USGS reports that West Nile virus goes beyond crows

November 16, 1999

NOTE TO EDITORS: Dr. Robert G. McLean, Director of the USGS National Wildlife Health Center, Madison, Wis., will testify at a congressional field hearing on the West Nile virus, slated for Thursday, November 18, at 11 a.m., at Fairfield University, Fairfield, Conn. The hearing will be in the School of Business Dining Hall. Dr. McLean will be available following the hearing. For more information on the hearing, call Olivia Ferriter, 703-648-4054.

The virus that can cause West Nile encephalitis in humans is not only found in crows, according to scientists at the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis. USGS has recently released a list of 18 bird species that the Centers for Disease Control and Prevention (CDC) in Fort Collins, Colo. confirmed as having tested positive for the West Nile virus, including:"Although the cause of death has not been determined in all cases, we can assume that these birds were exposed to the virus in the tri-state area of New York, New Jersey and Connecticut," said Dr. Robert McLean, USGS wildlife biologist and director of the Madison Center. "Some of these species could play a significant role as we track the spread of this disease."

McLean added, "The variety of birds is disturbing because many of these species migrate to other areas and could potentially disseminate the disease elsewhere. The good news is, with the exception of American crows, there were only a few individual birds from each respective species that tested positive. It is still too early to tell if this is because they are not as susceptible or they were not as readily observed as crows."

Monitoring mortality in American crows is especially important because crows appear to be highly sensitive to the virus. Since many crows normally travel less than 200 miles they can act as sentinels for local transmission of the disease.

"At this time it is difficult to assess how many birds have died from this disease. While some of the affected areas have reported very high bird mortality, especially in crows, a number of the birds have died from other causes," said USGS wildlife disease specialist Linda Glaser. "About 49 percent of the 392 birds tested by the USGS and CDC have tested positive for West Nile virus."

McLean and his team of USGS biologists have been taking blood samples from migratory and non-migratory birds in the New York area to get a sense of the extent that bird populations and various species were exposed to the virus. "This information, in addition to being shared with the CDC, will be used in establishing an effective surveillance network to track the virus and ultimately learn how it could impact our native bird populations," said McLean.

There is still much that is unknown about the ecology of the virus and how it will respond to its new environment in the United States. "Once a bird is infected, the virus can be transmitted to mosquitoes only for about 4 to 5 days. Although this appears to be a short time, many birds can migrate hundreds of miles within that time. It will be important to find out what species are involved in the transmission of the virus and if migratory birds, including crows, are capable of moving the virus to new locations," said McLean.

The USGS will continue to focus on collecting information that will help determine the extent of the wildlife species involved, the geographic and temporal distribution of the virus, and whether the disease is continuing to expand to new sites.

McLean emphasized that with so many state and federal agencies involved in the West Nile investigation, he is optimistic that scientists will learn what role migratory birds may play in determining if the West Nile virus will find a permanent home in the western hemisphere and what impact it will have on our native species.

Periodically, the USGS issues Wildlife Health Alerts to keep natural resource agencies appraised of wildlife health or disease issues that may threaten free-ranging and captive wildlife. USGS, along with several state and federal and local natural resource, public and animal health agencies, is diligently monitoring the spread of the virus by conducting field investigations, processing wildlife specimens, and keeping these agencies informed through the USGS Wildlife Health Alerts.

The USGS is also developing digital maps to monitor the expansion and range of the disease. The maps can be quickly updated and allow scientists to electronically overlay maps showing other pertinent information such as climate, vegetation, bird and mosquito species range for the area being studied.

West Nile virus is closely related to St. Louis encephalitis virus, but generally causes a milder disease in humans, according to information from the CDC. The West Nile virus is transmitted by a mosquito, which acquires the virus when it bites an infected bird for the blood meal needed to produce mosquito eggs. If the same mosquito subsequently bites a human, it can pass the virus to a human host. Like St. Louis encephalitis, West Nile virus is not transmitted directly from person to person or from birds to persons. This is the first time West Nile virus has been recognized in the United States or any other area of the Western Hemisphere.
For more information on West Nile virus, see the following webpages:

USGS Wildlife Health Alert

USGS National Wildlife Health Center

USGS Biological Resources Page on West Nile Encephalitis

Centers for Disease Control and Prevention

As the nation's largest water, earth and biological science and civilian mapping agency the USGS works in cooperation with more than 2000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to sound economic and physical development of the nation's natural resources, and enhance the quality of life by monitoring water, biological, energy, and mineral resources. In-depth information about USGS programs may be found on the USGS home page at To receive the latest USGS news releases automatically by e-mail, send a request to Specify the listserver(s) of interest from the following names: water-pr; geologic-pr, geologic-hazards-pr; biological-pr; mapping-pr; products-pr; lecture-pr. In the body of the message write: subscribe (name of listserver) (your name). Example: water-pr joe smith.

US Geological Survey

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