Prospective immunization of the endangered California condors (Gymnogyps californianus) protects this species from lethal West Nile virus infection.

West Nile virus (WNV) has caused significant morbidity and mortality in humans, mammals, and both native and exotic birds in North America since its emergence in New York City in 1999 and its subsequent spread westward. Prior to the arrival of WNV to the western United States, prospective vaccination was conducted for the entire population of endangered California condors, both in captivity and in the wild. Here we show that this vaccine is safe for condors, stimulates protective immunity in adults, nestlings, and newly hatched chicks. Most importantly, we demonstrate protection of captive birds exposed to naturally circulating WNV during the 2004 transmission season. The prospective vaccination of the entire population of California condors before the arrival of WNV has thus potentially saved this endangered species from subsequent lethal WNV encephalitis, and possible extinction.