 80% of the horses infected, there's very little chance of controlling the outbreak within the facility, although it's still very important to take precautions (such as closing the facility) to prevent the disease from spreading to other farms. Unless there is a discrete group of horses that is well removed from everyone else on the property, you have to assume that all the horses have been exposed. At that point, you basically just have to let the outbreak burn itself out, and try to manage any complications that develop in individual animals.

In some ways, if you're going to have a major disease outbreak, influenza's a good one to have. It doesn't often cause serious illness, it rarely causes death, it's not transmissible to people and it doesn't have a "silent" carrier state. The lack of a carrier state is very important since, unlike strangles (Streptococcus equi), once horses recover from influenza, they only shed the virus for a short period of time. With strangles, some recovered horses appear healthy but will continue to shed the bacterium for long periods of time, and may continue to infect other susceptible horses.

Facility manager Sally Howard is quoted as saying "Staff are always extremely careful to ensure all animals that are received by the charity are put into quarantine for at least four weeks and then thoroughly tested before being allowed to mix with the herd."

• This is a great quarantine plan. Unfortunately, something went wrong here. If there was a true 4 week quarantine and if good protocols were used, influenza would not have made it into the herd. An investigation should look into why influenza was able to break through this quarantine. Some possible explanations are neglecting to quarantine a horse, not quarantining a horse for the proper length of time, mixing quarantined horses (i.e. a horse could be quarantined for 4 weeks, but if it gets exposed to a newly quarantined horse near the end of this time period, it could become infected and be introduced to the herd before it shows clinical signs of disease), and people spreading the virus from quarantined horses to resident horses on clothing or other items. 

"The source of this infection is not yet known but we will try to discover and eliminate it."

• I understand the sentiment but it's not practical. The initial source is no longer an issue, at least in terms of managing this outbreak. There are no long-term influenza shedders, so the horse that brought influenza onto the farm is likely influenza-free now. What they need to do now is ride out the outbreak and figure out why it happened.

"Vaccines are available but the ongoing cost of protecting 271 horses would add significantly to the already high costs of maintaining the herd."

• That's a tough situation and one that is a constant challenge for centres such as this. If resources are limited, what stays and what goes? Proper herd health for that many horses is certainly expensive, and good infection control practices are very important when you have a lot of horses. This outbreak has shown what can happen when you don't vaccinate, and the costs and disruption of the outbreak certainly would have paid for a lot of vaccine, but hind sight is always 20/20.  Deciding how to spend limited resources is a difficult decision.

Fortunately, this outbreak should be controlled soon, in part because it has spread so widely on the farm. There are few susceptible horses left, and when the influenza virus cannot find any more horses it can infect, it will disappear.

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While the manufacturer doesn't seem to be providing much information, Dr. Carolyn Cooper from the Canadian Food Inspection Agency provided some additional details about the PreveNile vaccine recall in a ProMed Mail post:

"The stop sale and recall of Intervet's equine West Nile vaccine PreveNile also affects Canada. It is a voluntary stop sale and recall, initiated by Intervet after an increase in the number of immediate hypersensitivity type reactions (anaphylaxis, colic) was noticed in the United States. The Canadian stop sale and recall is a precautionary measure as such an increase in reported adverse reactions has not been noted in Canada. Canadian distributors and end users of the product have been notified of
the recall."

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Iceland's sure having a tough time lately. From financial crises to volcanos, it's been in the news a lot lately. Now there's more bad news, at least for horse owners in the country. An apparent outbreak of respiratory disease is spreading in horses across the country. This is causing concern because of the potential for disruption of the big national horse shows that take place in the summer.

I haven't been able to find much specific information at this point. One news article states that the disease hasn't been "analyzed" yet, but it appears to be an infection that has not been identified in the country before. Playing the odds, an outbreak of respiratory disease that spreads quickly in horses in a region is probably equine influenza. Iceland is considered equine influenza-free, so if the virus made it into the country, it could spread quickly and widely, as was shown in Australia a couple of years ago. Containing influenza in a country where it has not be present before can be difficult, because few to no horses have any natural immunity.

Prompt determination of the cause and aggressive infection control measures are required to contain the problem, whether it's influenza or something else. Vaccination can be a useful tool in response to influenza outbreaks, particularly intranasal vaccination. However, vaccination is only part of the response and can't be the sole measure. Vaccination takes time to have an effect. It also takes time to get the vaccine into the country and into the horses. This can be particularly problematic if there are restrictions on importation of the vaccine. This was and still is a contentious issue in Australia.

Hopefully, a specific diagnosis will soon be determined and proper infection control measures will be implemented. While the summer show season is presumably important to many people, it's critical that control measures, including quarantine of affected properties and restriction of horse movement, take precedence. Failure to properly implement required precautions based on a desire to avoid disrupting shows usually just leads to a prolonged and larger outbreak.  Short-term pain for long-term gain needs to be the philosophy.

Image: Overview of the 2nd fissure on Fimmvörðuháls, close to Eyjafjallajökull (the volcano in Iceland that erupted on April 14, 2010 causing major disruption to international air travel). (Photo credit: Boaworm)

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Intervet-Schering Plough has issued an "urgent recall notice" regarding PreveNile West Nile virus vaccine. The recall notice states that there has been an increased incidence of adverse effects, but there is no further explanation. TheHorse.com quotes an Intervet-Schering Plough spokesperson as stating "Reports included symptoms of acute anaphylaxis, gastrointestinal symptoms (colic), respiratory distress, and death following vaccination either with PreveNile alone or in combination with other vaccinations." Any of these can occur (rarely) with any vaccine, and some can also occur coincidentally with vaccination (i.e. unrelated to vaccination but occurring at the same time). Presumably, their data indicate a high enough rate of adverse effects that they feel it could be related to the vaccine, and they're occurring at a higher than acceptable rate.

• If your horse has been vaccinated against West Nile virus using this vaccine, relax. Most vaccine reactions occur shortly after vaccination, and if you don't see any problems now, you probably won't. Hopefully more information will be forthcoming from the company, explaining the problems, but delayed adverse effects would be quite surprising. Don't rush to call your veterinarian because you've heard of the recall. If your horse has been recently vaccinated and you think there have been some adverse effects, inform your veterinarian so it can be reported to the company. 
• There's no indication that the recalled vaccine lots are not effective.
• Other West Nile vaccines, and other Intervet-Schering Plough vaccines, are not involved in the recall.
• If you are planning on having your horse vaccinated against West Nile virus, don't worry. The recalled vaccine shouldn't be used by veterinarians now that the recall is in place.

Hopefully Intervet-SP will provide some more information about the problems, such as what exactly has been reported, as well as how common and widespread the issue is. This could be nothing more than a very cautious and prudent response to adverse reaction reports, but it could also be an indication that there truly is a problem.

On a related note, this is a perfect example of why it is important to report any adverse drug reactions. Any time your horse has an apparent reaction to a drug, let your veterinarian know, and make sure the information gets reported to the company or other authorities. It's only through evaluating adverse-reaction reports that problems are identified. If good data aren't provided, proper action can't be taken.

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We've taken the plunge into (the time- and money-sucking abyss of) pony ownership for our 3 daughters. Foxie Casie, a 4-yr-old Welsh cross, arrived yesterday, much to the delight of the girls and mixed delight and trepidation to their parents. I figured I might as well discuss some of the infectious disease-related issues dealing with acquisition of a new pony.

Vaccinations

• She came vaccinated against rabies and tetanus, with the last round having been done this spring. I'll re-do rabies and tetanus next spring. She'll get a rabies vaccination every year, but I will probably stretch out the tetanus vaccine since yearly vaccination is likely not necessary (especially considering tetanus vaccination is labeled for every 5 years in Australia).
• I always debate about the necessity of West Nile virus (WNV) vaccination, since it's very rare now in Ontario and I'm not aware of any mammalian cases in the general vicinity. I'll probably err on the side of caution and vaccinate her, but will do so in late summer, in advance of the true high risk period here. Typically, any WNV cases that occur here develop late August through October, so vaccinating the pony in late July, to get peak immunity during the main risk period, makes sense.
• I will probably not vaccinate her against eastern equine encephalitis (EEE) since it's so rare and hasn't been found around here.
• She lives with no other horses and won't be traveling off the farm in the near future, so there's no need to vaccinate against things like influenza, equine herpesvirus or strangles.

Deworming

• She was dewormed about one month ago. I'll check a fecal egg count soon and go from there. Since she's the only horse on the farm (the sheep are low risk for passing parasites to her), if we get started right and keep her from contaminating the pasture, a pretty conservative parasite control program with periodic fecal egg counts and deworming as needed should suffice.

