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

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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A colleague mentioned a rumour that is apparently going around some areas about horses getting H1N1 influenza. While we never say never with infectious diseases, there is no evidence that H1N1 can be spread to horses, nor do I have much concern about this. H1N1 can infect different species, including humans, pigs, poultry, cats, ferrets and dogs. That's a pretty impressive range, but it's mostly because the virus is made up of genes from human, swine and avian influenza viruses, and we know that other species like ferrets and cats are prone to occasionally (rarely) catching human or avian influenza. Horses have their own influenza, H3N8 equine influenza A. This virus has been remarkably stable in the horse population in that H3N8 has been the predominant equine influenza strain for a long time. There is little information indicating that horses are susceptible to the variety of seasonal flu viruses that circulate amongst the human population every year, or H5N1 avian influenza. No one has specifically tested H1N1on horses, and equine infections are not theoretically impossible, but it's pretty unlikely that this strain would be a major concern in horses, given what we know right now. Considering the number of people that have been infected with H1N1, horses have certainly been exposed to this virus, but there are no reports of suspected equine infections.

While the risk of horses contracting H1N1 is very low, it can be reduced further with common sense practices to reduce the risk of exposure. If you may have influenza, avoid contact with people and other animals, including pets and horses, and get a flu shot.

Image source: http://graphicshunt.com

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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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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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Dr. Bob Wright of the Ontario Ministry of Agriculture Food and Rural Affairs has compiled a document with various pieces of information about disease occurences in horses in Ontario and Canada. While there's nothing too earth-shattering in it, it does give a nice synopsis of certain diseases and shows how Ontario is a pretty good place to be in terms of some nasty diseases. The report can be viewed by clicking here.

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I'm sure there are few people that haven't heard about the current worldwide flu epidemic (although you’d pretty much have to have not listened to or watched the news nor read a newspaper nor been on the internet for the last week to have missed it!). It's been called “swine flu” until the last few days, but that terminology is gradually disappearing and being replaced with the virus’s more formal name, H1N1 influenza A. It's been covered extensively in the press and we have numerous posts about it on our sister site, WormsAndGermsBlog.

Influenza is an important disease in horses, so what is the risk to horses from this outbreak?

The real answer is we don't know. However, it's extremely unlikely that it will become a concern in horses. Most flu viruses tend to be host specific, meaning they usually don't infect multiple species. Pigs are a noted exception to this, because they are susceptible to swine, avian and human influenza viruses. This makes pigs a great “mixing pot” for influenza viruses – if a pig is infected by more than one virus at the same time, the viruses can exchange DNA and recombine to form new virus strains with new properties. The virus causing the current epidemic appears to be related to known swine influenza viruses, but also contains genes from avian and human influenza viruses, which may account for why it is able to so readily infect humans.

We don't know what other species this virus may infect, but it's very unlikely that it could infect horses. Horses tend to only be infected by equine influenza viruses. There's no evidence that this virus has any greater chance of jumping to horses than any of the seasonal human influenza viruses, or other influenza viruses. Personally, I'm more concerned about it spreading to cats, since we know that cats are susceptible to different influenza strains such as the H5N1 avian flu virus. While we can’t yet say for sure that horses are completely safe from this infection, I'm not concerned about this flu strain and horses.

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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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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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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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The latest version of the DEFRA/AHT/BEVA Equine Quarterly Disease Report has been released. This is a joint venture of three British organizations, and often contains some interesting infectious disease information. This edition contains brief information about diagnoses of selected diseases such as equine herpesvirus (EHV) -1 abortion, EHV-1 neurological disease and equine influenza. There is also a brief review of the eradication of equine infectious anemia (EIA) following the 2006 outbreak in Ireland, which was also discussed in a previous equIDblog post.

Another useful piece of information in this report is the data regarding strangles (Streptococcus equi subsp. equi) diagnoses. They report 280 positive S. equi culture results and 167 positive S. equi PCR test results during the last quarter. This shows once again that strangles is still a rather common disease. This is important to remember when considering routine infection control practices, because the risk of strangles exposure is ever-present, it is not just concern during outbreaks.   Also, people shouldn’t overreact to single cases of strangles (as they often do), since this disease is far from rare.

The full Equine Quarterly Disease Report can be downloaded by clicking here, or through the Animal Health Trust’s website.
 

Image from http://snapshot.parade.com/mainemb.php?g2_itemId=783542.

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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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The December 2008 newsletter from the University of Guelph's Animal Health Laboratory contains an article about diagnoses of equine viral infections from 1998-2008. Equine herpesvirus type 1 (EHV-1) was the most commonly detected virus and the most commonly diagnosed infectious cause of abortion. Three cases of neuropathic EHV-1 (the strain that was recently identified as being an important cause of the neurological form of this disease) have been diagnosed since specific testing for this strain started late last year. Small numbers of influenza cases were confirmed and, as expected, they all involved the H3N8 type virus. Eastern equine encephalitis (EEE) is a rare disease in Ontario, but an outbreak occurred in 2008, with 8 cases identified. Equine rhinitis viruses A and B (formerly equine rhinovirus 1 and 2) were found in a few horses with fever, respiratory tract disease and swelling (edema) of the limbs and abdomen.

This newsletter, along with previous versions, can be found here. (Note: at the time of writing, the December newlsetter was not yet available online. If you look and it's not there, check back again later because it should be available soon).

Image: TEM of equine herpesvirus (source: Gluck Equine Research Center)

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