Eastern Equine Encephalitis

Last Update: March 2000

Author: F. A. Leighton

Reviewer: H. Artsob

The Virus

EEE virus is classified as a member of the genus Alphavirus in the family Togaviridae. It is an enveloped, single-stranded RNA virus. It was first isolated and described in 1933.

Geographic Distribution

In Canada, EEE virus has been detected in Quebec and Ontario. The centre of its distribution in North America is the eastern and southeastern United States. It also occurs in the islands of the Caribbean, and is widely distributed in South America.. There appear to be different subtypes in North and South America that vary considerably in their virulence, and in their ecology and epidemiology.


EEE virus normally cycles among passerine birds and the bird-feeding mosquito Culiseta melanura. The virus replicates rapidly in nestling passerines, and, while there is no evidence that these birds suffer clinical disease because of infection, they serve as a source of virus for uninfected mosquitoes. The prevalence of infected mosquitoes rises progressively over the summer as multiple cycles of infection and amplification occur between passerine birds and mosquitoes The natural passerine-Cs. melanura cycle occurs in forested wetlands which are the habitat of this mosquito species. Cs. melanura feeds exclusively on birds and, thus, does not transmit EEE virus to mammals such as horses or people, in which infection with EEE can result in severe disease. Other species of mosquito that will feed on both passerine birds and on mammals are the vectors that transmit virus to mammals. Such transmission generally occurs only within a range of up to 8 Km from the wet forested habitat of Cs. melanura. Various different species of mosquito appear to be able to transmit EEE from birds to mammals. These include native species such as Coquillettidia perturbans and some species of the genus Aedes, and also the Asian mosquito Aedes albopictus which was imported and became established in North America in the early 1980's and is extending its range throughout the southern United States.

Although humans and horses can become infected with EEE and may suffer disease as a consequence, these abnormal vertebrate hosts appear to play no role in the ecology or maintenance of the virus in nature. EEE is not contagious from human to human or horse to horse. Infected people and horses have too little virus in their blood sterams to infect new mosquitoes. It is reported that EEE has been transmitted among pheasants when uninfected birds have fed on the carcasses of birds that died of EEE.

It is not known how EEE is maintained in the mosquito-passerine bird cycle during winter. It may persist in birds, mosquitoes, or other vertebrates in the temperate environment where it multiplies rapidly in summer, or it may over-winter in migratory birds and be re-introduced to temperate regions each spring.

Disease in Animals

Wildlife: There is no evidence that EEE virus causes disease in its normal passerine bird host species nor in other wildlife native to the geographic area in which EEE exists in North America. Whooping Cranes brought into eastern North America for captive propagation have suffered fatal disease from EEE virus. Introduced species such as House Sparrow, Rock Dove, Chukkar Partridge, Ring-necked Pheasant, Emu, and Ostrich also have been reported to suffer disease from EEE.

Domestic Animals: EEE causes periodic epidemic mortality in horses. Descriptions of disease outbreaks typical of EEE in horses were made as early as 1831 in the eastern United States. Most outbreaks in horses are small and localized, but some have been large. In 1947, an outbreak in Texas and Louisiana caused illness in 14,000 horses and mules, of which 12,000 died. Typically, 80%-90% of horses that develop clinical disease die of the infection. Disease in horses due to EEE has been recognized in Quebec and Ontario, but only rarely.

As noted above, non-indiginous captive wild bird species raised for comercial purposes within the geographic range of EEE virus have suffered disease from EEE infection, usually encephalitis but sometimes also more generalized disease. Species include Chukkar Partridge, Ring-necked Pheasant, Emu, and Ostrich. Clinical disease also has occurred in domestic ducks and turkeys.

Human Disease

Of North American arboviruses, EEE appears to be the most severe human pathogen; approximately 33% of people who develop EEE encephalitis die of the disease, and many survivors are permanently incapacitated. However, EEE is not a common human disease; there have been only 153 cases recorded in the United States in the past 35 years - 4 to 5 cases per year. In Canada, there have been no recognized cases of human disease caused by EEE, but human disease has occurred is several states bordering Ontario and Quebec, and EEE has occurred occasionally in horses in Ontario and Quebec.

Surveillance and Management

Because of its annual cycle of amplification, risk of disease in people and horses varies greatly from year to year, depending on the prevalence of infected mosquitoes of species that will feed on both birds and mammals. Surveillance programs of various kinds are in place or have been used in the past to monitor the prevalence of infected mosquitoes, particularly in the eastern United States. When a high prevalence is detected, education programs to urge people to avoid exposure to mosquitoes and mosquito control programs can be undertaken. Earlier assessment of the magnitude of the amplification cycle is possible by surveys of nestling birds in EEE habitat or use of sentinel bird flocks which are monitored for exposure to EEE virus. Since mosquito control generally is not very effective when undertaken late in the summer, early detection can trigger control efforts earlier in the season which are more likely to succeed.

Effective vaccines are available for use in horses but none is available for general use in humans. The small number of human cases of EEE is insufficient to justify a public vaccination program.

Vaccination has proved effective for captive Whooping Cranes when an experimental vaccine intended for humans was used. Commercially-available vaccines for horses may not be effective for birds and are not licensed for use in birds.

General References

Calisher, C.H. 1994. Medically important arboviruses of the United States and Canada. Clinical Microbiology Reviews 7: 89-116.

Gibbs, E.P.J., and Tsai, T.F. 1994. Eastern encephalitis. In: Beran, G.W.(Editor-in-chief). Handbook of Zoonoses. 2nd Edition. Section B Viral. CRC Press Inc. Boca Raton. p.11-24.

Morris, C.D. 1988. Eastern equine encephalitis. In: Monath, T.P. (ed.). The Arboviruses: Epidemiology and Ecology. Vol. III. CRC Press, Boca Raton. p. 1-20.

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