New horse infectious disease issues

• She didn't have much of a trailer ride and appeared healthy when I saw her the day before, so shipping associated diseases like pleuropneumonia ("shipping fever") are pretty unlikely.
• Diarrhea's always a concern. Risk factors for diarrhea include shipping, stress, diet change and antibiotic use. Well, she's been shipped, presumably stressed, had her diet changed and been introduced to pasture, so she has some risk factors. She's not exposed to other horses, which helps, and she has no need for antibiotics (at least at the moment). So, basically, I'll give her a good diet (good hay and not-very-lush pasture) and keep an eye on her. My diarrhea research background probably makes me look at her feces more than the average person, but so far, so good.

I'm sure there'll be more to come... hopefully not much from the actual infectious disease standpoint. 

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A rabid horse has been identified in Bandera County, Texas - this is the first case of rabies in a Texas horse in almost 40 years. The horse started acting strangely, erratically and at times violently. Rabies was eventually diagnosed (diagnosis involves testing the brain after the horse dies or is euthanized) and the owners are undergoing post-exposure treatment.

Rabies is a rare disease in horses, but there is always a risk of exposure in regions where rabies is present in wildlife. The fact that equine rabies is rare is a good thing, but it can also breed complacency and lead to decreased vaccination rates.

Decreasing vaccination is a bad thing. Vaccination is cheap insurance for a very serious disease. Rabies is invariably fatal in horses. It's also a risk to anyone around the horse. Transmission of rabies from horses to people is theoretically possible through bites or other contacts that result in introduction of saliva into wounds, cuts or possibly the mouth, nose or eyes. I'm not aware of any documented cases of equine-to-human transmission of rabies, but given the severity of rabies, we need to be careful. Additionally, rabid horses have killed people because they can be unpredictable and aggressive. Based on all these factors, it's important that we not become complacent about rabies in horses, and a key component of rabies control is vaccination. However, vaccination is not the only aspect of rabies control. Using various management practices to reduce the risk of wildlife exposure and controlling rabies in wildlife in the area are other important measures, but individual horse owners can have the greatest impact on their horses' health by ensuring they are properly vaccinated against rabies.

If you own a horse, make sure it's vaccinated against rabies unless you live in a rabies-free region.  More information about rabies in horses is available on the equIDblog Resources page.

Image source: www.bbc.co.uk

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It's common for people to wipe injection sites in horses with alcohol before inserting the needle. It's so ingrained into some peoples' minds that they may complain if their veterinarian doesn't use an alcohol wipe before injecting. But what does that little swab really do, and is it needed?

Alcohol is a disinfectant and can kill many (but not all) bacteria and viruses. Therefore, wiping an injection site with alcohol could theoretically reduce bacterial counts and maybe reduce the risk of an injection site infection. However, alcohol does not work well in the presence of organic debris (i.e. dirt), and may not (and likely won't in most cases) penetrate through a horse's haircoat down to the skin. The best aspect of alcohol is some situations is wetting down the hair to help see the area you're injecting (for example, when visualizing the vein for an intravenous injection).

The risk of injection site infection is very low in the average horse, and the true benefit of wiping the site with alcohol is unknown. There is no clear evidence that this practice reduces infections. The most serious injection-associated infection, clostridial myonecrosis, is caused by either inoculation of clostridial spores (which are resistant to alcohol) or by spores that are dormant in the muscle tissue. In either case, wiping the skin with alcohol won't help prevent it.

Bottom line: There's no evidence that alcohol wiping is needed, or useful. There's also no reason not to do it, so it's really a matter of personal preference. Not using alcohol wipes is not an indication of poor practice, but a lot of people will use them on the chance that they are effective, knowing that at worst they will do no harm.

More information about clostridial myonecrosis can be found on the equIDblog Resources page.

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I received newsletter today from Intervet (a pharmaceutical company) that is targeted at equine veterinarians. One article discussed rabies in horses. It wasn't bad overall, but I thought the section on what to do when a horse might have been exposed to rabies was worth discussing.

The article asks, "If your client suspects that a horse has been bitten by a rabies-infected animal, what should be done?"

Answer: "Contacting you as the veterinarian is always the first step."

Great first step.  A second step that wasn't mentioned should be, "Try to identify and (safely) capture the animal that bit the horse." This is often impossible but certainly worthwhile if it can be done.  However, if you're trying to catch the offending animal, make sure you don't put yourself at risk of exposure to rabies in the process.  If the animal can be caught, it's rabies status at the time of the bite can be determined (either through testing or quarantine). If it can be shown that the animal wasn't rabid, a lot of stress, hassle and expense can be saved.

"If the horse was previously vaccinated... Then isolate and observe the animal for 45 to 90 days (your clinical evaluation will involve gait analysis, radiography and a spinal tap)."

Boosting the rabies vaccine is also a good idea. The next step, however, needs to be contacting local regulatory officials to find out what you have to do. They determine if, how and how long an animal needs to be quarantined - this is NOT the decision of the local veterinarian nor the animal's owner. Most likely, they will recommend a 45 day quarantine for a vaccinated horse, since this is what is recommended in the NASPHV Compendium on Rabies. The discussion of diagnostic testing makes no sense. There is absolutely no indication to perform diagnostic tests on a horse that has been bitten by a rabies suspect. None. There are no tests that can be used to diagnose rabies in live horses (also exposed horses don't instantly develop signs of rabies). Horses should be monitored closely for signs of rabies during the quarantine period, but that's it.

"...and have the client make a list of all people who had contact with the horse."

This is often done when horses have or are suspected of having rabies, but not horses that are potentially exposed. It is done to help public health personnel contact people that may have been exposed to rabies. A horse that was just bitten by an animal is not a risk for transmission of rabies.  (However, keeping a list of people who have contact with the horse after it's been bitten (i.e. durng the quarantine period) - which should be as short a list as possible - is a reasonable precaution in the unlikely event that the horse does develop rabies.)

"If the animal was not vaccinated, your options are to euthanize and perform a postmortem examination of the brain (the only way to definitely confirm rabies)..."

Euthanasia is one of the options that needs to be considered in an unvaccinated horse that has been exposed, which is one of the reasons that identifying the biting animal and testing it is critical, if it can be done. The last part of the above sentence (from the atricle) is complete nonsense. Why would you test the brain of a normal horse that has been euthanized because it's just been bitten by a potentially rabid animal? The horse isn't being euthanized because it has rabies, it's being euthanized because of the likelihood  of it developing rabies weeks to months later. Testing of the brain will tell you absolutely nothing if the animal was only bitten recently.

"...or isolate and observe the horse for six months and develop the human contact list."

Again, this needs to be decided based on discussions with regulatory personnel who are responsible for dictating what is to be done. A six-month quarantine is a pretty standard recommendation for an unvaccinated animal. Creating a human contact list should not be necessary, since quarantine involves severely restricting contact of people with the horse and only a few (ideally one) person would have any type of contact.

The article wraps up with the very true emphasis on vaccinating horses. It's a cheap measure to prevent a relatively rare but invariably fatal disease.

Click image for source.

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Eradication of infectious diseases is a great goal, but it's rarely practical. The best known (and perhaps only) example of infectious disease eradication is the elimination of smallpox. So, why is it so hard to do?

The following general criteria need to be in place to eradicate a disease:

• It must have a clearly defined host range and that range is ideally only one species. A disease that can affect multiple species is very hard to control.
• It must predictably cause disease in individuals that are infected.
• There must be no long-term carriage state. Once a person/animal gets over the illness, he/she/it must get rid of the infection completely in a defined and predictable period of time.
• A highly effective vaccine should be available.
• There must be a commitment to put in lots of time, money and effort everywhere the disease exists.
  

This isn't the case with most diseases, and equine herpesvirus (EHV) has many characteristics that make eradication impossible:

• Unpredictable disease: EHV infection doesn't always cause signs of disease. When it does cause disease the signs can be quite variable and difficult to easily differentiate from other infections.
• Longterm carriage: This is the biggest problem with herpesviruses. EHV is able to survive in a latent (dormant) state in the body after infection. It can lie dormant for a long period of time, but infected animals can always start shedding the virus again. A large percentage of horses are carrying EHV in their bodies and there's no way to get rid of it.
• Vaccine: Vaccines are available but they are by no means 100% effective at preventing infection.
• Time, effort, money and cooperation are terms that are not commonly associated with disease control in horses. Getting everyone to follow a standard recommendation (if one were able to control disease) would be difficult to impossible. The entire horse-owning population would not be willing to spend the money for broad control measures, and there's no real impetus for governments to do so. Even getting people to agree to follow basic vaccination and infection control recommendations is difficult.  If there is any negative impact on use of horses, ease of management or any other minor inconvenience, 100% compliance with any recommendation becomes impossible to obtain.

We have to live with EHV. It will always be a risk to horses. Good infection control measures and vaccination of certain groups (e.g. pregnant mares) can help control the impact of the virus.

