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The West Nile virus (WNV) epidemic in the United States
in 2002 was the largest epidemic of meningoencephalitis (swelling
of brain and spinal cord tissue) ever documented in the Western
Hemisphere. WNV is now infamous for its deadliness, the unusually
broad range of animals it can infect, and the remarkable speed with
which it advanced across a large geographic area.
In 2002, West Nile virus spread from 27 primarily
eastern states and the District of Columbia to 44 states and 5 Canadian
provinces, spanning the continent. In this one year, the virus killed
a minimum of hundreds of thousands of birds (and at least 241 humans
and 4,300 horses). It is impossible as yet to gauge the short- and
long-term consequences of the loss of so many birds.
West Nile is an "emerging" virus in the
United States (a disease-causing virus that has recently become
dramatically more common), and as such, we still have much to learn
about understanding how this virus works and what it will mean for
North, Central, and South American wildlife. Scientists from many
fields are contributing to the effort to understand and potentially
combat this insidious virus.
First there was the silence of the crows. Then horses
fell ill – more than 14,000 this past summer [2002] alone –
along with squirrels, chipmunks, and mountain goats. Even mighty
raptors – eagles and hawks – and Great Horned Owls dropped
from the sky. Now scientists and the public alike have serious concerns
about the rapid spread of the West Nile virus’ North American
invasion, and are amazed at the scale and sweep of its ecological
impact.
While the human toll dominated the nation's attention
during 2002 – the virus killed at least 241 people and infected
thousands more – and the effects on wildlife were far worse.
The virus swept westward with alarming rapidity, appearing
in almost every state in the nation – an astonishing expansion
for a virus that was not even detected in the Western Hemisphere
until 3 years ago.
Equally unexpected, nearly 200 species of birds, reptiles,
and mammals fell ill, infected by West Nile virus during 2002. West
Nile virus infection is not fatal to all animals, and over time
some species are expected to adapt. But even partial declines in
certain populations could have serious consequences. Rodent populations
could increase substantially in areas where raptors die off, and
pest birds such as House Sparrows may increase where crows are absent.
The worst problems are still ahead, scientists say.
This spring, West Nile virus is expected to complete its push to
the West Coast, and may spread to the subtropics, where rare birds
and other vulnerable creatures already face formidable threats to
their survival.
"Once it gets to the tropics, where you have species already
stressed by habitat destruction and you have the potential for year-round
mosquito transmission, some of those populations are not going to
make it," said Peter Marra, an animal ecologist and West Nile
specialist with the Smithsonian Environmental Research Center in
Edgewater, Maryland. "I'm concerned about parrots and hummingbird
populations. There's not that many of them left.”
West Nile virus made its North American debut during
the fall of 1999, when it was discovered in a dead American Crow
in New York. Scientists don't know how the virus reached the United
States – perhaps it was hidden inside a single infected bird
imported from the Middle East. But one thing is certain, said Stephen
Ostroff of the Centers for Disease Control and Prevention (CDC)
in Atlanta: "There's no way that West Nile virus is going to
go away."
The virus does not appear to be any more virulent
among Americans than other people around the world, and scientists
suspect that the American population will gradually gain immunity
through low-level exposures. That is the situation today in countries
where the virus has been active for many years. Most people in those
countries have antibodies to combat the virus from early childhood,
and serious complications or death from West Nile virus infection
are rare.
But the virus has proven to be unusually aggressive
among North American wildlife and is capable of infecting a surprisingly
diverse array of animals. "Most viruses tend to be rather host-specific,
but that's not what we were seeing," said Tracey McNamara,
chief of pathology for the Wildlife Conservation Society.
It is still unclear how many of the more than 200
species infected by West Nile virus have suffered significant population
declines. But a consensus is emerging that among birds, in particular,
far more species are being affected than scientists originally predicted
– not just the crows, ravens, and jays that were known to be
especially vulnerable.
