West Nile virus in mosquitoes of northern Ohio, 2003.

From June 19, 2003 to August 18, 2003, we surveyed the mosquitoes of Oberlin, OH, for West Nile Virus (WNV) infection using reverse transcriptase-polymerase chain reaction. A total of 12,055 mosquitoes, representing 17 species or species groups and 4 genera, were collected in gravid traps at seven sites throughout the city, with Culex pipiens/restuans being the most abundant and showing the highest minimum infection rate (MIR) of 0.78. This represents a decrease in WNV enzootic activity from the previous year. Both Cx. pipiens/restuans abundance and MIR increased significantly with date. However, we found no correlation between Cx. pipiens/restuans abundance and MIR.

West Nile Virus detection in kidney, cloacal, and nasopharyngeal specimens.

We compared kidney tissue samples and cloacal and nasopharyngeal swab samples from field-collected dead crows and blue jays for West Nile virus surveillance. Compared to tissue samples, 35% more swab samples were false negative. Swab samples were usually positive only when the corresponding tissue sample was strongly positive.

West Nile virus in mosquitoes of Northern Ohio, 2001-2002.

From June 14 to August 30, 2001 and June 10 to August 22, 2002, the mosquitoes of Oberlin, Ohio were surveyed and tested for West Nile virus (WNV). Mosquitoes were trapped weekly using gravid traps and CO(2)-baited Centers for Disease Control light traps at seven sites in 2001 and eight sites in 2002 in woodlots within the city limits. A total of 12,151 mosquitoes, representing 14 species and one species group, were collected in gravid traps, with Culex pipiens/restuans being the most abundant, followed by Ochlerotatus triseriatus. In light traps, 12,510 mosquitoes were collected, with Aedes vexans being the most abundant, followed by Culex pipiens/restuans. All gravid trap collections were tested for WNV via reverse transcriptase-polymerase chain reaction. West Nile virus was first detected in July 2002. Of all species tested, the Cx. pipiens/restuans species group exhibited the highest minimum infection rate (MIR) and the greatest percentage of positive pools. Both the MIR and percent of positive pools increased significantly with the date, although abundance of Cx. pipiens/restuans in gravid trap collections did not.

Patterns of West Nile virus infection in Ohio blue jays: implications for initiation of the annual cycle.

West Nile virus (WNV) was first detected in North America in New York City in 1999 and rapidly moved westward. Understanding the mechanisms by which the amplification cycle is reinitiated each year increases our ability to predict epizootics and geographic expansion of the disease. Such understanding is enhanced by knowledge of the patterns of infection in the vertebrate reservoir hosts. Blue jays (Cyanocitta cristata) may serve as reservoir hosts for WNV. We examined the influence of age and date on the prevalence of WNV in jay carcasses in Ohio during May-August 2002. Percent of carcasses that were infected increased significantly with time from 3% in May to more than 90% by August. We found no difference in prevalence between juvenile (nestlings and fledglings) and adult jays early in the season, which contradicts the expected pattern if the majority of the adults sampled in 2002 had been exposed to the virus in 2001. Therefore, jays infected in 2001 were unlikely to have been important in initiating the 2002 virus cycle in Ohio.