Several possible viruses could cause this sort of encephalitis in horses, with EEE 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 percent of the horses infected.
Any horse surviving the disease almost always has long term brain damage.
Fortunately, there is an excellent vaccine available to prevent this disease in horses. 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.