West Nile — More Than Just a Threat to Humans
A report released in late June by wildlife health experts at the University of California, Davis (UCD), warns that West Nile virus could pose a serious threat to some species, especially rare and endangered birds, and encourages officials to broaden existing monitoring efforts to track the virus' movement in the state.

Prepared by a team led by Walter Boyce, director of the UCD Wildlife Health Center, at the request of the California Department of Fish and Game, 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.

Areas of greatest concern are the Central Valley, coastal regions, western Sierra Nevada, the Salton Sea and the lower Colorado River basin. However, with the current limited knowledge of West Nile virus, it's nearly impossible to know which species may be hardest hit.

"Some level of mortality due to West Nile virus will occur in a substantial number of different bird species in these areas," Boyce said. "Just which ones we can't say with certainty. 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."

For example, yellow-billed magpies are not listed as threatened or endangered. However, their distribution is limited to the Central Valley, an area high in mosquitoes, which puts magpies at risk. Furthermore, they may be subject to high levels of mortality because they belong to the highly susceptible family of birds that includes crows and jays. Most of the dead birds found to be infected with West Nile virus have been crows — nearly 500 already in California this year.

Crows and related birds such as 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 said. "We may see local 'hotspots' of disease around areas with large numbers of susceptible hosts, such as crow roosts."

The virus, first detected in mosquitoes near the Salton Sea in July 2003, is expected to move into the northern part of the state this year. Ten people and four horses have been diagnosed with West Nile virus in California as of June 29.

Little is known about the impact of West Nile virus on free-ranging wildlife populations since its introduction to North America in 1999. It has killed individuals in more than 200 species of native and exotic birds and 20 species of mammals. The threat to reptiles and amphibians is not believed to be as great as for birds because the mosquito species that feed on them rarely feed on birds.

Source: California Waterfowl Association
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