Picture of an epidemic
How far? How fast? What's next? Who's next? Even with a human epidemic like SARS, answering these questions can be gnarly. When the subject is, er, birds, research money dries up faster than a prairie pothole in a July heat wave. But even if your avian interests tend more toward KFC and turkey dinner than American crows and great horned owls, you might want to consider the wild birds. They are, after all, the most likely mechanism for West Nile's geographic spread. "There is some evidence, and a fair amount of belief, that it's being carried around in migratory birds," says Paul Slota, who's in charge of public information at the National Wildlife Health Center in Madison, Wis. "We think it's pretty clear that is happening," Slota says.

Birds, like people and alligators, are a "host" of West Nile. A mosquito is the "vector" that carries the virus between animals. Mosquitoes carry innumerable pathogens, including viruses that cause other kind of encephalitis, and the malaria parasite. Oddly, at least 37 mosquito species can carry the virus.

West Nile virus was first seen in the West Nile district of Uganda in 1937. It has since spread widely in Asia, Africa and the Middle East. Map from U.S. Department of State.

As birds both spread West Nile and die of it, a multi-pronged effort to study the disease in wildlife is starting to yield results:
Lab studies of West Nile at the Centers for Disease Control in Fort Collins, Colo. pinpointed blue jays, common grackles, house finches, American crows and house sparrows as the birds that were most "competent" (able to infect new mosquitoes). Four species, again including American crows and blue jays, could also pass the disease to other animals by contact. And five species got infected by eating food containing pathogens. Overall, the studies showed that mosquitoes are one of several possible pathways to infection.
Field studies by scientists from the National Wildlife Health Center are looking at virus and antibodies in the blood of migratory birds. Robert Dusek, a field biologist with the center, plans to recapture birds banded in previous years, "to see what species are recovering, which were exposed but survived." (Because infection does not always cause disease symptoms, it's unclear how many of the birds actually got sick.) The studies should clarify the role of migrating birds in carrying West Nile, and flesh out the picture of disease dynamics. "Learning as many facts as we can about West Nile virus will help us manage and predict outbreaks," says Dusek, "whether it's for endangered birds, domestic animals or human health."

Population studies track groups of animals year after year, and are the best way to pin down the ecological effects of a disease when animal numbers naturally fluctuate. Last year, Caffrey of the Audubon Society noticed a population crash among American crows she's studied in Oklahoma for six years. Within two months after West Nile reached the area in September 2002, she says,"40 percent of 120 marked individuals disappeared." Three of the six corpses she found had evidence of West Nile, she says.
Anecdotal observations by experts at wildlife rehabilitation centers indicate that certain birds of prey may be suffering from West Nile. Patrick Redig, director of the Raptor Center at the University of Minnesota, says that rehab centers across the upper Midwest reported that "a lot of great horned owls died" in 2002. "If you let your imagination run wild," he adds, "there could have been a lot of dead great horned owls out there, but in wildlife rehabilitation, we never know if what we are seeing is the major portion, or just the tip of the iceberg."
Annual surveys give a broader and hopefully more scientific picture of bird populations nationwide. The biggest, the breeding bird survey, and the Audubon Christmas Bird Count, use volunteer ornithologists and a rigorous approach to document long-term population changes. Still, the West Nile epidemic may be too recent to appear in the results. "There are certainly some places around New York City that showed declines in crows in conjunction with the first occurrence of West Nile virus," says John Sauer, an ornithologist who specializes in survey data at the Patuxent Wildlife Research Center. "There were sites with big declines, but ... from 2000 to 2002, there are not really any strong, slap-in-the-face type conclusions ..." Nonetheless, the 2003 results should be "interesting," he adds.

Got any good news?

There are 1 2 3 pages in this feature.
Bibliography | Credits | Feedback | Search

©2003, University of Wisconsin, Board of Regents.