West Nile Virus Activity in Kern County and the Factors Leading to the 2007 Outbreak.

West Nile virus (WNV) reappeared in Kern County in late-May 2007, amplified rapidly and was detected concurrently by all surveillance methods. Enzootic activity during 2007 had some similarities to that of the previous three years, with 77 seropositive sentinel chickens in 9 flocks, 207 positive mosquito pools, 124 dead birds that tested positive, and 168 seropositive wild birds. WNV disease in equines remained infrequent, with only 4 cases reported. In contrast, Kern County had a significant increase in human disease, with 138 laboratory confirmed fever and neuroinvasive cases, combined incidence = 17.8 per 100,000 population. The standard surveillance indicators, sentinel chickens and mosquito pools, indicated that WNV enzootic activity was on the decline, yet there were epidemic numbers of human cases. During this fourth year of virus activity, WNV was found throughout Kern County on the floor of the Central Valley.