Mosquito surveillance for West Nile virus in southeastern Wisconsin--2002.

In 2001, West Nile virus (WNV) was identified among dead American crows and bluejays in five counties in southeastern Wisconsin. In response to the introduction of WNV, a pilot mosquito surveillance program was initiated in these five southeastern Wisconsin counties during the summer of 2002. Forty sites were selected for surveillance one night each week during a 17-week period. Mosquitoes were collected in carbon dioxide-baited light traps and gravid traps. During the study period 31,419 mosquitoes were collected, identified to species level and pooled into groups of up to 50 mosquitoes of like species from each collection site. Twenty-five different mosquito species were identified with the common pest mosquitoes, Aedes vexans and Ochlerotatus trivittatus, being the most abundant. Seventeen of the 25 mosquito species found in southeastern Wisconsin have previously been shown to be carriers of WNV in other parts of the U.S. Only 2/1,592 (0.126%) mosquito pools from Wisconsin were positive for WNV by cell culture and reverse transcription polymerase chain reaction (RT-PCR). Active mosquito surveillance is useful for identifying potential mosquito vectors of arboviruses in defined geographic areas, and to monitor population densities of those vectors. This information coupled with infection rate data can help guide public health policies related to vector control, and may help reduce the impact on human, veterinary and bird mortality.