Eastern Equine Encephalitis
Charlotte M. Newell, DVM

Well, it's spring, and time for "spring shots". It seems like every year, there's a new vaccine for yet another equine illness. EWT, PHF, EHV, EVA--it all starts to sound like alphabet soup.Which ones does your horse actually need? The answer to this question depends on where your horse lives and what you do with him--do you show every weekend? Is there a lot of traffic in and out of your barn? If she's a mare, are you breeding her this year? Probably the best course of action is to discuss this with your veterinarian and then formulate a schedule. However, no matter what your horse does, he should receive an EWT "shot". These letters stand for Eastern and Western Encephalomyelitis (EEE and WEE) and Tetanus. If you've ever stepped on a rusty nail, you know what a tetanus shot is. But what exactly is the encephalomyelitis part of this? Does he really need it? Well, if by some miracle, your horse is never going to be bitten by a mosquito, maybe not. But if you live in the eastern United States, your horse may be exposed to EEE, and the small expense of vaccination is cheap insurance against a disease with a mortality rate of 75 to 90%. (WEE, which is more often seen in central and western parts of the country has a lower mortality, from 19 to 50%) Even if a horse recovers from EEE, there are usually persistent neurological deficits that may render him useless. EEE is a member of a family of insect-transmitted viruses that infect horses, birds and humans. The insect in this particular case is any of several species of mosquitoes. These pests first bite a wild bird or reptile, animals that are able to carry the virus without suffering ill effects.The ring necked pheasant and the English sparrow are two common hosts. The virus then multiplies in the salivary glands of the mosquito, and is passed on to the horse at a later time. This means that your horse does not have to be in close proximity to the original host. The mosquito vector means that the incidence of EEE coincides with the mosquito season, with the peak occurence in late summer. EEE tends to occur in cycles , with outbreaks occuring two or three years in a row, followed by several years without a reported case. Because EEE can also cause encephalitis in humans (who get it in the same way horses do--from the wild host via a mosquito), it is a reportable disease to state public health departments, most of which maintain survey programs which trap and examine mosquitos to check for the presence of the virus. The earliest clinical signs of EEE are vague, and appear within days of infection. There may be fever, stiffness, and a lack of appetite. The fever may subside and then reappear. These first signs are followed by others more suggestive of a neurologic problem--they include behavior changes such as aggression, excitability, and continual chewing activity. Overreaction to sounds and to touch may cause horses to crash through fences and even stalls. Some horses are depressed. As the infection progresses, these signs become more pronounced, and others appear. Animals may press their heads against walls or other stationary objects, or walk aimlessly in circles. They may have difficulty swallowing due to paralysis of the muscles of the tongue or pharynx; they may become blind. This is followed by an inability to stand, coma, convulsions, and usually death. Rarely, horses may survive, but most have lingering neurologic abnormalities. There is no specific treatment for the disease; the most that can be done is supportive care. As you can see , it's a lot better to prevent this disease than to deal with a case of it. Vaccination should be done before the start of mosquito season; if the season is more than six months long in your area, or if there is an outbreak near you, your veterinarian may advise a second vaccination in mid or late summer. Foals which are vaccinated before the age of six months should be revaccinated at one year. Other measures to protect your horses include use of fly repellents, and keeping horses stabled during times of peak mosquito activity.

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