ENZOOTIC TRANSMISSION OF WESTERN EQUINE ENCEPHALOMYELITIS VIRUS IN THE SACRAMENTO VALLEY OF CALIFORNIA DURING 1993 AND 1994

The temporal and spatial distribution of western equine encephalomyeli tis virus (WEEV) in California is described during 1993 and 1994 based on seroconversions in sentinel chickens and infected pools of Culex t arsalis Cog. A single horse and no human cases were reported despite w idespread enzootic transmission in the Sacramento Valley. During 1993, elevated sentinel chicken seroconversion rates in the Sacramento Vall ey were related to cool vernal temperature, above normal rainfall and river discharge, and elevated infection rates in Cx. tarsalis. Western equine encephalitis virus appeared to overwinter in the Sacramento Va lley and was detected again in 1994; however, warm vernal temperatures and reduced water availability limited the intensity and distribution of transmission to sentinel chickens. The absence of human cases and possible mechanisms for WEEV persistence are discussed.