Common House Sparrows Potential Reservoir For West Nile Virus

Among avian species, the virus has been isolated most frequently from dead crows, but "crows may be just the tip of the iceberg," said Komar, of the Arbovirus Disease Branch, Centers for Disease Control and Prevention, Ft. Collins, Col.

A reservoir host can harbor the virus without being killed by it and serve as a source of the virus for mosquitoes, he explained. Studies undertaken in the Northeastern United States since the West Nile virus was first discovered there have implicated wild birds as the vertebrate reservoir. The finding that common house sparrows can be a reservoir does not mean that humans are at increased risk, he emphasized.

Komar and his colleagues evaluated the reservoir competence of some wild bird species by exposing them to bites of mosquitoes carrying the NY99 strain of the virus and analyzing their blood at 24-hr intervals. Species studied included house sparrows, pigeons, and several others that are common residents of the Northeastern United States..

Most birds survived the infection and developed neutralizing antibodies. House sparrows developed the highest levels of circulating West Nile virus and had the longest duration of infectious viremia, up to a maximum of 5 days, compared with the other species. For example, in pigeons viremia lasted only 1 day.

To evaluate the influence of the virus strain on the magnitude and duration of viremia, two groups of house sparrows were inoculated with either the NY99 or the EG101 strain. The mean peak viremia in house sparrows was greater and duration of viremia longer with the NY99 strain, suggesting that further evaluation of virus strain differences is warranted, he said.

"Sparrows are resident birds and do not migrate, so they are probably not involved in transporting the virus," Komar said. "However, as reservoir hosts the virus can multiply in the sparrows without killing them." It is likely that other birds are also reservoir hosts for the virus, and the species that is responsible for the migration of the virus along the East Coast has yet to be identified, he noted.

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