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Herd immunity is an important infectious disease concept. Basically, it involves trying to ensure that a high enough percentage of a population is resistant to an infectious disease so that the disease cannot be spread easily through the group. Ensuring that a large percentage of the population is vaccinated helps protect individuals that cannot be vaccinated (because of allergy, disease or other reasons) or that did not properly respond to vaccination (not all vaccines protect all vaccinated individuals).

"Herd immunity" usually refers to this concept when applied to herds of animals (such as horses or cows), but the "herd" can be a a small local population, a regional population, or broader, and it can be people or animals. For some human infectious diseases, it's been shown that vaccination of 75-95% of the population is required to prevent outbreaks. If vaccination rates start to slip, the chance of an outbreak increases. This is best seen in some areas where vaccination rates decline in certain groups of kids because parents are reluctant to have their children vaccinated (for one reason or another), and subsequently outbreaks of disease start occurring (or increasing).

Vaccination is an important (but not the only!) infection control tool. For diseases that are transmissible between horses (or dogs, or people, or whatever other "herd" is being considered), vaccination of a single horse helps protect that individual from disease, and also helps protect the rest of the population.

During a public health infectious disease course that I teach, a student showed a link to this interesting and amusing demonstration of herd immunity from the UK. Make sure you have your sound turned on. It's an entertaining description of the concept of herd immunity.

When you're deciding on your vaccination program, for both individual horses or a farm, be conscious of the herd immunity concept.

This equIDblog entry was originally posted on the Worms & Germs blog on 05-Oct-09.

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A Nova Scotia horse has died of eastern equine encephalitis (EEE). EEE is a rare disease in Canada, but it is sporadically identified in some provinces. It was reported that this is the "first case" in Nova Scotia, although it wasn't clear if they meant in 2009 or ever.

EEE is a highly fatal mosquito-borne viral disease of horses. There is no specific treatment. The virus can also affect (and kill) people, but horses cannot transmit the infection to people -  horses and people (and some other species) are infected by being bitten by infected mosquitoes.

The veterinary clinic that diagnosed the case reported that 120 doses of EEE vaccine will be delivered there this week. A ProMed moderator commented that he/she hoped a major vaccination campaign will be started. However, it's far from certain that any vaccination efforts will be effective, particularly for this season. EEE is a very sporadic disease in Canada, but since it's so severe, vaccination certainly should be considered. However, considering most horses in Nova Scotia have never been vaccinated previously for EEE, it will likely take a couple of doses and several weeks before adequate antibody levels are present. By that time, the risk of exposure may be even lower because the mosquito season is ending in the maritimes. I'm definitely not saying don't vaccinate, however, because of the short remaining mosquito season and the lag time from vaccination to effective immunity, the immediate focus should really be on mosquito avoidance.

Deciding whether or not to vaccinate against EEE in areas where it is very rare and sporadic can be tough. From a population standpoint, it's somewhat difficult to recommend vaccinating all horses against a disease that may only affect a handful or horses, if any. However, from an individual horse standpoint, it's bad news if you happen to be that rare horse that gets infected. Really, it comes down to risk aversity and economics, and it's up to each horse owner (in consultation with their veterinarian) to determine what diseases they should vaccinate against.

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In the wake of the death of Dr. Alister Rodgers from Hendra virus, there have been increasing calls for the Australian government to put significant resources into Hendra virus research. Various areas need to be investigated, including how this virus is maintained in the bat population, how it is transmitted from bats to horses, ways to treat infection and ways to prevent infection. Vaccination is an obvious topic, and creation of a vaccine appears to be possible. However, as I wrote the other day, there's a question about whether a company would put millions of dollars into development of a Hendra virus vaccine for people, given that the disease is very rare, is currently limited to one region, and only appears to be a risk for people in close contact with sick horses.

One thing that needs to be considered is whether it may be better to develop a vaccine for horses rather than people. Think about it:

• All reported human Hendra virus infections have come from people in close contact with sick horses.
• Human vaccines are very expensive to develop, test, get approved and market.
• Vaccines for animals are much cheaper to make because testing and regulatory requirements are not as strict. (This can lead to marketing of vaccines for animals with limited evidence of effectiveness, but the upside is that vaccines can get to market quicker and with less expense.)
• People are often more willing to get their horses vaccinated than to get vaccinated themselves.

So, even though it might sound strange, development of a Hendra virus vaccine for horses may be a more effective way to protect people.

If this approach is taken, a key step would be continued research into the epidemiology of Hendra virus infection to investigate other routes of human exposure. If people can get infected by other routes, vaccination of horses obviously wouldn't address the entire problem. However, based on what we know currently, vaccination of horses might be the most effective, timely and economic response to this pressing problem.

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Hendra virus, as you've probably seen here and elsewhere, is back in the news as the cause of another outbreak in horses in Australia with subsequent transmission to people. This bat-borne disease is very rare but devastating, with high mortality rates in both horses and people. Currently, a veterinarian is in critical condition in the ICU of a Brisbane hospital fighting this virus, while other exposed horse farm personnel are waiting to see whether they get sick too. Last year, another veterinarian died. Because of the severity of disease, vaccination gets discussed. Development of a Hendra virus vaccine is possible and a prototype vaccine has been produced. However, the question is will such a vaccine ever get used?

Vaccines are very expensive to develop, test and market. Given the nature of the pharmaceutical industry, there usually has to be a reasonable expectation that the costs will be recovered through sale of the vaccine. Some treatments for rare diseases that have no chance at making money are produced by pharmaceutical companies as a service to society, but not every money-losing product can be made. The problem with Hendra virus, in terms of vaccination, is that it is such a rare disease. Only 7 people have been diagnosed with the infection. It's tough for companies to justify spending millions on a vaccine for such a rare disease if they don't think people will use it. Vaccines for rare diseases can be profitable if a lot of people get vaccinated (e.g. rabies). However, given the sporadic nature of Hendra virus, the limited geographic range where it occurs and the fact that it seems only people with close contact with horses are at risk, the market would be very small. Unless things change with respect to how common this disease is (and we all hope not to see any more of such a terrible disease), I doubt we'll ever see this vaccine on the market.

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A recent post about mandatory vaccination in show horses sparked a discussion about concerns regarding adverse reactions to vaccines. Dr. Carolyn Cooper of the Canadian Food Inspection Agency posted the following comment.

"There is mandatory reporting of adverse vaccine reactions in Canada by the vaccine manufacturers to the Veterinary Biologics Section (VBS) of the Canadian Food Inspection Agency. VBS monitors the type and frequency of reports in order to identify any potential issues with the safety and efficacy of licensed products. In order to increase the usefulness of mandatory adverse reaction reporting, veterinarians should report all suspected adverse reactions to the vaccine's manufacturer. Any comments regarding the licensing of veterinary vaccines in Canada, or adverse reaction reporting in Canada, can be directed to VBS at http://www.inspection.gc.ca/english/anima/vetbio/conpere.shtml "

This is a very important reminder for veterinarians and horse owners. While there is mandatory reporting, adverse reactions to drugs and vaccines are rarely reported by veterinarians. There's nothing underhanded about the lack of reporting, it's mainly that people don't think about it when it happens. Without good data, however, we are left to debate the true risks of vaccines and drugs with only incomplete information. Drugs and vaccines can be marketed for animals with rather minimal safety data. Even with large studies, safety issues can be missed until the drug/vaccine is mass marketed, as has been shown with some high-profile human drugs. If regulatory agencies don't get the data (i.e. if the reactions aren't reported to them), then they can't adequately monitor safety concerns. If your horse has an adverse reaction to a pharmaceutical product, you should make sure this is reported. It doesn't take your vet much time and shouldn't take any effort on your part.

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Last year, there was a large equine influenza outbreak at the Crawford County Fair in Pennsylvania. In response to that, a mandatory influenza vaccination policy was implemented. However, registrations for this year’s fair have dropped by 1/3 from 457 to 310. One horse owner says “It’s because of the shots.”

This is a pretty unfortunate response by horse owners. Mandatory vaccination is an entirely reasonable component of an infection control program that should be more widespread. Shows are optimal environments for transmission of disease. Huge numbers of horses get sick ever year from infections acquired at shows and outbreaks are not exactly rare.

One horse owner explains her decision not to show as “It’s the expense, plus there’s the risk of an adverse reaction to the shots.” That’s code for “It’s the expense.”

Vaccines are pretty cheap, especially when you consider how much money is spent on the horse and showing each year. People often spend huge amounts on boarding, (largely unnecessary) supplements, tack and other supplies, along with the costs of registering and traveling to shows. Concerns about adverse reactions are often used to justify non-vaccination, but it’s usually just an excuse. Adverse reactions to vaccines are quite rare and typically minor. Adverse reactions to intranasal influenza vaccination are extremely rare to non-existent.

