Crow Deaths from West Nile Virus as Early Indicators of Future Human Infection in Virginia

Mina Tabibi
Advisor: Elizabeth P. Eutis-Turf Ph.D.

Objective: To examine the temporal correlation between early season crow mortality from West Nile Virus and subsequent human infection in Virginia.

Methods: Avian and human data was obtained from the Virginia Department of Health (VDH) Office of Epidemiology Excel files. A relative risk ratio was calculated for 2002 and 2003 separately determining how many times counties/cities that found an infected crow early in the transmission season were more likely to report a human case than counties/cities that did not report early crows. Bivariate statistical analyses were also performed to see the strength of the relationship between early season infected crows and human cases in counties/cities. ArcView Geographic Information System (GIS) software (E SRI, Redlands, CA) was also used to map infected crow clusters and human cases throughout the state for 2002 and 2003.

Results: In 2002 of the 20 counties/cities that reported an infected bird prior to August 2, 6 (30%) subsequently reported a human WNV case compared to 5 (5%) of the 86 counties that reported a human case but no positive crow prior to August 2 (RR 5.16, 95% confidence interval 1.75, 15.23). In 2003 of the 84 counties/cities that reported an infected bird prior to August 2, 15 (17%) subsequently reported a human WNV case compared to the 2 (5%) of the 35 counties that reported a human case but no positive crow prior to August 2 (RR 2.22, 95% confidence interval 0.69, 7.15). In 2002 unlike 2003, there appeared to be a strong correlation between the numbers infected crows tested prior to August 2 and the number of human cases. The maps also showed that in 2002 and 2003 counties/cities with early season crow clusters had an increased risk of human cases.

Conclusion: This study supported the hypothesis that in 2002 and 2003 areas with early occurrences of WNV infected birds had an increased risk of human infection. This relationship was apparent based on the statistical analyses and maps.