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.
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:
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.
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."
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.
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."
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,"
Got any good news?