THE SPATIAL-TEMPORAL DISTRIBUTION OF DROUGHT, WETTING, AND HUMAN CASES OF ST. LOUIS ENCEPHALITIS IN SOUTHCENTRAL FLORIDA JEFFREY SHAMAN, JONATHAN F. DAY, AND MARC STIEGLITZ Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts; Florida Medical Entomology Laboratory, Institute of Food and Agricultural Sciences, University of Florida, Vero Beach, Florida; Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York Abstract. Using a dynamic hydrology model, we simulated land surface wetness conditions at 42 sites in 28 counties in southcentral Florida from 1990 to 1998 and compared these simulations with the incidence of human cases of St. Louis encephalitis (SLE) within these counties. Within counties, drought four months prior and wetting one-half month prior were significantly associated with human cases of SLE. Simulated land surface wetness conditions resolved transmission loci in both space and time, and May drought was significantly associated with the subsequent occurrence of human SLE cases. These findings are consistent with previous results associating simulated land surface wetness conditions with the transmission of SLE virus as measured in sentinel chickens, and support our working hypothesis that springtime drought facilitates SLE virus amplification in mosquito and wild bird populations. INTRODUCTION Within Florida, four general patterns of transmission of St. Louis encephalitis (SLE) virus to humans have been observed since 1952. 1 During the vast majority of years, no human cases have been reported. During some years, such as 1993, small, focal outbreaks, usually involving fewer than 10 human cases, were reported in various locations throughout the state. The 1993 SLE outbreak involved eight human cases from a small area on the border of Lee and Collier Counties. Another transmission pattern involves sporadic human cases reported over a wider geographic area. This type of transmission was most recently observed in 1997 when nine human SLE cases were reported from Brevard, Charlotte, Hillsborough, Lee, Palm Beach, and Polk Counties. Sporadic transmission of West Nile virus to humans was also reported throughout Florida in 2001 and 2002. The final transmission pattern is of greatest concern: a full-blown epidemic in which hundreds or thousands of cases, some appearing in well-defined clusters, are reported over large parts of south Florida. The most re- cent Florida SLE epidemic occurred in 1990 when 226 clinical cases and as many as 30,000 infections were reported through- out southcentral Florida. Indian River County was the epi- center of this outbreak. 1 Recently, we found an association between antecedent drought, coincident wetting, and transmission of SLE virus in Indian River County Florida. 2 We used a dynamic hydrology model 3 to hindcast mean area water table depth (WTD) in Indian River County, and compared this model simulation to sentinel chicken seroconversion data. Seroconversion of sen- tinel chickens, as measured by serum assay for hemagglutina- tion inhibition (HI) antibodies to SLE virus, has been strongly correlated with the clinical disease in humans. 4 Using logistic regression, we found the probability of sentinel chicken seroconversion, i.e., transmission of SLE virus, to be strongly associated with low WTDs 11-17 weeks prior and higher WTDs 0-2 weeks prior. 2 A mechanism for this empirical relationship was suggested by mosquito collection data obtained in Indian River County from 1986 to 1991. Culex nigripalpus Theobald is the demon- strated enzootic and epidemic vector of SLE virus in south Florida. 5–7 Collections of Cx. nigripalpus were made in densely vegetated “hammock” (an island of dense native veg- etation) habitats used by this species for daytime resting. 8 During the driest conditions (low modeled WTD) preceding heavy SLE virus transmission, Cx. nigripalpus collections dra- matically increased. 2 Rather than indicate an increase of mos- quito abundance, these data suggest that drought restricts Cx. nigripalpus activity to the more humid hammock habitats. Extreme drought periods in south Florida tend to occur dur- ing the spring, a time when nesting wild birds also make use of the hammocks. Thus, drought drives the mosquitoes and birds into contact with one another. This forced interaction of vector mosquitoes and susceptible avian amplification hosts provides an ideal environment for the rapid epizootic ampli- fication of SLE virus. In addition, confinement of blood fed and gravid Cx. nigripalpus females to the hammock habitats for extended periods forces more infected females to com- plete the extrinsic incubation of acquired arboviruses during a single gonotrophic cycle. 1,8 Subsequently, when the drought ends and water resources increase, infected mosquitoes and birds disperse and carry the virus from the hammocks. Gravid female mosquitoes oviposit, re-feed, and if infective, they ini- tiate the early transmission phase of the Florida SLE cycle. Additional supporting evidence was found from wild bird serologic data obtained during the 1990 SLE epidemic. We showed that the presence of HI antibodies to SLE virus in wild birds was significantly associated with modeled anteced- ent drought, coincident wetting, and the emergence of the Florida SLE virus vector, Cx. nigripalpus, five weeks prior. 9 Our findings suggest that three factors conspired to create the 1990 epidemic: 1) a large population of susceptible wild birds; 2) severe springtime drought, which facilitated amplification of the SLE virus among the Cx. nigripalpus and a portion of the wild bird population; 3) continued rainfall and wetting of the land surface in the summer and early fall, which sustained a large, active Cx. nigripalpus population. The continued bit- ing and reproductive activity of Cx. nigripalpus maintained levels of epizootic transmission throughout the summer and early fall in Indian River County. These constant high levels of SLE virus amplification facilitated spillover transmission to humans throughout much of southcentral Florida. In this study, we shift our analysis to human cases of SLE reported throughout southcentral Florida. We hypothesize that the spatial-temporal organization of human SLE cases in Florida is tied to hydrologic variability. Specifically, we pro- pose that the epidemic transmission with Indian River County as the epicenter in 1990, the focal transmission with Lee and Am. J. Trop. Med. Hyg., 71(3), 2004, pp. 251–261 Copyright © 2004 by The American Society of Tropical Medicine and Hygiene 251