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Epidemiologic Notes and Reports Eastern Equine Encephalitis -- Florida, Eastern United States, 1991

The Florida Department of Health and Rehabilitative Services (HRS) has confirmed five human cases of eastern equine encephalitis (EEE) in elderly residents of Bradford, Duval, and Washington counties in northern Florida (Figure 1). Dates of illness onset were in mid-June and early July (Figure 2). One patient partially recovered and has residual neurologic deficits, two patients remain comatose, and the other two patients died.

From July 1 through July 19, the Duval, Bradford, Leon, and Saint Johns county health departments issued public health alerts after high seroconversion rates in sentinel chicken flocks were detected or after human or equine cases were confirmed. On July 26, the Florida HRS issued an alert for all counties in the state's panhandle. Local mosquito-control districts in affected counties have increased applications of adulticides.

Although human EEE cases have been reported only from northern Florida, an extensive epizootic in horses has been observed over a wide area of the southeastern United States (Figure 1). As of July 29, 246 laboratory-confirmed equine cases and more than 80 unconfirmed but histopathologically compatible equine cases have been reported. The Florida Department of Agriculture and Consumer Services has reported 173 equine cases scattered statewide; 70 of these were reported by the beginning of July--the most ever reported in a season by this time (Figure 2). Subsequently, a new state rule requiring reporting of equine cases was promulgated.

Other states reporting equine cases are Georgia (41 cases), South Carolina (19 cases), Alabama and North Carolina (five cases each), Mississippi and New York (two cases each), and Kentucky (one case). In Georgia, epornitic infections were reported in commercial quail, and fatal cases occurred in two dogs and 70 piglets.

In the northeast, a localized EEE epizootic has been reported in counties bordering the Cicero swamp in upstate New York. EEE was confirmed in one fatal equine case from Oswego County, and four suspected cases from Onondaga and Oswego counties are under investigation. Mosquito surveillance in the two counties detected six EEE viral isolates from Culiseta melanura, the principal enzootic vector; three isolates from Coquillitidea perturbans; and one isolate from Aedes canadensis. The latter two species can function as epizootic vectors. The counties sprayed the swamp preemptively in June and twice in July. Reported by: WE Birch, DVM, Alabama Dept of Public Health. HL Rubin, DVM, Florida Dept of Agriculture and Consumer Svcs; WG Hlady, MD, AL Lewis, DVM, R Mullen, MPH, JA Mulrennan, PhD, EE Buff, RS Hopkins, MD, State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs. JR Cole, PhD, Univ of Georgia Coll of Veterinary Medicine, Tifton; JD Smith, Georgia Dept of Human Resources. C Palmer, MD, Kentucky Dept for Health Svcs. M Currier, MD, Mississippi State Dept of Health. D White, PhD, MA Grayson, PhD, DL Morse, MD, State Epidemiologist, New York State Dept of Health. JK Atwell, DVM, North Carolina Dept of Agriculture. LA Williams, Jr, JL Jones, MD, State Epidemiologist, South Carolina Dept of Health and Environmental Control. D Alstad, DVM, Animal and Plant Health Inspection Svc, US Dept of Agriculture. Div of VectorBorne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: In the United States, EEE is the rarest of the mosquitoborne arboviral infections (1). A median of five sporadically occurring infections among humans are reported annually; however, the illness is fatal in 30% of cases overall, and even higher case-fatality rates are observed at the extremes of age.

Numerous mosquito species have been implicated as potential epizootic vectors of EEE (2,3). In the southeast, these species include salt-water-marsh mosquitoes such as Aedes sollicitans, which are abundant in coastal areas, and fresh-water mosquitoes, such as Culex nigripalpus, Coquillitidea perturbans, and Aedes atlanticus. Heavy spring rains in northern Florida have led to exceptionally large populations of Culiseta melanura, the principal vector of EEE virus in the enzootic cycle, and floodwater species that potentially are epizootic vectors.

An effective EEE vaccine for horses is commercially available, but cases continue to occur because of failures to vaccinate foals and to revaccinate older horses. An experimental EEE vaccine for humans is available to laboratory workers. In many areas where EEE is enzootic, control programs to reduce vector mosquitoes rely on larvicides and adulticides and long-term projects to reduce breeding sites. Personal protective measures to reduce mosquito bites are an important approach to prevention. These measures include the use of repellents, appropriate dress, and avoidance of outdoor activity during twilight hours when many mosquitoes are most active.


  1. Tsai TF. Arboviral infections in the United States. Infect Dis Clin North Am 1991;5:73-102.

  2. Morris CD. Eastern equine encephalomyelitis. In: Monath TP, ed. The arboviruses, epidemiology and ecology. Boca Raton, Florida: CRC Press, Inc, 1988:1-20.

  3. Scott TW, Weaver SC. Eastern equine encephalomyelitis virus: epidemiology and evolution of mosquito transmission. Adv Virus Res 1989;37:277-328.

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