West Nile Virus in Birds

Since 1999, the presence of WNV has been confirmed in 96 species of wild birds native to North America and in an additional 25 species of domestic, exotic, or introduced birds. In 2002, WNV has been detected in birds in 42 States (15 for the first time), the District of Columbia, 4 Canadian provinces, and possibly Mexico. The incidence with which WNV occurs in wild birds is poorly known; in the 15 cases in which this statistic has been reported, values range from <1 percent to 69 percent, with a median of 30 percent. In recent weeks there have been unconfirmed reports of a large die-off of hawks and owls (up to 400 individuals) in Ohio, but the role of WNV in this mortality event remains unclear. What we do know is that, as of September 6, 21 percent of 4,398 dead and live birds tested in Ohio in 2002 have been WNV-positive. By comparison, 30 percent of 942 dead birds and 0.3 percent of 1,848 live birds tested in Ohio in 2001 were WNV-positive (a combined rate of 10 percent). Predictions of dire consequences for migratory birds due to WNV are clearly premature; it is more likely that WNV-related mortality will taper off in subsequent years as more susceptible individuals are eliminated from the population.