Hopefully this fair sticks with this policy. They should be commended for taking a responsible action and more fairs should be doing the same thing. Personally, this is a fair that I’d want to show at. The vaccination requirement means there’s less chance of a horse acquiring influenza. Also, weeding out people who are too cheap or otherwise unwilling to use good preventive medicine strategies should decrease the pool of horses that are carrying other infectious diseases. Those people can organized their own vaccine-free show (the biohazard games).

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A horse in Harford county Maryland has been euthanized because of rabies. The horse first starting showing signs of disease in mid-July, which manifested as "striking changes in behaviour." The report doesn't say when the horse died, but animals typically die within a few days of the onset of neurological disease. The horse was transferred to the New Bolton Center where rabies was diagnosed. Subsequent testing showed it was a raccoon rabies strain, although that does not mean that a raccoon was the actual source of infection.

Public health officials implemented a 45 day quarantine of the farm. Stray cats (about 25) were caught and euthanized. Fortunately, the family pets were properly vaccinated and have received booster shots (plus presumably a period of observation at home... a much better situation than if they were not vaccinated).

People that had contact with the horse have received rabies post-exposure treatment. This includes one person who had to be tracked down overseas.

Harford County Health Department spokesperson Bill Wiseman said "There was never a risk to public safety. This incident was a great example of public health work in action and cooperation between local, state and in this case, international authorities." I don't buy the statement that there was no risk to public health. While the risk of rabies transmission from infected horses is very low, it's not zero. Rabid horses have killed people because of their abnormal and sometimes aggressive behaviour. Further, the fact that this horse had rabies means that it got it from something. Rabies can have a long incubation period so it's not guaranteed that it acquired it on the farm, but you have to be prudent and assume that there is infected wildlife in the area that could pose a risk for other animals or people. Public health authorities managed the situation well and reduced the public health risks, but there were certainly still risks.

Rabies vaccination is highly effective. There is no statement about whether this horse was adequately vaccinated but it's unlikely. Proper vaccination would likely have prevented this horse's death, as well as the death of the stray animals, cost of vaccination of people, cost of veterinary care for this horse, quarantine of the farm and the associated financial and emotional costs. A dose of vaccine that costs a few dollars could have saved thousands of dollars and emotional stress.

Rabies is a rare disease in horses but its severity means it should not be ingored. Vaccinate your horses.

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Here's a question I received the other day:

"Do people who work with animals and who work in barns need a tetanus shot as a result of this type of work?  We have Therapeutic Riding Programs in the region and there is a sense that perhaps the volunteers and those who frequently tend the horses need to receive this.  Is this the case?"

Tetanus is a disease that we are quite concerned about in horses because horses are very susceptible to it. That's why we vaccinate them yearly. Tetanus can also affect people, but very rarely because of vaccination and because people have lower susceptibility to the disease. While we pay a lot of attention to tetanus in horses, this does not mean that being around horses increases a person's likelihood of exposure to tetanus. The bacterium that causes tetanus, Clostridium tetani, lives in soil and commonly present in the environment. The more environmental exposure that you have (especially to soil), the greater your risk of exposure to C. tetani. Being around horses doesn't increase your risk any more than doing other things outside.

Whether you have contact with horses or not should not change your approach towards tetanus prevention. You should be vaccinated against tetanus every 10 years. Many (probably most, actually) adults are not up-to-date on tetanus vaccination. Adults tend not to get booster shots on schedule, and often only receive them when they have had a wound that requires medical care. For example, If you get stitches, the medical staff will almost certainly inquire about your last tetanus shot, and give you another one if you haven't been vaccinated in the past 10 years (or if you can't remember).

More information about tetanus in horses is available on the equIDblog Resources page.

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It was only a matter of time, but eastern equine encephalitis (EEE) has been found in more US states this year. EEE is an seasonally important disease in some areas, including Louisiana. More cases in more states, and perhaps even some regions of Canada, are likely over the course of the summer and fall.

The latest case of EEE infection was reported in a horse in Rapides Parish, Louisiana. Not surprisingly, the horse died, as EEE has a very high mortality rate in horses - and in people.

If you live in a region where EEE cases have been identified or typically occur, you should have your horse vaccinated. Really, you should have already had your horse vaccinated, because it takes time for the vaccine to work after administration. However, even if your horse hasn't been vaccinated yet this year, get it done (better late than never).

Another important aspect of preventing EEE cases is mosquito control. EEE, like Western equine encephalitis and West Nile virus, is transmitted by mosquitoes. Measures should be taken to reduce mosquito populations in the area and mosquito exposure, both for your horse and yourself.

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The American College of Veterinary Internal Medicine (ACVIM) has released a consensus statement on equine herpesvirus type 1 (EHV-1). The ACVIM is the specialty organization for veterinary internal medicine in North America, and every year it produces consensus statements on selected large and small animal topics. These are developed by a group of experts in the field, who review current knowledge (and knowledge gaps) and come up with a comprehensive overview of the subject. This consensus statement has detailed information on various aspects of EHV-1, including diagnosis, vaccination and infection control. It can be accessed through the ACVIM website, on the equIDblog Resources page, or by clicking here.

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Botulism is suspected in the deaths of 10 horses on a farm in Wyoming, USA. Very few details are available to indicate why botulism is suspected or what the source might be. It was reported that six horses died initially, and four others that were lethargic and unable to stand died within 24 hours. These signs fit with botulism, which is characterized by profound muscle weakness and progressive paralysis. Death is caused by an inability to breathe when the paralysis starts to affect the muscles that control breathing. Making a diagnosis of botulism can be difficult in horses - it is often a "diagnosis of exclusion" whereby the diagnosis is made because everything else that could cause the condition has been ruled out.  Testing of some of the affected horses is underway. It was not reported whether possible sources of the disease (e.g. feed) are also being tested.

Botulism is caused by a toxin produced by the anaerobic bacterium Clostridium botulinum. In adult horses, the disease almost always occurs from consuming the toxin in contaminated feed.  If the bacterium (which can be found in the soil in many regions) gets incorporated into feed, under the right conditions the organism will grow and multiply, and produce the powerful neurotoxins that cause botulism. Fermented feeds like haylage and silage are common sources, because the way they are stored is more likely to create an environment where C. botulinum can grow.  However, various other food sources have also been implicated in some outbreaks.

Botulism is rare, but typically fatal in adult horses. Some basic tips to reduce the risk of botulism are:

• Avoid feeding horses haylage and silage, especially in areas where botulism is more common. There are some benefits to using feeds, so they are commonly used in some regions, but if you use fermented feeds you should discuss the risk of botulism with your local veterinarian, and take measures to reduce the risk of C. botulinum growth.
• Vaccines against botulism are available. Adult horses are usually not vaccinated, other than mares which are vaccinated to reduce the risk of botulism in their foals (known as "shaker foal syndrome"). Adult horses being fed high-risk feeds could be considered for vaccination, but if the feed is so high risk that vaccination is warranted, perhaps the feed program should be reconsidered.  Furthermore, there are several types of C. botulinum, but the vaccines only protect against some of them. If you are thinking about vaccinating, you should make sure that the vaccine you're going to use protects against the botulism strains found in your area.

If you ever have a horse that is showing signs of botulism (e.g. weakness, difficulty rising, dropping food out of its mouth, difficulty swallowing), or if botulism is suspected in a sudden death, immediately stop feeding any potential high risk feeds until the cause of the disease is identified. This might help reduce the number of affected horses.

More information on botulism can be found in our botulism archives. An information sheet about botulism in foals can be found on the equIDblog Resources page.

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Here's a recap of West Nile virus activity in Canada in 2008.

Humans: There were 36 people diagnosed with disease caused by West Nile virus. Five had neurological disease, 29 had non-neurological disease (i.e. West Nile fever) and in two cases the clinical syndrome was not defined. Most affected people were from Saskatchewan (17) and Manitoba (12), with three from Ontario and two from Quebec. There were single cases in Alberta and British Columbia, and both of those were thought to have been acquired outside of the respective provinces.

Horses: West Nile virus infection is an immediately notifable disease in Canada, whereby the Canadian Food Inspection Agency must be notified of all diagnosed cases in animals. Six positive horses were reported in Canada in 2008, including two in Ontario horses, both in eastern Ontario. Two other cases were reported in Saskatchewan and two more were reported in Quebec. Whether this low number of diagnosed cases is because the disease is now actually this uncommon in canadian horses, or because there is less testing for the disease is unclear. Based on anecdotal information from other veterinarians and horse owners, I suspect that West Nile virus infection in horses is truly rare in Ontario, but is more common in Saskatchewan. This makes sense considering the different types of mosquitoes in the two provinces, as well as the corresponding difference in the number of human cases. These questions highlight the need for proper diagnostic testing to be performed on all horses with neurological disease. The more confidence we have in the surveillance data, the better conclusions we can make about patterns of disease and the need for vaccination.

