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

The researchers have tracked the spread of West Nile Virus (WNV) in Florida during its first outbreak in 2001. The virus was found in 65 out of 67 counties, infecting various animal species and a small number of humans. The spread of the virus was primarily monitored by observing bird mortality which proved most efficient for tracking. The study showed that rural areas were largely affected, unlike previous outbreaks that were mostly urban.

Survey Results and Geographic Scope

• The study was conducted over five months, starting from the initial detection of West Nile Virus (WNV) in July 2001. The virus was found across an extensive area, with infection detected in 65 out of Florida’s 67 counties.
• The transmission was mostly concentrated in the northern part of the state, and most of the transmission sites were rural, a pattern that contrasted with the typical urban outbreaks previously seen in 1999 and 2000.

Species Affected and Surveillance Methodology

• Various species were infected by the virus, including 1,106 wild birds, 492 horses, and 194 sentinel chickens. Humans were also affected, with 12 confirmed cases of the virus.
• The researchers isolated the virus from mosquito samples, with the majority (13 out of 28) originating from Culex mosquitoes.
• Similar to other studies done in the northeastern United States, the study found that monitoring wild bird mortality was the most sensitive and effective method to track the spread of the virus.

Timeline and Prediction of Human Cases

• All human cases followed the detection of WNV in animals. However, WNV activity was reported in the county of residence for only eight of the twelve cases before the people were infected.
• Seven of the twelve human cases were preceded by detection of West Nile virus-positive animals around two weeks or more in advance, indicating that animal surveillance could provide an early warning for potential human cases.