Transmission cycles of WNV. Transmission occurs between ornithophilic mosquito species (Culex spp.) and passeriform avian hosts. Birds typically manifest high viremias capable of exceeding oral infection thresholds for many mosquito vectors. Humans and equines fail to generate sufficient viremia for the infection of mosquito hosts and are deemed “dead end” or tangential hosts. Red: Avian mortality was first identified in a migratory stork and subsequently in commercially farmed geese in the late 1990s and has subsequently been associated with widespread mortality among numerous North American avian species (in particular, corvids). (A color version of this figure is available at www.vetres.org.)