Volume 6 No. 2
July 2009

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Encephalo
Easter & Western

Encephalo
E&W w/ Tetanus

Eastern Equine Encephalitis / Western Equine Encephalitis

Linda Hansen, DVM
Staff Veterinarian

As spring and summer approach, it is time to think about preventative vaccinations for horses.  Included in the core vaccinations recommended by the American Association of Equine Practitioners are Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), and Tetanus Toxoid.  

The encephalitis viruses are alpha viruses and are spread to horses by mosquitoes from wild birds and rodents.  Humans are also susceptible to these viruses, although they are not likely to contract them from contact with horses.  Both viruses are known to occur in North America, with EEE being more prevalent east of the Mississippi and WEE west of the Mississippi .  Venezuelan Equine Encephalitis (VEE) occurs in South America and is considered a reportable foreign animal disease in the United States .  There is some evidence the EEE/WEE vaccines provide cross protection against VEE.¹  

EEE and WEE are serious and frequently fatal in the horse.  Symptoms include fever, depression and neurologic signs which can include altered mentation and vision, wandering, head pressing, circling, ataxia (stumbling) leading to paresis, paralysis, convulsions and often death.  Horses that become recumbent generally have a poor prognosis. Those that recover often have residual effects.²  

EEE and WEE vaccines contain inactivated virus with adjuvant, to stimulate immune response, and preservatives.  These vaccines have demonstrated to be effective in protecting against EEE and WEE infection.  Horses should be vaccinated in the spring, prior to mosquito season to provide the best protection when transmission is likely.  In areas where the virus is endemic and the mosquito season can be year round ( Southeast U.S. ), vaccination in the fall may also be recommended by your veterinarian.  

Initially, unvaccinated horses should receive one dose followed by a booster in three weeks.  They should be revaccinated yearly (or twice yearly) and also in outbreak situations when exposure is likely.  

EEE and WEE vaccines are often available in combination with other recommended core vaccines such as Tetanus Toxoid.  Colorado Serum has both EEE/WEE combination and an EEE/WEE/Tetanus combination available for horses.  

¹American Association of Equine Practitioners, www.AAEP.org, Eastern/Western Equine Encephalomyelitis, Copyright AAEP 2008  

²The Merck Veterinary Manual Ninth Edition, pages 1027-1031, Merck and Company, Inc., Whitehouse Station , NJ , USA 2005