In 1999, West Nile virus in the Western Hemisphere was mostly confined to a 50-mile radius of New York City. In just three years, this mosquito-borne disease infected birds and was carried by migrants across the continent to 44 states and 5 Canadian provinces. Last summer, the Lab of Ornithology received several reports from concerned bird watchers in the Midwest about deaths of captive raptors from West Nile virus and an eerie silence in neighborhoods normally busy with the birdlife of late summer.

Henry V. Huang, Washington U. School of Medicine. www.cfe.cornell.edu/erap/wnv/maps.cfm

The shaded areas on the maps above show the counties where West Nile virus was detected during 1999–2002. The movement of migratory birds rapidly carried the virus to new regions.

But hard data on the effects of West Nile virus are few and far between. Thousands of birds, whether they die from West Nile virus or other causes, are never found. Those found are often not tested for the virus. Since 1999, however, New York State's Department of Environmental Conservation's Wildlife Pathology Unit examined more than 12,500 specimens of 213 bird species, providing important regional data on casualties possibly caused by the disease. And although most researchers lack the comprehensive data on bird populations needed to document widespread declines, citizen-science participants are already helping to fill in some of the gaps.

So far the results suggest that the impact of West Nile virus on birds is scattershot. Although the virus has spread far and rapidly, severe outbreaks are confined mostly to localized areas. Also, mortality rates appear to vary considerably among species.

Of the birds tested in New York State, most died of causes other than West Nile virus, and this includes crows, which are particularly susceptible. One study showed that when American Crows were infected in the laboratory, 100 percent died within 7 days of infection. Of the 5,950 wild American Crows tested, 44 percent were diagnosed with West Nile virus. None died from West Nile virus during winter when mosquitoes that transmit the disease are inactive, and few tested positive in early spring. Thus, transmission of West Nile virus through fecal contamination at winter roosts appears to be of little importance. However, increasing numbers succumbed as summer and the mosquito season approached, peaking in September when 78 percent of the dead American Crows tested positive for West Nile virus.

Thirty-four percent of dead Fish Crows (N=116) also tested positive for West Nile virus as did 29 percent of Blue Jays (N=1,284), relatives of crows. The sample of House Finches was small but suggests that they may be vulnerable;

23 percent (N=43) tested positive. Of House Sparrows, 14 percent tested positive (N=427). Excluding these four species and American Crows, only 3 percent of dead songbirds (N=2,466) were positive for West Nile virus.

Concerns about captive birds of prey appear to be substantiated by a preliminary report from the USGS National Wildlife Health Center, which confirmed that 9 of 30 dead raptors at rehabilitation projects in several states tested positive for West Nile virus.

Among wild raptors in New York State, the statistics were uneven. Twelve percent of dead hawks tested positive (N=359). The sample size was small for falcons, but indicates so far a high incidence of the virus among dead Merlins (33 percent, N=15) and American Kestrels (33 percent, N=33). None of the 18 Peregrine Falcons, 25 Bald Eagles, 13 ospreys, 24 Barred Owls, and 37 Eastern Screech-Owls tested positive for West Nile virus. However, 14 percent of 63 Great Horned Owls tested positive.

Most raptors died from traumatic injuries (including gunshots, electrocution from powerlines, and collisions with cars and trains), poisonings (lead, rodenticides, pesticides), starvation, other diseases, and parasites. Nevertheless, continued monitoring of raptor populations is essential to determine how mortality from the virus, in addition to other causes, impacts raptors over the long-term.

The greatest threat from West Nile virus is to endangered species such as the Florida Scrub-Jay, California Condor, and Whooping Crane, already so rare that an outbreak could hamper the species' recovery. For birds in captivity, new vaccinations may provide some protection. In the first round of testing, the Centers for Disease Control (CDC) found a 60 percent increase in survival rates of inoculated birds, according to the American Bird Conservancy. The CDC is also investigating an oral vaccine for wild birds. However, vaccinating even limited numbers of wild birds will be expensive and time-consuming.

For widespread birds such as crows, the patchy pattern of West Nile virus outbreaks means that even if local populations are devastated, the species has a good chance of persisting. A preliminary analysis of Christmas Bird Count data by Lab of Ornithology researchers showed that the numbers of crows counted dropped by as much as 90 percent in some New York City locales but parts of Long Island just to the east showed no discernible declines. Scattered locations along the New England coast began showing declines in 2000, the year that migrant birds began to carry the virus along the Eastern seaboard.

This patchy pattern also characterizes West Nile virus where it originated in the Old World. In many regions of the Old World, West Nile virus is mostly a benign infection in birds—the result of perhaps thousands of years of evolutionary selection for birds that survived by producing antibodies. The numbers of Hooded Crows in Egypt with antibodies varied widely across distances as short as 62 miles, evidence of varying exposure.

West Nile virus primarily infects birds because the mosquitoes that transmit it specialize on birds. The disease is rare among humans and is only known to be transmitted through mosquito bites. In infected areas very few mosquitoes carry the virus, and of people who become infected, fewer than 1 percent become seriously ill. West Nile virus claimed 246 lives in 2002. In contrast, some 20,000 deaths in the United States are attributed to influenza annually, and some 500,000 to cancer.