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1. Spreading like wild virus

2. The study of birds

3. Making sense of West Nile


The main interaction in West Nile virus is between birds and mosquitoes. Once in a while, skeeters feel a need to act like vampires, and they infect people, horses or other animals.Graphic: CDC.














Pair of whooping cranes at the International Crane Foundation, Baraboo, Wis. So far, these highly endangered wetland birds have not been harmed by West Nile virus. Photo: Joel Trick, U.S. Fish and Wildlife Service.




Will West Nile cause birds to go extinct?


Necking our cranes
It's easy to get worked up about West Nile. Courtesy of birds that carry it to new locations, and mosquitoes that inject it into new species, the virus has reached large parts of the Americas, and infected 175 species of vertebrates.

Not bad for less than four years work.

Diagram shows how West Nile virus circulates between birds and mosquitoes, sometimes infecting people or livestock incidentally.

In the future, the virus could appear almost anywhere in the hemisphere, says USGS biologist Paul Slota. "There's an awful lot of [bird] migration to Alaska, and to Central and South America, the Caribbean, and there's a lot of concern that it will follow bird populations north and south."

The peril of bird extinction is particularly acute in Hawaii, where the native birds have already been decimated by mosquitoes carrying bird pox and bird malaria.

Epidemics and other ecological changes are most threatening to rare species like the Hawaiian honeycreepers or the California condor since small populations are more susceptible to any problem.

Two tall, long-billed, white whooping cranes stand in clear water, with grass in background.Of particular concern is the whooping crane, a majestic wading bird that almost went extinct in the 1940s. Although whoopers have come back -- about 400 now live in captivity or in the wild -- they remain highly endangered. Fortunately, while West Nile has infected 158 bird species, no whoopers are known to have gotten sick from it, says veterinarian Barry Hartup, who minds the health of dozens of cranes from around the world living at the International Crane Foundation.

It's a slightly different story with the more numerous sandhill cranes, he adds. In 2002, seven chicks of the Mississippi subspecies died at the Audubon Center for Research of Endangered Species. The center is in New Orleans, where the virus is active.

Cranes shot
The threat to endangered birds has sparked test vaccinations against West Nile. At the Audubon Center, six captive whooping cranes, and some Mississippi sandhills, have gotten a vaccine originally designed to protect horses from West Nile. California condors have also gotten an experimental, recombinant DNA vaccine.

Tests of West Nile vaccine on greater sandhill cranes, a non-endangered subspecies, at the Patuxent Wildlife Research Center and the National Wildlife Health Center were inconclusive. Neither the control nor the experimental animals got sick after being experimentally infected, but the vaccinated birds "developed higher antibody levels and had shorter periods of virus in the bloodstream," says Hartup.

Map of the United States shows the Pacific Flyway, Central Flyway, Mississippi Flyway, and Atlantic Flyway migration corridors, all running north-south.
A map of the migration routes of North American birds shows where birds may have transported West Nile virus.Image: USGS.

Confounding virus
Predicting the future of West Nile is a fool's errand - perfect for Why Filers, in other words. But responsible scientists shy away from predictions. "Nobody really knows, anything anybody says is speculation," says CDC expert Nicholas Komar. So far, West Nile has not followed a path toward lower mortality seen in some epidemics. This welcome, gradual reduction in mortality confers an evolutionary advantage: A virus survives better by infecting its host without killing it. Dead hosts spread less disease than live ones.

The virulence -- infectivity and mortality -- of West Nile has been fairly stable since it arrived, says Komar. And while it's true that mutations can help diseases lose their viciousness, he says, "It can work the other way around as well. They occasionally evolve into being more deadly, and this may be one of those instances. ... In this case, we are probably dealing with a mutation that leads to greater virulence, allows the virus to spread more quickly."

A jet airplane soars amid stormy clouds.For long-distance disease movement, nothing beats a jet plane. A jet may well have imported West Nile to New York in 1999. City of New York.

A flying start
One thing is certain, however. West Nile, like SARS, is a disease of the jet-plane era. Just as SARS jumped the Pacific Ocean from China to Canada, West Nile apparently moved from Israel -- site of an identical viral strain -- to the United States via jet plane. The outbreak "is a consequence of environmental change," says Komar. "Something happened in the environment that allowed it to come to this country."

Like the Spanish galleons of old, the jet an equal-opportunity transporter, hauling people and pathogens alike and creating opportunities for other emerging infections.

But if the future of West Nile is not predictable, history does offer a sobering caution. "West Nile has spread a lot in the last four years, and it will probably continue to spread," says Komar. "It has infected a large number of species, and will probably continue to infect a large number; the affected species list will probably continue to grow. But whatever happens from here is anybody's guess."

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