A Cross-Sectional Study of West Nile Virus in Clinic-Admitted Raptors

West Nile Virus (WNV) is an arbovirus that is typically transmitted through a mosquito vector via direct inoculation. This virus had never been seen in the U.S. prior to 1999 when an outbreak occurred in New York. Since then, it has spread across the U.S. and is now considered endemic among many bird species. Birds are the natural reservoir for this virus, and transmission occurs when a mosquito takes a blood meal from an infected bird and proceeds to bite other birds or humans. Although a small percentage of human West Nile cases exhibit moderate to severe symptoms, most cases are asymptomatic. Avian WNV research has important public health implications because of the relationship between birds and humans in the transmission cycle. Studies have shown that raptors are particularly sensitive to WNV; therefore, understanding how the virus affects raptor species may be a useful indicator of regional virus activity.  In this study, 1,455 injured raptors were admitted to a rehabilitation clinic from 2003-2008. Species, symptoms, and season of admission were documented, and each specimen was tested for WNV antibody. Antibody seroprevalence was then compared to the number of reported human cases in the same region.  Seroprevalence varied greatly depending on the year of admission. Seroprevalence was as low as 3.4% (9/264) in 2008, and as high as 46.8% (109/233) in 2005.  Overall, WNV seroprevalence for all years was 17.9% (260/1455). Specimens were more likely to be WNV positive in the summer months. The number of local human cases and the proportion of infected raptors did not appear to correlate annually. Reported human cases were by far the highest in 2003 (n=948), when raptor seroprevalence was 32.6% (76/233). However, in 2005, when raptor seroprevalence was highest (46.8%), there were only 30 human cases reported. Our data agree with prior studies showing that WNV is highly prevalent among raptor populations. Although the relationship between human and bird cases did not correlate annually in the present study, possibly because most human cases are never reported, continued monitoring of the annual fluctuations in avian WNV prevalence could prove useful for protecting human health.