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Eastern Equine Encephalitis -- United States, 1989

As of August 31, 11 states have reported to CDC one fatal human case and 65 equine cases of eastern equine encephalitis (EEE) in 1989 (Figures 1 and 2). The human case, confirmed serologically, occurred in an 11-year-old boy in Mississippi who died of encephalitis on August 6. Equine cases have been confirmed serologically (40 cases) or by viral isolation from brain (25 cases).

Despite the routine application of larvicides and adulticides by state and local agencies, pest and vector mosquitoes, including those that transmit EEE, have been abundant this year in several eastern coastal locations. For example, on Maryland's eastern shore, Aedes sollicitans mosquito collections in light traps have exceeded 50,000 per trap per night. In late July, an EEE epornitic (i.e., an outbreak in a bird population) occurred in a pheasant flock on Maryland's eastern shore, leading to 20 deaths among approximately 1500 fowl. Four equine cases from the DelMarVa Peninsula and on Assateague and Chincoteague islands off the Maryland and Virginia coast have been confirmed, and other clinically suspected cases have been reported from this area. Emergency widespread aerial applications of adulticides over Assateague Island and adjacent mainland recreational areas were conducted in the last week of August. Reported by: State and local health departments. Div of Vector-Borne Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: EEE occurs sporadically in the United States, principally in coastal locations in mid-Atlantic and southeastern states. In most years, fewer than five human cases have been reported; however, clinical disease is associated with a case-fatality rate of 30%-70%, and most surviving patients have serious neurologic sequelae (1-3). EEE in horses is usually fatal.

EEE is transmitted in an enzootic cycle among birds and Culiseta melanura mosquitoes, an ornithophilic species that seldom bites people. Various mosquito species with catholic feeding habits, including Ae. vexans, Ae. sollicitans, and Coquillittidia perturbans, are chiefly responsible for transmitting infections to humans and horses (1,2,4). This year, following heavy spring and summer rains, several northeastern states that continuously monitor vector mosquito activity have reported the highest numbers of enzootic and epizootic vector mosquitoes ever recorded in their areas.

With onset of cooler weather the abundance of mosquitoes is declining in many areas of the eastern seaboard; EEE cases generally decline in late summer, although cases have occurred as late as mid-October (5).

No specific preventive or therapeutic measures against EEE are available. In areas with a potential risk for the disease, protective measures against mosquito exposure --including the use of repellents, appropriate dress, and avoiding outdoor activity in the evening (the peak period of biting activity)--are prudent.


  1. Shope RE. Alphaviruses. In: Fields BN, ed. Virology. New York: Raven Press, 1985:931-53.

  2. Tsai TF, Monath TP. Viral diseases in North America transmitted by arthropods or from vertebrate reservoirs. In: Feigin RD, Cherry JD, eds. Textbook of pediatric infectious diseases. Vol 2. 2nd ed. Philadelphia: WB Saunders, 1987:1417-56.

  3. Przelomski MM, O'Rourke E, Grady GF, Berardi VP, Markley HG. Eastern equine encephalitis in Massachusetts: a report of 16 cases, 1970-1984. Neurology 1988;38:736-9.

  4. Morris CD. Eastern equine encephalomyelitis. In: Monath TP, ed. The arboviruses: epidemiology and ecology. Vol 3. Boca Raton, Florida: CRC Press, 1989:1-20.

  5. Goldfield M, Sussman O. The 1959 outbreak of eastern encephalitis in New Jersey: I. Intro duction and description of the outbreak. Am J Epidemiol 1968;87:1-10.

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