"There's been a huge die-off of raptors,"
said Robert McLean with the U.S. Department of Agriculture's National
Wildlife Research Center in Fort Collins, Colorado. The experience
of the University of Minnesota's Raptor Center, which treats sick
and injured raptors, was typical. "In mid-August 2002, we had
our first case," said spokeswoman Sue Kirchoff. "By September
and October, we were just inundated."
The center took in 70 ailing birds of prey, including
eagles, Red-tailed Hawks, and Great Horned Owls. Officials presume
that if that many were found and brought to the Raptor Center, countless
others died in the wild, with potentially far-ranging repercussions.
"From a biological standpoint, raptors take longer
to mature and raise fewer offspring" than smaller birds, said
Patti Bright from the American Bird Conservancy. "Whether they'll
be able to rebound, well, we just don't know." It will take
a while longer, Bright and others said, before it is known whether
rodent populations increase due to the West Nile virus's impact
on birds of prey.
The evidence for declines in songbirds and other small
bird species is less direct, in part because they are so much less
visible. "We're simply not going to know for a while about
the smaller birds, because we're not going to find the bodies,"
said David Wilcove, a professor of ecology at Princeton University
who has been studying West Nile virus disease.
Still, researchers this year found more than 140 bird
species sick or dead as a result of West Nile virus, including chickadees,
doves, grackles, gulls, herons, kingfishers, pelicans, sparrows,
swans, turkeys, warblers, woodpeckers, and wrens. While most of
these species will probably pull through as resistant individuals
breed, passing anti-viral vigor to their offspring, ornithologists
expect that others will not be so lucky.
They point to a similar problem in Hawaii, where the
arrival of an avian pox virus in the 1890s and avian malaria in
the 1930s forcing dozens of species to extinction, or close to it.
"Those microbes just hammered native Hawaiian forest birds,"
Wilcove said.
Several unexpected aspects of the epidemic feed Wilcove's
and others' pessimism. One surprise is that the virus can be transmitted
directly from bird to bird, not only via mosquitoes. Raptors can
acquire the virus by eating infected prey, and some birds can apparently
spread the virus in their droppings. There's also evidence that
some birds can pass the virus directly to their developing eggs.
Another surprise is that West Nile virus can be transmitted
directly from adult mosquitoes to their eggs, so that newly hatched
aquatic mosquito larvae hatch infected. That could make insecticides,
which typically kill only adults, less effective.
Scientists have also been surprised to learn that
the virus can persevere through the winter, even in many northern
states. Researchers are not sure what animals are serving as the
virus's winter host, but the phenomenon is allowing the disease
to spread year-round and is giving the summer viral eruption an
earlier start each year.
Yet another surprise is the number of mosquito species
that carry the virus – 36 at last count. "This is a virus
that's never seen a mosquito it doesn't like," said Ostroff
of the CDC. "That's not typical for most pathogenic viruses."
If that’s not enough, some researchers suspect
that West Nile virus might be capable of mixing its genetic material
with that of a closely related virus, such as the one that causes
St. Louis encephalitis, if both viruses were to infect a single
animal. Other viruses have periodically produced such hybrids, creating
in the process an entirely new and dangerous virus.
"This virus is going to spread to the West Coast
big time by next year, no question," USDA's McLean said. "Each
habitat is different, but California seems to be an area that has
all the factors you need for a major spread. I think they're going
to be facing major problems in humans, horses, birds, and other
animals. I just don't see any barriers."
Whether West Nile virus ends up decimating any animal populations
or settling in as a temporary high-grade ecological disturbance,
the epidemic should be a wake-up call to beef up the nation's surveillance
and quarantine network, said Princeton's Wilcove.
This article was prepared with information from the article, “West
Nile Virus Update,“ by Rick Weiss, published in the Washington
Post, December 27, 2002
and an Audubon web page posted in February 2003 at http://www.audubon.org
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