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Scott and I are having a difference of opinion. Speaking from experience, it’s certainly difficult to win a debate with him most days (that might be a huge understatement), but I’ll weigh in with my two cents on this one. If Scott were to vaccinate a horse for West Nile virus (WNV), he’s stated that he wouldn’t do so until July, based on when the peak incidence of disease and therefore presumably the period of highest risk is. I would still vaccinate a southern-Ontario horse in April. Here’s why:

West Nile vaccines are labeled to provide protection for 12 months. Before they can be marketed with such a label, these vaccines have to be tested to prove that they still offer some protection for the animal for at least that long. For some vaccines, like rabies, protection likely lasts much longer than the label claim, but until recently no one’s bothered to study most vaccines beyond one year. I have no doubt that the protective immunity does decrease with time – the protective effects of vaccine are likely highest (as Scott said) about 30 days post vaccination, and lowest at the end of the 12 months. But there is no evidence that the immunity drops off so fast that after 4-6 months the vaccine would require a booster to be adequately, if not maximally, effective. There are vaccines, like herpesvirus and influenza, for which we recommend boosters for horses semi-annually, but this is for animals that are at ongoing high-risk for exposure to these diseases, which are very common. Six months after mid-April is mid-October, and in this part of the world there are very few mosquitoes still flying around at that point.

Scott pointed out that the first part of the summer is likely lower risk in terms of WNV transmission, at least for horses. But there is a time of year when the risk is even lower – October to April, when (as I just said) there are virtually no mosquitoes. If you vaccinate a horse in July, its immunity will be lowest over the first three months of the summer, when there are still birds and mosquitoes around that are carrying WNV. If you vaccinate a horse in April, its immunity is lowest in the late winter, when there’s almost no risk of transmission, so it doesn’t matter! I would rather have a horse protected for the entire mosquito season (May-September), and in the vast majority of cases the animal’s immunity will still be quite adequate come peak season in August, even without an extra booster in July. (Although we want to protect our animals from infectious disease, we also don’t want to give them any more vaccines than we have to.)

I don’t think there’s a right or wrong answer in this case. The best thing to do is talk to your veterinarian about the pros and cons of doing things either way, while taking into consideration the conditions in your specific region in terms of vector populations, disease prevalence, and the health management priorities for your own animals.

Photo: Transmission electron micrograph (TEM) of the West Nile virus (WNV). (Credit: Cynthia Goldsmith, CDC Public Health Image Library ID#10701)
 

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As spring approaches (slowly... at least here in Ontario!), people once again start thinking about vaccination programs for their horses. A question that comes up every year is when is the best time to vaccinate against mosquito-borne diseases? Depending on your region, the pathogens of concern may include West Nile virus (WNV), as well as Eastern/Western or Venezuelan equine encephalitis virus (EEE, WEE and VEE, respectively). Often people get the generic response of "30 days before mosquito season," which isn't always very helpful.

Why? It comes down to some basic mosquito biology and timing.

All mosquitoes are not alike. There are two main groups of mosquitoes when it comes to  transmission of diseases like WNV that circulate primarily in the bird population. One group is called "amplifiying vectors." These mosquitoes feed on multiple birds, thereby spreading WNV and increasing the number of WNV infected birds. However, these mosquitoes rarely feed on other types of animals, so they are not very important in transmission of WNV to horses and people. The other group of mosquitoes is the "bridging vectors." These mosquitoes feed on many different types of animals, not just birds. They can pick up WNV by feeding on infected birds, and then transmit the virus when they subsequently feed on horses or people. This is the most important type of mosquito in terms of disease transmission. Areas that have larger populations of more efficient bridging vectors tend to have greater problems with WNV.

The "30-day" part of the recommendation comes from the concept that we want to vaccinate animals about 30 days prior to the risk of exposure, so that the vaccine has ample time to work. The hard part is deciding when to start counting. One of the important things to consider is when do cases typically occur? If, as in many areas, WNV cases are not seen until late August or September at the earliest, vaccinating in April is probably not optimal. Your veterinarian and/or public health department may be able to tell you when West Nile virus activity is typically highest in your area. While there's no guarantee that things will be the same every year, it's unlikely that the WNV season would start well in advance of earlier years. Here in Ontario, disease due to West Nile is rarely reported in horses any more, but the cases that do occur still tend to appear in late August, at the earliest. Therefore, I'd be hesitant to vaccinate for WNV in the spring. If a horse is vaccinated in late April, it will be four months between vaccination and the time of peak risk. The period of peak protection would occur over a few months when there is little evidence of risk of exposure. So, if I was going to vaccinate against WNV, I'd start in late July. If a horse had been vaccinated in the spring, I'd consider a booster in the fall to make sure that the periods of peak immunity and peak risk conincide.

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A mandatory influenza vaccination policy at the Crawford City Fairgrounds (Pennsylvania), implemented this year following an outbreak of influenza at the facility in 2008 is being met with disdain.

Among the complaints:

"A two-day horse show is different than an eight-day fair exhibition.'" Two days is lots of time to transmit infectious diseases.

"If a horse is sick and can’t perform, people won’t bring them to a show." Unfortunately, that's not the case. Also, not all horses that are shedding infectious diseases look sick.

"All these requirements will not guarantee healthy horses." Absolutely true. But, with infection control, we are trying to reduce the risk and incidence of diseases. We know we'll never eliminate the risk.

"There’s no sense giving a shot that will do more damage than the disease." Influenza vaccination is very safe. While adverse effects can occur with any vaccine, influenza vaccines are quite low risk.

"Potentially, requiring a 30-day vaccination for each show will cost horse exhibitors and 4-H members in horse clubs too much money." The cost of vaccination is dwarfed by other costs of participating in such events. People spend a lot of money on board, feed, tack, supplements (many of which are useless), trailering and many other items. In the grand scheme of things, vaccines don't add much to the total yearly cost.

No one knows whether mandatory vaccination against influenza ought to required for all horse shows as an infection control precaution. However, given the fact that outbreaks can occur, that all events involving mixing of large numbers of horses pose a high disease risk, and that vaccination is relatively cheap and effective, it seems like a logical requirement to me.

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Following an influenza outbreak that sickened at approximately 70 horses last year, the Crawford Country Fair in Pennsylvania is requiring influenza vaccination for all equine participants this year. This year, all horses must be vaccinated between 6 months and 15 days prior to the fair. Vaccination against such an infectious disease is a sound recommendation to be sure, although the six-month window might be a little long. I'd prefer to see the horses vaccinated closer to the time of the fair, in order to maximize the protective effect during the highest-risk period.

The fair board is also requiring a certificate of veterinary inspection for each horse at the fair, issued within 30 days prior to opening day. I'm not sure that this kind of inspection will do much in terms of preventing sick horses from participating in the fair, as most infectious diseases (including influenza infection) don't last for 30 days.

• If a horse is clinically healthy 30 days before the fair, it certainly doesn't mean the animal won't be infectious at the time of the fair if it is exposed to a virus (or other pathogen) in the interim.
• If a horse has influenza 30 days before the fair, there's not much chance it will still be shedding the virus by the time of the fair.

While I wholeheartedly agree with the sentiment that regular veterinary examination is important for infectious disease control, but the time frame in this case just won't do much for control of influenza. If the fair board really wanted to reduce the risk of sick horses attending, they'd have to require inspection much closer to the date of the fair.  However, such a requirement could turn into a logistical nightmare, since a large number of horses would need to be evaluated by a small number of veterinarians over a short period of time. Furthermore, if someone competed at many fairs during the year and they all had similar requirements, a horse might have to undergo a weekly exam before each event, which could be cost-prohibitive.

Despite the limitations, this is still a good, proactive step on the part of the fair board, and similar precautions ought to be considered elsewhere. We need to stop simply accepting that some horses will get sick following fairs and that outbreaks will happen, and realize that there are practical measures that we can do the reduce (although not eliminate) the risks. One of the best things that could be done would be to require examination of all horses upon arrival at a show or event. The examination could be done by a veterinarian or veterinary technician.  At a minimum,  requirements could include that horses have no signs of respiratory disease, diarrhea or fever. Horses with these problems could then be immediately send home, greatly reducing the risk of disease transmission to other animals at the event. Yes, it would take some effort and money, but it really wouldn't be that hard to do, and could be a very useful preventive measure.  Compared to the cost and headache of quarantining and treating sick animals when an outbreak does occur, the idea really sells itself.

I'm not trying to be negative about the approach taken by this particular fair board. I think their awareness of the need to do things to reduce the risk of infectious diseases at their event is great. 

