Eastern Equine Encephalitis

Eastern equine encephalitis virus (EEE) is transmitted to humans and equines by the bite of an infected mosquito. EEE is an alphavirus that was first identified in the 1930's and currently occurs in focal locations along the Eastern seaboard, the Gulf Coast and some inland Midwestern locations of the United States, where equine epizootics can be a common occurrence during the summer and fall. Small outbreaks of human disease have also occurred.

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

In addition to humans, EEE virus can produce severe disease in horses, birds, and dogs. 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. Human infections are usually preceded by, and exceded in numbers by horse infections, which may be used as a surveillance tool.

EEE virus occurs in natural cycles involving birds and Culiseta melanura, in some swampy areas nearly every year during the warm months. Where the virus resides or how it survives between these cycles 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 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.

The virus may escape from enzootic foci in swamp areas in birds or bridge vectors such as Coquilletidia perturbans and Aedes sollicitans. These species feed on both birds and mammals and can transmit the virus to humans, horses, and other hosts. Other mosquito species such as Ae. vexans and Culex nigripalpus can also transmit EEE virus. When health officials maintain surveillance for EEE virus activity, this movement out of the swamp can be detected.

No human vaccines are commercially available for this disease. Arboviral encephalitis can be prevented in two major ways: personal protective measures and public health measures to reduce the population of infected mosquitoes. Personal protective measures include reducing time outdoors, particularly in early evening hours; wearing long pants and long sleeved shirts, and applying mosquito repellent to exposed skin areas. Public health measures often require insecticide spraying.