TRANSMISSION PATTERNS OF ST-LOUIS ENCEPHALITIS AND EASTERN EQUINE ENCEPHALITIS VIRUSES IN FLORIDA - 1978-1993

Sentinel chickens were maintained at field sites in 40 Florida countie s for varying periods between 1978 and 1993. For each county, the tota l number of St. Louis encephalitis virus (SLE) or eastern equine encep halitis (EEE) virus seroconversions were divided by the number of chic kens exposed to calculate a mean annual seroconversion rate. These rat es were used to evaluate the annual and geographical distributions of these viruses within Florida. For SLE, the rates in counties that repo rted human SLE cases during a widespread epidemic in 1990 were compare d with adjusted mean annual seroconversion rates calculated by excludi ng the epidemic year seroconversion data. In general, actual rates wer e higher than adjusted rates in counties where human SLE cases were re ported in 1990. Similar calculations were made for EEE virus that was unusually abundant in northern Florida during the spring and summer of 1991. In general, EEE virus was distributed in the panhandle and nort hern regions and SLE virus in the central and southern regions of the state. However, this distribution was not exclusive, and during years of high transmission, either virus can extend beyond its normal range. The annual distribution of SLE and EEE viruses was sporadic, ranging from years with little detectable activity to years with widespread, h igh-level transmission.