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 counties for varying periods between 1978 and 1993. For each county, the total number of St. Louis encephalitis virus (SLE) or eastern equine encephalitis (EEE) virus seroconversions were divided by the number of chickens exposed to calculate a mean annual seroconversion rate. These rates were used to evaluate the annual and geographical distributions of these viruses within Florida. For SLE, the rates in counties that reported human SLE cases during a widespread epidemic in 1990 were compared with adjusted mean annual seroconversion rates calculated by excluding the epidemic year seroconversion data. In general, actual rates were higher than adjusted rates in counties where human SLE cases were reported 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 northern 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, high-level transmission.