In the last two weeks, six horses that died in the Chesapeake and Suffolk area with central nervous system symptoms have been referred to the Virginia Department of Agriculture and Consumer Services’ Regional Animal Health Laboratory System for testing. Testing is not yet complete, but three of those horses exhibited microscopic changes in the brain that indicate a viral encephalitis problem. Two of the horses have not yet had their brains examined microscopically, and one horse had no microscopic evidence of encephalitis. Samples from all six horses have been sent to the U.S. Department of Agriculture’s National Veterinary Services Laboratory in Ames, Iowa for virus isolation and identification attempts.

Even though veterinary officials caution that the current outbreak has not yet been confirmed to be Eastern Equine Encephalitis (EEE), they recommend that horse owners vaccinate their animals every six to twelve months against the disease. This is particularly important in the Southeastern region of Virginia, where they recommend the six-month vaccination interval.

Although a few cases of equine encephalitis tend to occur almost every year in Southeastern Virginia, this is an unusually high number of cases in a short period of time. Several possible viruses could cause this sort of encephalitis in horses, with Eastern Equine Encephalitis being the most likely. All of the equine encephalitis viruses follow a similar pattern of residing most of the year in wild bird or mammal hosts and only becoming a problem when there are enough mosquitoes present to transmit the virus from the reservoir into horses or people. This in turn is related to rainfall and temperature patterns, which this year have been favorable for the production of mosquitoes.

Typical symptoms of encephalitis include staggering, circling, depression, loss of appetite and sometimes fever and blindness. There is no cure for the disease, which can kill 50-90% of the horses infected. Any horse that survives the disease almost always has long term brain damage. Fortunately, there is an excellent vaccine available to prevent this disease in horses. The vaccine is only effective for 6 – 12 months, so horses must be re-vaccinated at least annually and, in an area where the disease occurs frequently, vaccination every 6 months is probably best. Other prevention methods include destroying standing water breeding sites for mosquitoes, use of insect repellents such as DEET, and removing animals from mosquito infested areas during peak biting times, usually dusk to dawn