CODY, WYOMING — Wildlife biologist Terry Creekmore was studying sick prairie dogs in the South Dakota Badlands in 1999, when the National Wildlife Health Center asked him to head east to New York City. Hundreds of crows were turning up dead in the city, and nobody knew why.
Creekmore took blood samples from 175 New York crows, and helped discover that the wild birds were suddenly being infected by something that was supposed to be far, far away: West Nile virus, which originated in Africa and the Middle East.
“West Nile virus was not diagnosed (initially) by the public-health people,” says Creekmore, who’s now with the Wyoming Department of Health, testing dead birds for traces of the virus by extracting their brain tissues. “It was diagnosed by the wildlife people.”
Since the New York outbreak, West Nile virus has spread westward all the way to California, carried long distances by birds, and transmitted mainly by mosquitoes, which bite infected birds and then bite other animals. More than 8,000 people have been infected nationwide; more than 200 have died.
It’s not clear why, but six of the nine states with the greatest number of human cases are in the West: Colorado, Wyoming, both Dakotas, Montana and New Mexico. Some speculate that the region’s recent weather patterns have favored the virus. It just reached Utah, Nevada and Arizona this year, and though it has waned in the cold weather, it will likely show up in Oregon and Washington next spring.
Most of the attention focuses on people, as health departments issue warnings to use mosquito repellent, and TV news headlines sensational stories about elderly people stricken with the disease. But the virus is also beginning to take a toll on Western wildlife — and nobody knows how heavy that toll could become.
Adapting may be difficult
Most people exposed to West Nile virus don’t get sick at all, or merely suffer flu-like symptoms. Less than 1 percent become severely ill, with symptoms that include meningoencephalitis (inflammation of the spinal cord and brain), high fever, convulsions, blindness, paralysis and coma.
Wildlife casualties are tougher to measure. Species that live around humans — bats, chipmunks, skunks and squirrels — have been discovered with the disease. Most wild birds that die are never found, however, and few are ever collected for analysis, especially in the wide-open spaces of the West. But as word of the problem spreads, wildlife agents, hunters, ranchers and hikers have become more alert for dead birds, turning them in by the dozens for analysis.
Some bird species seem to be largely immune to West Nile, but some are hit very hard. The little evidence that exists is mainly found in relatively limited studies and in individual cases, but it shows that the virus is fatal in more than 200 species of birds in this country, including bald eagles, magpies and pheasants.
Infection is almost always fatal for crows. Raptors, including hawks, owls and falcons, are also noticeably susceptible; their reported symptoms include blindness, paralysis and starvation.
Birds in Africa have adapted to the virus, and over time, North American birds may as well, says Todd Cornish, at the Wyoming State Veterinary Laboratory. A lot of factors play into resistance: genetics, the species’ immune system, the dose of the virus (which depends on the mosquito). “If you get hit and you have the right genetic makeup, you’re going to live and pass on those (resistant) genes to your offspring,” Cornish says. “The ones who aren’t resistant die.”
This virus may thrive on methane
One species at risk is the sage grouse, which already has been reduced to a small fraction of its historic population (HCN, 2/4/02: Last dance for the sage grouse?). Five studies in Montana, Wyoming and Canada show the virus beginning to hit grouse. About 20 grouse in Wyoming have been identified as casualties; most were found only because they were being monitored with radio-collars, in ongoing population studies.
The toll may become more noticeable over time — especially in areas impacted by the demand for natural gas. Tens of thousands of coalbed methane wells have been drilled recently from Montana to New Mexico, and all those wells pump groundwater to the surface along with the gas. This creates breeding grounds for virus-carrying mosquitoes.
Professors David Naugle at the University of Montana and Greg Johnson at Montana State University, who are researching for any possible link between methane development and the virus, say it’s too early to draw conclusions. But some environmentalists and ranchers say there’s bound to be a link.
Nancy Sorenson, a rancher near Gillette, Wyo., who sees the virus spreading in her area, told The New York Times, “The only standing water we had around here this summer was methane water.”
The West’s total grouse population still likely runs more than 100,000, but grouse expert Clait Braun, now retired from the Colorado Division of Wildlife, says small, isolated grouse populations are most at risk, because they have the lowest genetic diversity — the Gunnison sage grouse in western Colorado, for example, and about 1,000 greater sage grouse surviving in Washington. Braun says methane water may not be as much of a mosquito breeder as irrigated hayfields are, but adds that, in general, “My professional opinion is that a lot of birds are going to die.”
The question is: How much more can birds like the grouse — already under pressure — withstand? Cornish’s lab is testing grouse to see if any of their immune systems have created antibodies to resist the virus. But as of early November, the first 24 grouse had tested negative.
The author writes from Cody, Wyoming.
Centers for Disease Control, West Nile virus www.cdc.gov/ncidod/dvbid/westnile/index.htm
Terry Creekmore and Todd Cornish at the Wyoming State Veterinary Laboratory in Laramie, 307-742-6638