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.

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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.

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As eastern equine encephalitis (EEE) season is in full swing in some areas and West Nile virus cases should be starting soon, the need to avoid the vectors of these diseases, mosquitoes, is clear. There are various ways to reduce exposure to mosquitoes, but none of them are great in terms of their effectiveness and practicality. In peoples' quests to find alternative approaches, they often come across garlic. Ingestion of garlic has been advertised and used as a mosquito repellent in humans and horses. 

But, does it work?

I've heard many anecdotes about peoples' success (or lack thereof) with feeding their horses garlic or garlic components. Some people have said they thought it worked. Others, probably the majority, didn't think it did anything.

The scientific literature doesn't help much. Despite what you read on internet sites selling garlic, there's no published work on garlic as a mosquito repellent in horses. There's a little research involving humans but no significant effect has been identified. One study involved giving people garlic or a placebo, then exposing them to mosquitoes and counting the number of bites and the weights of the mosquitoes after exposure (the more the blood they ingest, the more they weigh), but the study showed no effect of garlic (Rajan et al 2005). Beyond that, there's nothing else out there.

Lack of scientific evidence doesn't necessarily mean lack of effectiveness. It could just be that adequate studies haven't been performed. However, it doesn't change the fact that there is currently nothing indicating that garlic is an effective mosquito repellent. There's probably no harm in giving reasonable doses of garlic as part of an overall mosquito control program, but it shouldn't be done in place of other activities such as reducing mosquito breeding grounds and avoiding areas where there are large numbers of mosquitoes (particularly at dawn and dusk).

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Eastern equine encephalitis (EEE) season is now in full swing in the US. Cases are being reported in horses in several US states, including Florida, Georgia and Louisiana. Infected mosquitoes have been found over a wider range, including as far north as Massachusetts. That’s not particularly surprising because this seasonal mosquito-borne viral infection typically occurs at this time of year in eastern/southeastern coastal regions.

The risk of EEE varies greatly across North America. For EEE to be a problem:

• EEE virus must be present in birds in the area. Birds are the reservoir hosts and are not typically affected by the virus.
• Mosquito populations must be large enough to facilitate transmission of EEE virus between birds and from birds to horses (or humans).
• The types of mosquitoes that are present must be those that like to feed off both birds and horses/humans. These "bridging vector" mosquitoes are critical since they are the ones that allow the infection to spread beyond birds.

Not all mosquitoes are the same in terms of their feeding patterns and likelihood of carrying EEE virus and transmitting it from birds to horses and people. Not all bird species have the same likelihood of carrying EEE virus. These factors play an important role in explaining why EEE can be a major problem in some areas but not in others.

Photo: Aedes vexans, one of the mosquito species capable of transmitting EEE from birds to mammals (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 five-year-old Quarter Horse mare was found down in the field in the mid-afternoon.  The horse was seen moving around the field normally less than eight hours earlier.  She was found near a fence, but there were no external signs of trauma on the mare's body.  Some green feed material was present at the nostrils.  With encouragement the mare was able to stand, but she was very unsteady and uncoordinated, particularly in the hind limbs. Upon examination by the veterinarian in the field, it was also noted that the horse could not open her mouth normally (lockjaw - which is often a sign of tetanus in horses), and the mare seemed "dazed".  There were three other horses kept in the same field, all of which appeared completely normal.  All the horses were fed the same round-bale hay (a new bale was just put out the previous day).  The mare was vaccinated for rabies in 2008 and West Nile in 2009 (spring).  There was no movement of horses on or off the farm (i.e. the horses were not taken to shows/competitions or off-site rides).

When the horse arrived at the hospital, she was down on the trailer.  By that time she had a fever, high heart and respiratory rates, and she was dehydrated.  Although she was still aware of things going on around her, she was very depressed.  She was still able to see, and she could move all four legs and her tail.  However, it was even difficult for her to lie on her chest (e.g. sternal recumbency), so she would roll to one side instead (e.g. lateral recumbency).  Her jaw remained rigid, but she could still move her tongue a little.

With a lot of encouragement and help the mare was able to stand up and stumble off the trailer, but she was so weak and uncoordinated in all four legs that she fell down again before she could even walk the 10 metres to her stall.  Eventually she made it to her stall, and she was able to stand for about an hour before she lied down (or fell down) again.  A urinary catheter had to be used to drain the mare's bladder because she did not seem to be able to urinate on her own.

The mare was treated with intravenous fluids (supportive therapy), and anti-inflammatories and steroids to try to reduce the inflammation that was suspected to be going on in her brain and spinal cord.  Despite all this, her condition continued to worsen, and by the next morning the mare could not even sit up and was becoming less aware of her surroundings.  The mare was therefore humanely euthanized.

Post-mortem tests in this mare confirmed there was inflammation in the brain based on a high number of inflammatory cells in her cerebral spinal fluid (CSF).  Because of the clinical presentation, some of the brain tissue also had to be sent away for rabies testing, which was (thankfully) negative.  Once that result was back, samples were also tested for evidence of infection with equine herpes virus (EHV-1), West Nile virus and Sarcocystis neurona (the cause of equine protozoal myeloencphalitis), all of which were also negative.  Botulism was also considered, but this disease is very difficult to test for in horses.  In the end, the final diagnosis, and the cause of the mare's neurological signs, was infection with eastern equine encephalitis (EEE) virus .

