| West Nile virus (WNV) activity was measured in areas with large fish crow (Corvus ossifragus) populations in New Castle County, Delaware, and compared to results from similar habitats without any resident fish crow populations. West Nile virus surveillance was conducted from June through September in 2003 and 2004. Four sentinel chickens were placed in cages at each site and observed throughout the season. Blood samples were collected from them every two weeks and tested for WNV using a hemaglutination inhibition test. Positive samples were confirmed by an enzyme-linked immunosorbent assay test. In 2003, 7 of 67 samples collected tested positive for WNV antibodies; six were from crow sites (17.1% seroconversion rate) and one was from non-crow sites (3.1% seroconversion rate). In 2004, 4 of 167 samples collected tested positive for WNV antibodies; two were from crow sites (2.3% seroconversion rate) and two were from non-crow sites (2.5% seroconversion rate). None of the differences in the sentinel chicken seroconversion rates were statistically significant. Mosquitoes were collected using dry ice-baited CDC miniature light traps in 2003 and dry ice-baited CDC miniature light traps, infusion-baited gravid traps, and resting boxes in 2004. Traps were placed at each site every two weeks and run overnight. Collected mosquitoes were identified to species, pooled, and analyzed for WNV using a real-time reverse transcriptase polymerase chain reaction test. In total, 76,899 mosquitoes in 2,599 pools of 32 species or species groups were tested for virus. The 16 positive pools were of Aedes vexans (1), Culex pipiens (1), Cx. restuans (1), mixed Cx. pipiens and Cx. restuans (7), Cx. salinarius (1), or mixed Culex spp. (5). In 2003, the overall maximum likelihood estimations of the infection rates were higher at the crow sites than the non-crow sites, 0.24 and 0.17 infected mosquitoes per 1,000 individuals, respectively. In 2004, the results were almost opposite with higher mosquito infection rates at the non-crow sites than the crow sites, 0.24 and 0.19 infected mosquitoes per 1,000 individuals, respectively. The elevated seroconversion rates and infection rates in 2003 suggest that fish crows were serving as reservoirs for WNV in 2003. |