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A recent case of rabies in a horse in Kentucky should serve as a reminder of the importance of vaccination against this disease. The affected horse was a Thoroughbred that was presented to a referral clinic for signs of severe colic. After surgery, the horse began showing signs of neurological disease, developed violent seizures and was euthanized. This type of presentation is not unusual, as rabid horses often start out looking like they have other problems such as colic. The scenario of taking a horse to surgery due to signs of severe colic and finding out later that it actually had rabies is nothing new. A major concern with the non-specific nature of the early signs of rabies in horses is that a large number of people may be exposed to the horse before rabies is even considered.

Rabies is always fatal in horses. Transmission of rabies from horses to humans is extremely rare, but is a potential concern. Rabid horses can also be very dangerous due to their unpredictable and sometimes very violent behavior, and rabid horses have killed people. At the same time, rabies vaccination is cheap and very effective.  All horses should be vaccinated against this devastating disease.

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While it's a little early in the foaling season, it never hurts to think about vaccination plans for foals. Vaccination is an important part of the disease prevention program - but it's only one part... general infection control practices are as, or often even more, important. Vaccination of foals is not the same as vaccination of adult horses. Foals may be more or less susceptible to certain infections. They may have greater or lower risks of exposure, and they may respond differently to vaccines.

There is no standard vaccination program for foals. Specific vaccination programs need to be designed for each horse on each farm based on risk of exposure and infection, cost and many other factors. Your veterinarian can help you determine what your animals should be vaccinated against. When it comes to foals, here are some important things to considered:

• Foals typically need to be vaccinated several times to get the desired immune response. There are at least a couple of reasons for this. First, if a foal has high antibody levels from colostrum, it may not respond properly to the vaccine until those antibodies are used up. These antibody levels drop over time, but the rate and timing of the drop is different for each foal. Vaccinating the foal several times (at appropriate intervals) helps reduce the risk of vaccine failure from colostral antibodies. Second, foals are born with a fully functional immune system, but the immune response to an infectious agent or vaccine is slow and low the first time an individual encounters it. The first dose of vaccine may produce some response, but a much higher response is generated with boosters.  This is actually true of adult horses as well.
• Over-vaccination can be a problem. Vaccination too early and/or too frequently could actually result in a decreased immune response or complete vaccine failure, a phenomenon known as immune tolerance. So, giving many doses of vaccine starting at a very young age may actually be counter-productive. This is probably a bigger concern with influenza vaccination compared to other vaccines.

Talk to your veterinarian about vaccination. Over-, under- and improper vaccination can lead to increased disease risks, plus wasted time and money.

Guidelines for vaccination of foals have been developed by the American Association of Equine Practitioners. These are also available from the link on the equIDblog Resources page.

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Here's a good question posted by a reader on equIDblog:

Could you please comment on adverse reactions to routine vaccinations? I have two mares - a  9 year old welsh cross pony and an eight year old paint - that have both experienced painful neck swelling for a week after their rabies and tetanus shots. They aren't able to stretch their heads to the ground to eat and I've had to elevate their food. The affected area involves the entire neck, not just the injection site, and there are no abscesses involved. My veterinarian suggested giving Banamine prior to the vaccination, but just the fact that he has developed a routine to deal with this tells me that it's not all that uncommon. I'm now debating whether to put my horses through the ordeal at all or perhaps vaccinating every other year. Do mature horses reach a point where they no longer need boosters? Your input would be greatly appreciated.

Vaccine reactions are fairly uncommon and are usually mild, but some horses are prone to more severe reactions and it's often hard to decide what to do with them. Vaccinating them obviously poses some risk, be it short term discomfort or the potential for more serious problems. Not vaccinating also poses risks, if the horses are left more susceptible to infectious diseases. As the veterinarian in this case suggested, sometimes pre-treating the horse with an anti-inflammatory like Banamine (flunixin meglumine) can help reduce the risk or severity of some adverse reactions. However, even this doesn't work in all cases.

The question of whether adult horses need to be continually re-vaccinated is a good one. The answer really is we don't know. It is likely that for many diseases, yearly vaccination of older horses may not be necessary. We simply don't have any good data on how long immunity persists in older horses following vaccination and the risks associated with extending vaccination intervals. In cats, vaccination recommendations have been changed in recent years to include decreased frequency of vaccination of older cats, largely because of concerns about serious adverse effects in some of these animals. These recommendations were made by the American Association of Feline Practitioners, a prominent feline veterinary group. I'm not aware of a similar initiative to investigate this issue in horses.

An important difference between vaccinating house pets and horses that we need to consider is herd immunity.  Protection of a large population of horses is increased by maintaining a high level of "herd immunity" through vaccation of a large percentage of horses. Since horses more often live in large groups, move between groups and are probably at greater risk of infectious disease exposure compared to your average household cat, extrapolating from vaccination recommendations for cats should be done with great caution. What we need are sound scientific studies of how long protective immunity lasts in most horses, and the potential implications of decreasing vaccination frequency. Unfortunately, it's unlikely that most pharmaceutical companies would put much time or money into this type of research.

So, there's no clear answer. In a situation where a horse is known to have adverse reactions to vaccines, I'd either pre-treat with anti-inflammatories or vaccinate less frequently. If the horse was low risk for infectious disease exposure based on its management, use and lifestyle, I'd be more likely to avoid vaccination or do it less frequently. However, you have to realize that there might be an increased risk of disease from this. Also, not maintaining rabies vaccination could have major implications if the horse is ever exposed to a rabid animal.

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Potomac Horse Fever (PHF), a disease named for the river near Washington DC where it was first identified, is caused by Neoricketsia risticii.  The technical name for PHF is equine monocytic ehrlichiosis (EME).  This causative organism is normally found in a trematode parasite with a somewhat complex life cycle involving bats, snails and certain aquatic insects. It is therefore associated with swampy/marshy areas in certain regions.  Horses are aberrant hosts of N. risticii, in which infections can cause a range of disease from very severe colitis and laminitis to mild non-specific illness.  The organism is typically very sensitive to treatment with tetracyclines, but the disease can progress very quickly, so treatment early in the course of disease (if PHF is strongly suspected) is crucial.

In general, PHF is an uncommon disease in horses, but there are certain areas of Ontario where cases occur with some regularity. For example, cases have been reported in Eastern Ontario (Brighton to Kemptville), Keswick area and Ancaster. The incidence is usually highest in late summer. Transmission is normally associated with horses grazing pastures containing infected flies (caddisflies).

Although vaccines for PHF are available and appear to be safe, their efficacy is very questionable. At least in part, this may be due to the fact that multiple strains of N. risticii occur naturally, but only a single strain (from the 1980s) is included in the vaccines.  Vaccination is not considered necessary for horses in areas where the disease has not been previously reported. Given the peak incidence of the disease in late summer and the nature of the vector, if someone wants to vaccinate, they should do so in late spring. Preventing access of horses to areas near swamps and other insect breeding grounds may be as (or more) effective than vaccination alone.

Photo Credit: Buck Cash, Great Falls, Potomac River, VA

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I was reading a fact sheet on foal care today that was available online. It was one of those sources that gives a mixture of good, mediocre and bad information. One thing that I didn't like was a reference to tetanus antitoxin administration being a common practice in newborn foals. That might have been the case a couple of decades ago, but it is certainly not recommended now.

Tetanus is certainly a serious problem in horses, and foals are at risk for exposure because the bacterium that causes the disease, Clostridium tetani, is widespread in the environment. Good measures need to be in place to reduce the risk of foals developing tetanus. The most important steps are:

• Ensure the mare is properly vaccinated for tetanus prior to foaling. This includes a booster that should be given approximately 30 days before foaling.
• Ensure that the foal gets enough good quality colostrum (see the information sheet on the Resources page for more details about colostrum for foals).
• Treat the umbilicus properly after birth (for more information, see our previous post entitled "Starting Out Right - Foal Umbilical Care").

If these things are done, there is no need to give tetanus antitoxin to a newborn foal. In fact, it's not only unnecessary, it's a potential health risk. Tetanus antitoxin neutralizes tetanus toxins that are in bloodstream. It used to be widely used in horses of all ages if they were wounded. However, administration of tetanus antitoxin has been associated with a disease called Theiler's Disease or serum hepatitis. While rare, this is a severe and almost always fatal liver disease.

Tetanus antitoxin is only potentially indicated in foals whose mares were not vaccinated against tetanus (or their vaccination status is unknown) and possibly in foals that did not receive adequate colostrum.

More information on tetanus, colostrum and vaccination recommendations can be found on the  equIDblog Resources page.

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Most pharmaceutical products are dosed on the basis of weight (e.g. milligrams of drug per kilogram of body weight). That means an animal twice the size of another gets twice the dose.

Other drugs (mainly chemotherapeutic drugs, like those used for cancer treatment) are dosed based on body surface area (e.g. milligrams of drug per square metre of body surface). With this type of dosing, large individuals get more than small individuals, but the differences are not as great as with weight-based dosing.

