VECTOR-BORNE DISEASES, SURVEILLANCE, PREVENTION St. Louis Encephalitis Virus in Wild Birds During the 1990 South Florida Epidemic: The Importance of Drought, Wetting Conditions, and the Emergence of Culex nigripalpus (Diptera: Culicidae) to Arboviral Amplification and Transmission

Weanalyzed theprevalenceofhemagglutination inhibition(HI) antibodies toSt. Louis encephalitis (SLE)virus inwildbirdsduring the1990SLEepidemic in IndianRiverCounty.The initial presence of SLE HI antibody was associated signiÞcantly with modeled drought 15 wk prior, wetting conditions 1wkprior, and the emergenceof theFlorida SLEvirus vector,Culex nigripalpus, 5wkprior. Our Þndings indicated that three factors conspired to create the 1990 epidemic: (1) a large population of susceptible wild birds; (2) severe springtime drought, which facilitated ampliÞcation of the SLE virus among the Cx. nigripalpus and a portion of the wild bird population; and (3) continued rainfall and wetting of the land surface in the summer and early fall, which sustained a large, host-seekingCx. nigripalpus population. The continued biting and reproductive activity of Cx. nigripalpusmaintained epizootic transmission throughout the summer and early fall in Indian River County. The high level of SLE virus ampliÞcation resulted in spillover transmission to humans. We hypothesize that without the continued reproductive activity of the vector mosquito, brought about by excessive summer and fall wetness, the unprecedented SLE virus ampliÞcation and consequent transmission to humans would not have been realized in 1990.