Eastern Equine Encephalitis Virus Associated with Aedes albopictus -- Florida, 1991

During June 6-10, 1991, as part of an ongoing study to identify the sources of Aedes albopictus bloodmeals, researchers from the University of Notre Dame collected mosquitoes in and around a tire dump in Polk County, Florida. The collections were made with a Nasci aspirator and yielded 9393 Ae. albopictus mosquitoes that were sent to CDC in December 1991; 9350 were tested, in 96 pools, for virus isolation by plaque assay in Vero cell culture. Forty-three blood-fed specimens were tested separately for bloodmeal identification. The specimens tested for virus yielded 14 virus strains identified as eastern equine encephalitis (EEE) virus by indirect fluorescent antibody test using a panel of alphavirus monoclonal antibodies including EEE virus complex-specific (1B1C-4) and North American EEE virus-specific (1B5C-3) monoclonal antibodies (1). The virus strains were reisolated from the original mosquito pools by intracranial inoculation into 1- to 3-day-old suckling mice. Two representative isolates were confirmed as EEE virus by plaque-reduction neutralization test. Results of the bloodmeal identification were 31% bovine, 24% unidentified mammal, 19% deer, 14% human, 7% raccoon, 5% rabbit and 2% passeriform birds.

During June 1991, Ae. albopictus mosquitoes were collected in two other Florida counties and tested for virus: 100 specimens, in two pools, from Gilchrist County and 430 specimens, in six pools, from Marion County were negative for EEE.

The tire dump, which has been closed since 1988, contains approximately 8 million tires. Scientists from the University of Notre Dame and the Florida Medical Entomology Laboratory in Vero Beach have been sampling Ae. albopictus populations at the dump since 1989.

Reported by: ML Niebylski MS, JP Mutebi, GB Craig Jr, PhD, Dept of Biological Sciences, Univ of Notre Dame, Notre Dame, Indiana. JA Mulrennan Jr, PhD, Florida Dept of Agriculture; RS Hopkins, MD, State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs. Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: This is the first documented isolation of EEE virus from Ae. albopictus collected in the United States. In the United States, EEE is the rarest of the mosquitoborne arboviral encephalitides but has a human case-fatality rate of approximately 30% (2). The virus is maintained in fresh-water-swamp habitats in an enzootic cycle involving mosquitoes, principally Culiseta melanura, and a variety of bird species. During 1991, heavy spring rains in northern Florida led to exceptionally large populations of Cs. melanura as well as mosquito species that serve as epizootic vectors (3,4). Consequently, Florida experienced early, widespread EEE virus activity with 70 equine cases reported by the beginning of July, the highest reported in a season by that time (3). Polk County reported four confirmed EEE cases in equines, three of them with onset in May and June; date of onset for the other case is unknown (CDC, unpublished data). Therefore, epizootic transmission of EEE virus was occurring in Polk County during the same period that infected Ae. albopictus mosquitoes were collected at the tire dump. In addition, the Florida Department of Health and Rehabilitative Services (HRS) confirmed five human cases of EEE among elderly residents during this period (3), but all were from an area approximately 125 miles north of Polk County.

From 1986 through 1991, Ae. albopictus extended its range from one to 61 of Florida's 67 counties (5) and is widely distributed in Polk County; it has been collected from 47 of 148 CDC light-trap locations (M. Mahler, Polk County Environmental Services, personal communication, 1991). In addition, results from vector-competence studies showed that a strain of Ae. albopictus mosquitoes from Houston became infected with EEE virus after feeding on viremic chicks, and 57% of those mosquitoes refeeding transmitted the virus 15 days postinfection (6). This information, along with findings reported here regarding virus isolations from field-collected specimens and with the opportunistic feeding habits of Ae. albopictus, suggest that Ae. albopictus may become an epizootic and epidemic vector of EEE virus. Plans are under way by the Florida HRS and CDC to improve surveillance for EEE in Polk County and to initiate collaborative studies during the summer of 1992 to more clearly define the role of Ae. albopictus in the EEE transmission cycle. States with Ae. albopictus infestations and a history of EEE should increase surveillance for both human and equine cases, as well as for EEE virus activity among mosquitoes and birds.

References

1. Roehrig JT, Hunt AR, Chang GJ, et al. Identification of

monoclonal antibodies capable of differentiating antigenic varieties of eastern equine encephalitis viruses. Am J Trop Med Hyg 1990;42:394-8. 2. Tsai TF. Arboviral infections in the United States. Infect Dis Clin North Am 1991;5:73-102. 3. CDC. Eastern equine encephalitis--Florida, Eastern United States, 1991. MMWR 1991;40:533-5. 4. Morris CD. Eastern equine encephalomyelitis. In: Monath TP, ed. The arboviruses: epidemiology and ecology. Boca Raton, Florida: CRC Press, Inc, 1988:1-20. 5. O'Meara GF, Gettman AD, Evans LF, Curtis GA. The spread of Aedes albopictus in Florida. American Entomologist (in press).

6. Scott TW, Lorenz LH, Weaver SC. Susceptibility of Aedes albopictus to infection with eastern equine encephalomyelitis virus. J Am Mosq Control Assoc 1990;6:274-8.

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