Vaccines are a different story. They are administered based on the "antigenic dose" which is independent of body size. Therefore, the same dose is required for a miniature horse and a clydesdale. While it may be tempting to split doses of vaccine between several miniature horses (and initially this may seem logical (based on their small size) to those who do not realize how the dose is determined), this may result in ineffective vaccination. Trying to save money by splitting vaccine doses can end up costing money through increased risk of disease. Always give the full dose of vaccine as described on the label.

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Recently, I discussed issues regarding equine influenza vaccines and how they are not updated as regularly as human vaccines.  The H3N8 type of equine influenza is the main concern in horses. This strain has two main groups, European and North American.  Standard WHO/OIE recommendations are that equine vaccines should include a representative of each of these groups. An outbreak of equine influenza in Croatia in 2004 (reported by Barbic et al. in the journal Veterinary Microbiology) highlighted the need to follow these recommendations, and the need to be diligent about looking for causes of vaccine failure.

The outbreak occurred at a racetrack in Zagreb. Not surprisingly, an H3N8 strain was responsible. Investigation of the cases determined that vaccine status had no influence on disease - both vaccinated and unvaccinated horses became equally sick. The vaccine used in Croatia at that time contained three influenza strains, including two different H3N8 strains, but the strains that were used were from 1963 and 1979. When they compared the strain that caused the outbreak and the vaccine strains, there were multiple genetic differences, which is not surprising given influenza's capacity to evolve over time. There were far fewer differences with the more recent strains used in vaccines in most other countries.

The study concluded "Further surveillance of the equine population and updating of equine influenza vaccine strains in accordance with the recommendations of the Expert Surveillance Panel is necessary in Croatia to reduce the likelihood of further outbreaks as a result of vaccine failure."

While this outbreak was clearly a result of not following standard recommendations, it's a good reminder that ongoing surveillance is needed to detect and control emerging strains against which horses will not be protected by current vaccines.

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This post originally appeared on the Worm & Germs Blog site on December 12, 2008.

In 2008, the American Association of Equine Practitioners (AAEP) published updated vaccination guidelines for horses.  One of the changes from the previous set of guidelines was the inclusion of rabies as a core vaccine (meaning every horse should receive it).  There was lots of discussion about this at the recent AAEP Annual Convention in San Diego, CA.

Some veterinarians don't like the idea of vaccinating every horse against rabies.  Just like veterinarians and owners of dogs and cats who are concerned about over-vaccination in these species, the same concerns exist in equine medicine.  Equine rabies vaccines are not approved for use every three years like some canine and feline vaccines, so they still need to be given every year until someone can determine for how long a vaccinated horse is protected from infection.   Furthermore, there has never been (to my knowledge) a case of human rabies due to transmission from a horse.  These are all valid points, but there are also a lot of reasons why including rabies as a core vaccine for horses is very good idea:

• Rabies is a very deadly disease, in both animals and people.  To some owners, their horse is every bit a part of their family as any dog or cat could be.  To other owners, their horses represent a great investment, and part of their livelihood.  Even if the risk of disease in horses is low, protecting them is safe and easy, so it just makes sense.  As the saying goes, an ounce of prevention is worth a pound of cure, but when there is no cure and prevention is so simple... you do the math.
• Rabies vaccination is extremely effective in horses, producing an excellent immune response even with a single dose.  It does not require complex adjuvants that some other vaccines need to stimulate the immune system, which also makes it less likely to cause an abnormal vaccine reaction.
• Rabies is not a seasonal disease like many of the respiratory viruses or insect-borne diseases (e.g. West Nile) for which horses are also typically vaccinated.  Rabies boosters only need to be given once a year, so this can be done during a time of year when no other vaccines are required, if there are concerns about giving too many vaccines at once.
• Horses live outside and in barns.  Most are far less supervised than dogs and cats, but even these animals are at risk of rabies exposure.  A rabid animal could easily be "brave" enough to attack a horse, even though it normally wouldn't.  Bats can also easily get into and out of many barns - you may never know one was there, and finding a bite mark from a bat on a horse would be like looking for a needle in a haystack, but that's all it takes to transmit the virus.  So it makes sense to give your horse added protection by vaccinating it.
  
• Rabies in horses may not look like rabies at first.  One of the most common early signs is actually colic.  A rabid horse that looks like a colic may expose the people who are trying to look after it before they realize what the horse has.  In other horses the signs may be recognized too late, like the rabid horse that was found at the Missouri State Fair earlier this year, that resulted in exposure of many people.
• While rabies transmission from horses to people has not been documented, rabid horses have killed people, particularly horses that develop the "furious" form of rabies, which can cause them to become very violent.

For more information on rabies, see our rabies archive or the information sheet available on the equIDblog Resources page.

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On December 9, the Infectious Diseases session was held at the 2008 AAEP Conference in San Diego, CA. Topics presented covered several conditions, including MRSA, Clostridium difficile, equine infectious anemia (EIA), parasite resistance and deworming protocols, and Lawsonia intracellularis.

Lawsonia intracellularis is a bacterium that causes a disease called (among other things) proliferative enteropathy in foals and weanlings. More information about this disease is also available on the equIDblog Resources page. This disease is relatively new in the horse world, being first identified in the 1990s. It has traditionally been considered a disease of pigs, where it causes several syndromes of clinical disease, including one similar to that seen in foals. In pigs that carry the bacteria in their intestine, clinical disease is brought on by stressful events, and the same may be true in foals. The infection causes severe thickening of parts of the small intestine, and the inflammation causes loss of large amounts of protein from the bloodstream. Affected foals are usually very “unthrifty” and quiet/depressed, and many often develop edema (non-painful, cool swelling) along their abdomens or under their jaws.

In pigs, prevention of disease due to Lawsonia infection involves decreasing stressful events and vaccination with an oral vaccine. Dr. Nicola Pusterla of the University of California (Davis) presented the results of a study that was done recently looking at the effects of vaccination of foals against Lawsonia in three different ways – giving the vaccine orally, giving the vaccine orally after treating the foals for three days with a gastroprotectant (omeprazole), and giving the vaccine rectally. The reason for the last two groups was to decrease the risk of the vaccine being destroyed by the acidic environment of the normal stomach. Indeed, they found the antibody response to the vaccine in the bloodstream was better in these groups than in the untreated foals that were given the vaccine orally. Of the 12 foals that received the vaccine, all of them stayed healthy throughout the study period (42 days).

Although seeing some work on developing a vaccine for this disease is great, we are still a long way from being able to use vaccination to prevent disease in foals. While the animals in the study produced antibodies in the bloodstream, Lawsonia actually lives inside cells, where antibodies usually cannot get to them. Immunity against intracellular pathogens like this requires cell-mediated immunity, which is much harder to measure. It’s also important to remember that even with a vaccine, control of this disease (as with any infectious disease) depends on a lot of other factors, like reducing stress on animals and preventing transmission and spread of the bacteria. Unfortunately, no one is even sure exactly what the source of the Lawsonia is in most foals (although there’s a good chance that they ingest it, possibly with fecal contamination from foals that are shedding Lawsonia).

As more and more cases of Lawsonia are seen, both in North America and now in Europe, researchers will continue to learn more about how this disease is spread, and hopefully one day develop a vaccine to help us prevent it.

Keep watching this site for more of the latest and greatest from the 2008 AAEP Conference!

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Influenza vaccination is an important preventive strategy in people, and much effort is made to vaccinate as many people as possible with effective vaccines. Every year, the World Health Organization makes recommendations about which influenza vaccine strains should be included in the upcoming year's vaccine. This year, three strains were recommended for the Northern Hemisphere: an A/Brisbane/59/2007 (H1N1)-like virus, an A/Brisbane/10/2007 (H3N2)-like virus and a B/Florida/4/2006-like virus.

The decision about what influenza strains to include is critical, because vaccination against one strain may not provide much protection against other strains. There are many human influenza strains in circulation, and the virus itself often changes slightly with time, which can decrease the  effectiveness of the vaccine. The decision of what strains to include in the human flu vaccine each year can be highly contentious.  It is based on knowledge of circulating influenza strains and informed "predictions" (guesses) about what strains will be the most important later in the year. Some years, their guesses are right on.  Other years... not so much.

People often ask the logical question: why do the strains included in equine influenza vaccines not get updated as often as those in the human vaccine? It appears that equine influenza is more stable and less likely to change, therefore the same vaccine remains effective for much longer. Currently there is also less variation in equine influenza strains found in different countries and on different continents compared to human influenza strains.  The H3N8 equine influenza strain has been the predominant strain for years. Therefore, changing of vaccine strains on a yearly basis is not required for horses as it is for humans.

