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The recent Encephalomyelitis outbreak...
ENCEPHALOMYELITIS OUTBREAK IN THE HORSE POPULATION
Many of you who read about horse health have seen the recent articles on the Eastern Equine Encephalomyelitis ( EEE) outbreaks in the Eastern US. Late last week there was confirmation of an outbreak in central Arkansas. As the outbreaks spread, as they usually do until cold weather, there will continue to be news. This week we would like to discuss EEE and its close relative, WEE ( I will let you figure out what WEE is an abbreviation for).
The life cycle of these two diseases is similar, as are the clinical signs of infection. EEE is far more aggressive then WEE, and will make a much higher percentage of horses infected sick (75-- 90 %). The range of activity for both EEE and WEE overlaps in the Midwest and either side of the Mississippi River. Of course, the river is a hot spot, since mosquitos are an important vector or carrier for the virus. The bird population is an equally important vector, because the mosquito alone cannot produce enough virus to impact the horse population:
An infectd mosquito bites the bird, who becomes a very compatible host in which the viruses can reproduce. They do this with little effect on the bird.
When the infected bird is bitten by another mosquitos, its blood will become loaded with the encephalomyelitis virus.
This mosquito then hurries over to the nearest horse (or human) and, in the process of feeding itself, infects them.
The virus does not spread from the horse, it is a “d_ _ _ end host” (we are not allowed to use the “d-word” in our clinic).
We can see why the spread of this disease is dependent on moist conditions, which create an ideal breeding ground for the mosquito. We can also see why the central Arkansas and Mississippi River areas would have the first viral outbreaks. If you have lived in or visited those areas, you know the mosquitos around the rice fields would carry us off if our feet weren't already stuck in the mud. The Mississippi River area also harbors the Japanese and St. Louis E virus.
The clinical signs for both EEE and WEE are similar. The first signs obvious enough to be noticed are usually seen 5 days after infection. Prior to that the horse will be running a fever due to virus proliferation in its organs. As you would expect, the early signs are of the nervous system. The first sign is usually a change in behavior from calm to irritable or from alert to being non-responsive. Other signs include not eating or not drinking, head pressing and leaning against the wall, and continuous walking. If the horse dies it is usually after being down and semicomatose for some time. If it survives, recovery may take a period of weeks. Even if the horse recovers it will exhibit dullness and reduced learning ability ("dummies").
Treatment is supportive and depends on the response the horse takes to the virus. Of equal importance is the challenge of differentiating this disease from others that have similar clinical signs, such as rabies.
Fortunately for the horse population, there are effective vaccines for both EEE and WEE. These were developed in the early 40’s after an outbreak resulted in the deaths of over 180,000 horses. We use the vaccines in our vaccination schedule. The EEE and WEE vaccine is given one time annually, and should afford the older horse good protection. However, both these diseases are due to a virus. As we know, viruses often do not stimulate strong long term protection. If outbreaks do start occurring close to home, young horses in particular should be boostered within six months of the previous vaccination.
EEE and WEE are interesting diseases of which we see little, until conditions are right for an outbreak. If vaccinations are up to date there should be little reason for concern, and your efforts will be rewarded by the peace of mind they provide.
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