West Nile virus threatening wildlife

West Nile virus could pose a serious threat to some species, especially rare and endangered birds, and officials are encouraged to broaden existing monitoring efforts to track the virus' movement, states a report re leased by wildlife health experts at the University of California, Davis. Prepared by a team led by Walter Boyce, director of the Wildlife Health Center, the report predicts where West Nile virus poses the greatest risk to wildlife by examining mosquito abundance in relation to bird species that "amplify" the virus and the location of rare amphibians, reptiles, birds, and mammals.

"Some level of mortality due to West Nile virus will occur in a substantial number of different bird species.... Just which ones we can't say with certainty," Boyce notes. "There is so much we don't know about this pathogen. The species that concern us most are those that have limited distribution overall or a limited population in areas with high numbers of mosquitoes."

The report includes a series of maps that vividly depict where West Nile virus poses risks to various groupings of species. Included are 27 birds, such as the brown pelican, California condor, and sandhill crane; 21 mammals, such as the kit fox, wolverine, and bighorn sheep; 10 amphibians, such as the Shasta salamander, black toad, and red-legged frog; and 10 reptiles, such as the desert tortoise, barefoot gecko, and giant garter snake.

The geographic overlap of abundant and rare bird species creates a situation that may increase the threat. Locally dense populations of crows are of particular concern because they raise the risk of infection for other, less common, birds in the same area. Most of the dead birds found to be infected with West Nile virus have been crows.

Crows and related birds like jays, magpies, and ravens, as well as house finches, house sparrows, and ring-billed gulls are considered likely amplifying hosts that are key, abundant sources of the virus. "The ability of West Nile virus to move within a given area is strongly influenced by the presence of hosts that serve as a source of virus for mosquitoes," Boyce points out. "We may see local 'hotspots' of disease around areas with large numbers of susceptible hosts, such as crow roosts."

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