Eastern Equine Encephalitis

NWDC is in the process of updating this fact sheet. 


Eastern Equine Encephalitis (EEE) is an infectious viral disease that is maintained in bird reservoirs, transmitted by mosquitoes, and sometimes causes fatal neurological disease in mammals. The first reports of EEE in horses in the eastern United States likely date back to the 1800’s, but the virus was not isolated until 1993.


EEE rarely infects humans and only a few cases a year are reported in the United States. Most people infected with this virus will show no clinical signs of illness; however, some will exhibit headache and fever followed by severe neurological disease. Overall, EEE has a 33% mortality rate in humans, and those who survive often suffer significant brain damage. EEE causes significant mortality in horses, with mortality rates ranging from 70%-90%. EEE has recently been reported in white-tailed deer, but it does not seem to be a substantial cause of mortality in deer populations. 

Species Affected

EEE causes subclinical infection in a wide variety of wild birds. The virus is known to cause mortality in the glossy ibis and several species of birds that are not native to the United States, including pigeons, house sparrows, pheasants, chukar partridges, white Peking ducklings, and emus. The virus is also known to cause disease in horses, humans, pigs, rodents, and more recently in white-tailed deer. In 2010, EEE antibodies were first discovered in moose in Maine and Vermont. 


EEE occurs throughout Eastern North America, primarily on the Atlantic and Gulf coasts. In 2010, the virus was discovered in Vermont in both moose and white-tailed deer, which marked the first evidence of EEE in that state in either animals or humans. The virus’s distribution extends down into Central and South America as far south as Argentina. EEE has been isolated in white-tailed deer in Michigan, Georgia, and Wisconsin.


The EEE virus is transmitted by mosquitos. The type of mosquito primarily responsible for transmission is the Culiseta genus, which feeds on birds. These mosquitoes pick up the virus when taking a blood meal from infected birds and then transmit the virus to uninfected birds in a similar way. Mammals can become infected by mosquito species (mainly from the Aedes group and the Coquillettidia group) that feed on both birds and mammals. Virus transmission occurs most frequently in the summer and fall when mosquito populations are largest. Mammals are generally considered dead-end hosts because they do not produce enough virus particles in their blood to infect a new mosquito. Only birds produce enough of the virus, which is why birds are a natural reservoir for EEE. Occasionally, infected horses may produce large enough amounts of the virus to infect a mosquito, but this only lasts for a short period of time. The potential for deer to be a host for this virus is being investigated. Humans may be exposed to the virus if brain or spinal cord tissue of an infected bird or mammal comes into contact with the eyes or an open wound. Infectious material may also become aerosolized and be inhaled when hunters saw through brain tissue to remove antlers. These modes of transmission of EEE virus to humans are unlikely, but it is important to be cautious.

Clinical Signs

White-tailed deer and horses may exhibit clinical signs of neurological disease 1-3 weeks following infection. Clinical signs include lethargy, confusion, loss of coordination, head tilt, circling, blindness, muscle paralysis, loss of fear, difficulty breathing, emaciation, and death. Most birds do not show clinical signs of disease. 


Virus isolation or other laboratory tests are needed to confirm a diagnosis of EEE. Clinical signs of EEE in white-tailed deer can be similar to those of chronic wasting disease (CWD) or other neurological conditions, so it is important to run the appropriate tests to find the cause of clinical illness.


While an effective vaccine exists for horses, there is no specific treatment for EEE. Horses may be treated with symptomatic supportive care, but treatment of wild, free-ranging animals is typically impractical.


Unlike with horses, there is currently no vaccine approved for use in deer. Mosquito control is the primary method of control and prevention of EEE. People should protect themselves by using insect repellents and wearing protective clothing while outside during mosquito season. It is also important to eliminate mosquito breeding sites (standing water) and make sure that all house screens are in tact to keep mosquitoes out. Hunters are also reminded not to shoot or consume sick animals. Since the 2009 outbreak of EEE in horses, llamas, and pheasants in 5 countries in Maine, the Maine Medical Center has been conducting surveillance of the disease in white-tailed deer through antibody testing of hunted deer. This surveillance helps to determine the distribution and prevalence of EEE within the state. Cases of horse and white-tailed deer infection may also be useful as sentinels for increased risk of human exposure to EEE virus.

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