However, all influenza strains are prone to change.  Small, gradual changes, due to a phenomenon called antigenic drift, can reduce vaccine effectiveness over time.  Major, sudden changes, due to antigenic shift, can result in new strains against which current vaccines provide no protection.   Ongoing surveillance of strains causing disease and vaccine effectiveness, in both humans and horses, is therefore a very important measure to ensure that vaccines are kept up-to-date. Companies that produce vaccines need to be diligent to ensure that vaccine strains are updated when necessary.

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Eastern equine encephalitis (EEE) was recently identified as the cause of an outbreak of disease in emus on a farm near Brockville, Ontario. EEE is a viral disease that is spread by mosquitoes, and can cause severe neurological disease in horses.  Previous equIDblog posts describe EEE in more detail.

Emus are quite susceptible to EEE.  Although EEE cannot be transmitted fro emus to horses, the relevance of this report to horse owners is that the emu outbreak indicates that the virus is present in mosquitoes in that region.

Most of the emu deaths in this outbreak occurred in mid-October.  It is unclear whether there is any further risk to horses, people or other animal species. Being a mosquito-borne disease, EEE transmission should decrease dramatically after the first few hard frosts in the fall. It is unlikely that there is a significant risk of further EEE transmission in the Brockville area this year.

Vaccination of horses in Ontario at this time of year is probably not useful because of the low risk of mosquito transmission and the time required for immunity to develop after vaccination. However, people should consider the risk of exposure in subsequent years, and try to reduce this risk. This can be done through a combination of avoiding and controlling mosquitoes (which are also important for prevention of West Nile virus infection) and vaccination against EEE. Since EEE is so rare in Ontario, it is not typically considered a "core" vaccine in horses. However, vaccination for EEE should be considered in regions where the disease has previously been identified.

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A common (and reasonable) question that I get asked periodically is “Why do I vaccinate my horse every year against tetanus but I only get a booster every 10 years?”   There are actually some good reasons for the difference.

Tetanus is a devastating disease caused by the bacterium Clostridium tetani. It used to be very common, but is now rare because of effective vaccination. Although the disease is rare, the bacterium that causes it is very common, and can be found in soil and manure almost anywhere. Horses typically get exposed to C. tetani through soil contamination of  wounds, especially puncture wounds of the foot or lower leg.

• Horses are probably more at risk of exposure to C. tetani than people, because these types puncture wounds happen frequently in horses, and the wounds are possibly more likely to be contaminated with the tetanus bacterium.
• Horses are extremely susceptible to tetanus (more than people and most animals), so situations that would not necessarily cause a problem in people could cause tetanus in a horse.

Given their risk of exposure, their susceptibility to the disease, and the difficulty (and high cost) of treating tetanus in horses, and the safety and effectiveness of vaccination, it is recommended that horses are vaccinated frequently for tetanus. It is possible that longer intervals for vaccination than every year could be used, but there is currently no information that tells us how long we can stretch it. So, erring on the side of caution, and considering tetanus vaccination is safe and cheap, it makes sense to vaccinate horses every year.

More information on tetanus is available on the equIDblog Resources page.

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Preliminary evidence has suggested that botulism might be the cause of botulism may be the cause of death of approximately 100 horses at a large breeding facility in Florida housing approximately 400 mares. The horses died over a period of about 7 days, with signs of neurological disease. There are no reports of affected horses at other facilities in the area.  The clinical signs in these horses and lack of evidence of another disease are strongly suggestive of botulism. This farm apparently fed the horses haylage (although some reports seem to alternate between using the words "hay" and "haylage"), which is a high-risk feed source for botulism.

In adult horses, botulism is caused by ingesting a toxin produced by the bacterium Clostridium botulinum. This bacterium will not grow in the presence of oxygen, however in can grow in conditions that are sometimes present in improperly fermented haylage and silage. As the bacterium grows, it produces botulinum toxin, one of the most potent toxins on the planet. Ingestion of botulinum toxin leads to progressive paralysis (i.e. severe weakness and flacid muslces).

Treatment of botulism is difficult and can be expensive, and the mortality rate for this disease is high. Some people choose to feed haylage and silage despite the risk of botulism. If you choose to feed haylage, silage or other high-risk feeds:

• Ensure that haylage/silage is properly prepared and stored.
• Consider vaccinating horses that are fed haylage/silage against botulism. Be aware, however, that vaccines do not protect against all strains of botulism.
• Immediately stop feeding haylage or silage if any horses show signs that could be consistent with botulism (e.g. weakness, problems eating).

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In some areas, vaccines are readily available from multiple sources, including the internet. Some people like to purchase vaccines and administer them to their horses themselves in order to save money. But are the savings really worth the risk? Here are some things to consider:

• Your veterinarian obtains vaccines through a reputable distribution system, which ensures quality control, tracking of products, and proper shipping and storage conditions. When you buy vaccines from other sources(especially the internet), you don't have that same level of assurance. Vaccines that have been improperly handled may not be effective.
• Some sources of vaccines and drugs are of questionable quality - in these cases you can't always be sure that you are even getting what you think you ordered. Ineffective vaccines or contaminated products are a concern. It's not saving you money if the vaccine doesn't work!
  
• While uncommon, vaccine reactions do occur. If your horse has an anaphylactic (severe allergic) reaction after being vaccinated, the reaction usually happens right away.  The horse's chances of survival are much greater if your veterinarian is there, because the needed expertise, drugs and equipment are readily available. If you administered the vaccine yourself, your horse may die by the time a veterinarian arrives.
• If your horse develops any problems associated with a vaccine administered by your veterinarian, the manufacturer may get involved and assist with the problem. This will NOT happen if you buy the vaccine from another source and give it yourself.
• Rabies vaccines MUST be given by a veterinarian. (In some areas it's illegal for a non-veterinarian to even possess rabies vaccine.)  A horse that has received a rabies vaccine from a non-veterinarian is considered unvaccinated by public health authorities.  If an unvaccinated animal is exposed to rabies, the repercussions may be much more severe, and may even include euthanasia.
• Vaccination is just one part of your horse’s "wellness program". Some of the pressure for people to vaccinate their own pets is a failure of the veterinary profession to adequately emphasize the importance of preventive medicine, of which vaccines are just one component. Simply charging an owner for "annual vaccines" leads people to want to vaccinate their pets themselves because they can get the vaccines for much less money. Veterinarians need to emphasize that what they are charging for (and what is the most important component of the preventative medicine program) is an annual physical examination and health consultation, and that only a small portion of the fee is for the vaccines.

Vaccination is a minor component of your horse's preventive medicine program. A careful physical examination and consultation about potential, developing and ongoing health issues are the most important parts of this program. Even if you vaccinate your horse yourself (which is still not recommended for the reasons above), it is still critical that your horse has an annual examination.  It's better for your horse's health, and it can be easier and cheaper in the long run because problems can be detected and treated early.

This equIDblog entry was originally posted (in modified format) on the Worms & Germs blog on 16-Oct-08.

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Over 50 horses have died from Eastern Equine Encephalitis in Florida this year. The disease, caused by a virus of the same name, affects the brain, resulting in a broad range of clinical signs from behaviour changes to blindness to irregular gait. The disease is also sometimes called “sleeping sickness” because some horses may become severely depressed, with low head carriage and droopy eyes, ears and lips. Almost all horses that develop neurological signs from this infection die. Only 35 cases were reported in Florida in 2006 and 2007 combined.

There are actually three related equine encephalitis viruses – Eastern, Western and Venezuelan – which are called EEE, WEE and VEE for short. VEE is found in South and Central America and Mexico, and occasionally in the southern United States, but has never been reported as far north as Canada (VEE is a reportable disease in Canada). It is unique among the three diseases as the only one in which an infected horse will carry enough virus in its bloodstream to infect a mosquito, which could then pass the virus on to another animal. The EEE and WEE viruses, just like the West Nile virus, do not reach high enough levels in the bloodstream of horses to do this. The mosquitoes usually pick up the viruses from passerine birds, which do not become ill from the viruses (unlike West Nile virus in birds from the family Corvidae).

People can also be infected by EEE, WEE and VEE. About 10 fatal cases of EEE in people are reported in the United States every year. But horses cannot transmit EEE or WEE to humans, even if they’re bitten by the same mosquito. A higher number of cases in horses, however, may mean a higher number of mosquitoes that are carrying the virus. There is no vaccine for these viruses for humans, but there are vaccines available for EEE, WEE and VEE for horses.

In the end, EEE is just one more good reason to make sure you wear mosquito repellent when you’re enjoying the great outdoors during the summer. Visit the Health Canada website for safety tips on using personal insect repellents. EEE is very uncommon in Ontario, but horses that live in or travel to the southern United States should be vaccinated. Talk to your veterinarian about whether or not your horse should be vaccinated. Remember that fly control is also important for our equine companions (and also helps protect them against West Nile!).

This equIDblog entry was originally posted on the Worms & Germs blog on 02-Aug-08.

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