This case of EEE was diagnosed in September 2009 in Southwestern Ontario.  The description of the presentation, and how rapidly this mare deteriorated, demonstrates just how devastating and severe this disease can be.    This case also tells us that there are mosquitos in the area that are carrying EEE.  Vaccination of the other horses in the region will not provide protection before the end of this mosquito season, but owners of horses in the same area should seriously consider (and discuss with their veterinarian) vaccination of their horses against EEE in the spring, prior to the next mosquito season.  No one can say for certain if vaccination of this mare would have prevented the infection, or decreased the severity of the infection, but it likely would have helped.  In the meantime, as always, decreasing exposure to mosquitos as much as possible (if there are any left) should be the priority.

Image: TEM of the salivary gland of a mosquito infected with eastern equine encephalitis (EEE) virus (source: CDC Public Health Image Library #7057).

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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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The latest USDA report on equine encephalitis surveillance data gives a good picture of what has happened so far this summer with West Nile virus, eastern equine encephalitis (EEE) virus and western equine encephalitis (WEE) virus in the US. The data, last updated July 28, report no cases of WEE anywhere in the US. In contrast, EEE has been found in seven states (Florida (43 cases), Georgia (22), Mississippi (16), Alabama (6), North Carolina (4), Louisiana (3), Virginia (1)). This is a pretty typical distribution, with most cases being found in a small number of states, and even in certain regions within those states. West Nile virus cases are, not surprisingly, low so far. This disease does not tend to occur until late summer and fall. As of July 28, only six cases have been reported (Washington (2), one each in West Virginia, Texas, Montana and Missouri). The important question is what will happen with West Nile virus for the rest of the year? Stay tuned.

Image: Scanning electron micrograph of a mosquito (source: www.cbc.ca)

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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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Two horses from different farms in the Savannah, Georgia area have been diagnosed with eastern equine encephalitis (EEE). This is a severe neurological disease that can strike very quickly, and is fatal in up to 90% of cases. There is no specific treatment, and affected horses often deteriorate rapidly and dramatically. The virus that causes EEE is transmitted from birds to horses by the bites of infected mosquitoes, which makes it a seasonal disease. The risk of EEE is also heavily influence by geography, largely due to the distribution of mosquito species that effectively transmit the virus. Some areas have recurrent (and often major) problems with EEE, other areas (like Ontario) have few or no cases on a yearly basis, but can experiece small clusters of disease from time to time, and still other areas never see cases at all.

Identification of EEE is important for a few reasons. Obviously, it's relevant for horse health. It indicates that EEE virus is in the area and mosquitoes capable of transmitting it between birds and horses are also present. Horse owners need to be aware of the EEE history in their area when deciding whether or not to vaccinate against this deadly disease. On a seasonal basis, waiting until the first cases are diagnosed to vaccinate isn't a good plan, because there is always a delay from vaccination until peak immunity, and your horse could be infected before the vaccine is given or takes effect. EEE vaccination is best performed about one month prior to the at-risk period/season, with a booster six months later if the risk of exposure is still present (e.g. in southern areas were mosquitoes are present year-round).

Identification of EEE also has implications for human health. EEE can also infect people and cause very serious (and often fatal) disease. People cannot acquire EEE from horses, but hey are infected the same way: from the bite of an infected mosquito. A vaccine is not available for people so mosquito avoidance is critical.  Identification of cases of EEE in the area should be a reminder to look at farm measures to reduce mosquiotes and decrease mosquito bites (more information on this is available in an earlier post about West Nile virus).

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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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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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This summer, a horse in Ontario was diagnosed with Eastern Equine Encephalitis (EEE), a serious neurological disease caused by a virus of the same name, which is transmitted by mosquitoes.  The horse was from the North Durham region.  The last reported cases of EEE in Ontario were in 2004.  A previous equIDblog entry talked about a large number of cases of EEE that have been reported in Florida this year.

Here are some of the key points to remember about EEE:

• Like West Nile, EEE is a seasonal disease.  It is more common in warmer areas, especially some regions of the southeastern US.  It is rare in cooler climates, but occasionally EEE is found in horses in Ontario.
• EEE is usually fatal in horses, and there is no effective treatment.
• EEE can also occur in people, and can be fatal in some cases.
• Infected horses cannot transmit the EEE virus to people, but if a horse gets EEE from the mosquitoes in the area, then people could also potentially be exposed to the virus by mosquitoes.
• A vaccine for EEE is available for horses, but most horses in Ontario are not vaccinated for EEE because it is so rare.  Nonetheless, vaccination can be considered because the disease is so devastating when it occurs.
• As for West Nile virus, avoiding mosquitoes - for both horses and people - is an important preventative measure for EEE.

For more information, see the equIDblog post "Eastern Equine Encephalitis – Not Just For Horses", or the CDC's website on arboviral encephalitides.

This equIDblog entry was originally posted on the Worms & Germs blog on 26-Aug-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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