Eastern Equine Encephalitis (EEE) is caused by a virus transmitted to humans and horses by the bite of an infected mosquito. EEE was first identified in the 1930's and currently occurs in locations along the eastern seaboard, the Gulf Coast and some inland Midwestern locations of the United States. While small outbreaks of human disease have occurred in the United States, equine outbreaks can be a common occurrence during the summer and fall.

It takes from 4-10 days after the bite of an infected mosquito for an individual to develop symptoms of EEE. These symptoms begin with a sudden onset of fever, general muscle pains, and a headache of increasing severity. Many individuals will progress to more severe symptoms such as seizures and coma. Approximately one-third of all people with clinical encephalitis caused by EEE will die from the disease. Of those who recover, many will suffer permanent brain damage requiring permanent institutional care.

In addition to humans, the EEE virus can produce severe disease in: horses, some birds such as pheasants, quail, ostriches and emus, and even puppies. Because horses are outdoors and attract hordes of biting mosquitoes, they are at high risk of contracting EEE when the virus is present in mosquitoes. Horses are often used as a surveillance tool.

EEE virus occurs in natural cycles involving birds and a mosquito, Culiseta melanura, in some swampy areas nearly every year during the warm months. Where the virus resides or how it survives in the winter is unknown. It may be introduced by migratory birds in the spring or it may remain dormant in some yet undiscovered part of its life cycle. With the onset of spring, the virus reappears in the birds (native bird species do not seem to be affected by the virus) and mosquitoes of the swamp. In this usual cycle of transmission, virus does not escape from these areas because the mosquito involved prefers to feed upon birds and does not usually bite humans or other mammals.

For reasons not fully understood, the virus may escape from these swamp areas in birds or bridge vectors such as Coquilletidia perturbans. These species feed on both birds and mammals and can transmit the virus to humans, horses, and other hosts. Other mosquito species such as Aedes vexans and Culex nigripalpus can also transmit EEE.

Below is a factsheet about EEE in Florida.

Eastern Equine Encephalitis (EEE) PDF