OK – so we’re supposed to be talking about this so-called EWT vaccine that you have to give your horse twice a year. Let’s start with the basics – the E represents Eastern Equine Encephalitis (also known as EEE). The W represents Western Equine Encephalitis (WEE), and the T represents Tetanus, which was discussed in our last blog. There is another in the EE group, Venezuelan Encephalitis (VEE), which has not been reported in the U.S. since 1971, and is not routinely vaccinated against in this country. So what is with EEE and WEE, and why do I have to give my horse a vaccine for this twice a year??
Both EEE and WEE are viral diseases causing severe neurological disease, and are carried from horse to horse by mosquito’s. As with West Nile virus, birds are crucial to maintaining the cycle of virus activity from year to year. While EEE occurs more frequently in the eastern U.S., and WEE is generally associated with the western part of the country, each has occurred in Florida. Administration of the vaccine two to three times a year here in Florida is directly a function of our never-ending mosquito season. In the northern parts of the country, EWT vaccination is generally performed once yearly in the spring, as hard freezes cause a break in the disease season. The vaccine appears to be effective at preventing disease for 4 to 6 months, so it is crucial to keep up with this schedule. The vaccine product is killed virus, meaning it has no ability to cause disease, and reactions are rare. Mortality associated with EEE is 75% to 95%, so it is a disease certainly worth preventing.
Horses affected by EEE initially show signs of fever and depression, and symptoms transition into neurological disease about 5 days after infection. The horse often dies 2-3 days after neurological signs set in. The first sign is often a change in behavior, which can be in either direction. Normally calm horses can become irritable and easily excited, attacking owners or themselves. Others may seem “sleepy,” or even ‘forget’ how to lead, in any case, the horse usually refuses feed and water. As the dementia progresses, the horse may compulsively walk in circles, press its head against things, grind his teeth, and develop blindness. Unsteadiness or clumsiness will begin to manifest, as can areas of paralysis (face, tongue, etc.). The horse will eventually go down, which more often than not signifies the end of the battle – a down horse with this disease has a grave outlook. WEE manifests similarly, although generally more slowly and less severely (mortality ranges from about 20 to 50%). It is important to remember that a horse with signs of encephalitis (brain inflammation) is not himself – so be extremely careful around these horses, and stay out of the stall until the vet arrives!
Horses are considered “dead end hosts” of the EE’s – meaning a horse with this disease is of no risk to infect other horses, mosquito’s or birds. Diagnosis involves a CSF tap, so a diagnosis may be presumed if vaccination was >4 months prior, and it occurs in an area where this disease is prevalent (like Florida). Treatment of this disease is supportive, and often ineffective (especially with EEE). It is aimed at symptomatically reducing the inflammation in the central nervous system – there is no cure. Euthanasia is recommended with horses that are down and unable to stand. Horses that do manage to recover often have lasting signs of brain damage. The staff at Springhill Equine firmly believe that every horse deserves at least twice a year EWT vaccination to prevent the devastating effects of these diseases! Check on your horse’s vaccination status, and schedule with us today if you are overdue! May your litter box be clean, and your food bowl full!
