Eastern Equine Encephalitis

This fact sheet has been updated on 9/18/19. 


Eastern Equine Encephalitis (EEE) is a zoonotic viral disease that is maintained in bird reservoirs, transmitted by mosquitoes, and sometimes causes fatal neurological disease in mammals.


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, 4-5% will exhibit flu-like symptoms like headache and fever, muscle aches, joint pain and fatigue for one to two weeks. When clinical disease does occur, severe cases can result in inflammation of the brain (encephalitis) and the spinal cord resulting in headache, disorientation, tremors, seizures, paralysis and death. Approximately a third of the people with the severe form of EEE die, but those who survive often suffer significant brain damage. Individuals over the age of 50 or under the age of 15, and those who are immunocompromised, are at greatest risk of developing severe neurological disease when infected with EEE. On average, 7 human cases of EEE are reported annually in the United States.


Horses appear to be particularly susceptible with mortality rates ranging from 75%-90%. Though rare, EEE is regarded as one of the most serious mosquito-borne diseases in the United States because of the high mortality rate in horses and humans.


In contrast mortality is thought to be uncommon in deer as they are thought to be resistant to the virus and produce adequate antibodies against it. EEE antibodies are widespread in the deer population in the Northeast (10% of sampled deer in Vermont and 7% in Maine). In 2009 Maine experienced a large outbreak of EEE in horses, llamas and pheasants. In 2010, the virus was detected in all but 2 counties in Vermont in both hunter-harvested moose and white-tailed deer. This marked the first evidence of EEE in Vermont in either animals or humans. Maine also detected EEE in moose for the first time that year.  EEE has been isolated in white-tailed deer in Michigan, Georgia, and Wisconsin, Massachusetts and Rhode Island. As of September 2019, there have been 3 reported cases in deer in Rhode Island and 1 in Massachusetts.

Species Affected

EEE causes subclinical infection in a wide variety of wild birds. However, the virus is known to cause mortality in the glossy ibis, whooping cranes and several species of wild and domestic birds that, while not native to the United States are now present here, including pigeons, house sparrows, pheasants, chukar partridges, white Peking ducklings, and emus.


While evidence of exposure to EEE virus has been reported in a variety of free-ranging wild birds and mammals, the virus is also known to cause clinical disease in humans, equine species (horses, mules, donkeys, zebras), pigs alpacas, lamas, bats, rodents, reptiles, amphibians, and more recently white-tailed deer. 


The first reports of EEE in horses in North America likely date back to the 1800’s, but the virus was not isolated until a major outbreak in horses in the coastal areas of Delaware, Maryland, New Jersey and Virginia in 1993. As the name suggests, EEE occurs throughout Eastern North America, primarily on the Atlantic and Gulf coasts, but the virus is now endemic to the entire east coast of North America. Its distribution also extends into Central and South America as far south as Argentina.


The EEE virus is primarily transmitted by mosquitoes. The type of mosquito most commonly responsible for transmission is Culiseta, which feeds on birds. These mosquitoes obtain the virus when taking a blood meal from infected birds and then transmit the virus to uninfected birds. Mammals become infected by mosquito species (mainly from the Aedes group and the Coquillettidia group) that feed on both birds and mammals. 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 after infection, and so are the natural reservoir for EEE. The potential for deer to act like birds as a host for this virus is being investigated. Virus transmission occurs most frequently in the summer and fall when mosquito populations are largest, and when the most virus has been produced by hosts. The speed of disease spread is governed by the density of mosquito populations, mosquito habitat, temperature, humidity, and the abundance of the virus, the vectors (birds) and the mammalian hosts.

Clinical Signs

Most wild birds and mammals do not show clinical signs of disease. Clinical signs reported in white-tailed deer occur 1-3 weeks following infection and may include hypersalivation, confusion, loss of coordination, head tilt, circling, blindness, loss of fear, difficulty breathing, emaciation, paralysis and seizures.


A preliminary diagnosis of EEE can be made on the basis of clinical signs and microscopic examination of the brain tissue that demonstrates the encephalitis. Isolating the virus from brain tissue, as well as other laboratory tests, are used to confirm a diagnosis. However, 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 complete the appropriate tests to confirm the cause of clinical illness.


While an effective commercial vaccine exists for horses and birds, there is no specific treatment for EEE. Treatment of wild, free-ranging animals is typically impractical.


Unlike horses and birds, there is currently no vaccine approved for use in deer or other wildlife. 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 intact to keep mosquitoes out. Ponds and pools can be treated with mosquito dunks that kills mosquito larvae.

Hunters are reminded not to shoot or consume sick animals, and to wear gloves and eye protection when processing game. Cooking game to an internal temperature of 165oF will kill this and any other viruses, as well as bacteria.

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