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Epidemiologic Notes and Reports Update: Arboviral Surveillance -- Florida, 1990

During the summer of 1990, surveillance of St. Louis encephalitis (SLE) and eastern equine encephalitis viruses in vector mosquitoes and avian hosts indicated a high level of viral transmission in nature and a potential for epidemic transmission in several states (1). The Florida Department of Health and Rehabilitative Services initiated weekly active surveillance of county public health units, hospitals, and clinical laboratories in August after rising seroconversion rates were detected in sentinel chickens. Since September 7, epidemic transmission of SLE in central Florida has led to 12 additional confirmed or presumptive cases in humans, bringing the total number of laboratory-documented cases to 18; 52 additional suspected cases are under investigation. Suspected and laboratory-documented cases have been reported from 13 counties, with most reports from Indian River (51%), Orange (12%), Brevard (10%), and Lake (7%) counties. The earliest onset date for a laboratory-documented case was July 28, and onset of illness for the most recent presumptive case was September 6. Public warnings have been issued, and larviciding and aerial and ground-based adulticiding have been intensified in the affected areas. Reported by: DL Wells, MD, E Buff, AL Lewis, RA Calder, MD, State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs. Div of Vector-Borne Infectious Diseases, Center for Infectious Diseases; Div of Field Svcs, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: The 18 laboratory-documented SLE cases reported from Florida in 1990 constitute the largest outbreak in the state since 1977, when 110 laboratory-documented cases were reported from the same central counties (2). Additional cases may occur because SLE viral transmission in Florida usually peaks in early October (3). Furthermore, in the fall, the principal mosquito vector of SLE in the state, Culex nigripalpus, shifts its host feeding preference from avians to mammals (4).


  1. CDC. Arboviral surveillance--United States, 1990. MMWR 1990;39:593-8.

  2. Nelson DB, Kappus KD, Janowski HT, Buff E, Wellings FM, Schneider NJ. St. Louis encephalitis--Florida, 1977, patterns of a widespread outbreak. Am J Trop Med Hyg 1983;32:412-6.

  3. Monath TP, Tsai TF. St. Louis encephalitis: lessons from the last decade. Am J Trop Med Hyg 1987;37(suppl):40S-59S.

  4. Edman JD, Taylor DJ. Culex nigripalpus: seasonal shift in the bird-mammal feeding ratio in a mosquito vector of human encephalitis. Science 1968;161:67-8.

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