Surveillance results from the first West Nile virus transmission season in Florida, 2001.

After West Nile virus (WNV) was first detected in Florida in July 2001, intensive surveillance efforts over the following five months uncovered virus activity in 65 of the state's 67 counties with 1,106 wild birds, 492 horses, 194 sentinel chickens, and 12 people found infected with the virus. Thirteen of 28 mosquito isolations came from Culex mosquitoes. As seen in the northeastern United States, wild bird mortality was the most sensitive surveillance method. However, unlike the predominantly urban 1999 and 2000 epizootics, the Florida transmission foci were rural with most activity detected in the northern part of the state. All human cases were preceded by the detection of WNV in animals; however, only eight of the twelve cases were preceded by reports of WNV activity in the county of residence. West Nile virus-positive animals detected by multiple surveillance systems preceded seven of these cases by two weeks or more.