West Nile virus has killed increasing numbers of birds each year since its U.S. debut in 1999; will this summer bring the worst outbreak yet?
The Growing Threat of Wildlife Disease: An Introduction
IN 1996, ECOLOGIST PETER DASZAK had the dubious honor of witnessing a species extinction firsthand. That winter, the last living member of a species of Polynesian tree snail, Partula turgida, was killed by a parasitic disease at the London Zoo. The animal's demise marked the first proven case of "extinction by infection," says Daszak, "so watching it was both an exciting and harrowing experience."
Daszak, executive director of the Consortium for Conservation Medicine at Wildlife Trust, says the unfortunate gastropod has plenty of company. While in most cases it's impossible to prove, he believes disease dealt the final blow to many species that have vanished forever, from Australia's marsupial carnivore, the thylacine, to the woolly mammoth. More recently, illnesses such as avian malaria and pox are thought to have killed off several forest bird species in Hawaii. Yet disease remains a largely unrecognized threat to the world's biodiversity. "It's time for conservation managers to realize that disease is as significant a threat to wildlife as is chemical pollution," says Daszak, who adds that the number of wildlife diseases is increasing today.
Like predators, disease-causing pathogens are natural components of ecosystems that help regulate animal populations. But frequently—and increasingly—the human species is responsible for the worst wildlife disease outbreaks. By transporting domestic and wild animals from one place to another, along with pathogen-contaminated products, people are continually exposing animals with naive immune systems to new and potentially deadly infections, just as Europeans brought in the smallpox and measles that devastated populations of native people in the New World.
One well-known case of what Daszak calls "pathogen pollution" was the introduction of cattle to Africa in the 1800s, which sparked an epidemic of a virus called rinderpest among native buffalo, wildebeest and other grazers. Daszak believes such spillover of pathogens from domestic livestock and pets is the leading cause of infectious diseases of wildlife. In the United States, the black-footed ferret was nearly wiped out by canine distemper from domestic dogs, and biologists recently discovered that cat feces, which end up in coastal waters via creeks and storm drains, are infecting threatened southern sea otters with deadly toxoplasma parasites.
People also contribute to wildlife disease outbreaks in more subtle ways. The destruction of wetlands, for example, has crowded ducks, geese and other waterfowl species into smaller and smaller habitats, spawning epidemics of cholera and botulism that kill tens of thousands of birds at a time. Human-induced climate changes, meanwhile, are shifting the distributions of both disease vectors and vulnerable hosts. Take the fungal disease that is wiping out frogs in remote Central American rain forests: Daszak suspects global warming contributes by increasing dryness and forcing water-loving frogs into smaller habitats.
As human populations increase, and both globalization and habitat destruction intensify, the risk of wildlife disease is growing. "We're seeing new diseases, as well as worrisome changes in the distribution and magnitude of more established ones," says Christopher Brand of the U.S. Geological Survey's National Wildlife Health Center. In this on-line issue of National Wildlife two such diseases are profiled: West Nile virus, an introduced pathogen of birds never seen in the United States until 1999 (see text below), and chronic wasting disease, an endemic affliction of western deer and elk that has suddenly begun showing up in other parts of the country (see "The Deer Slayer").
AS THE WEATHER warms across North America this spring, bird-watchers will not be the only ones dusting off their binoculars to track the season's avian migrations. Public health officials, fearing a repeat of last year's epidemic of West Nile virus, will be on the lookout for signs of sick or dying birds, sentinels that foreshadow upcoming outbreaks among people.
But many scientists argue that the real concern should focus on the birds themselves. "West Nile virus is much more a wildlife disease than it is a human disease," says Emi Saito, West Nile surveillance coordinator for the U.S. Geological Survey's National Wildlife Health Center. Though some 246 people died of the virus last year, hundreds of thousands of birds succumbed—mostly crows and blue jays, but also scores of less common birds, from bluebirds and hummingbirds to owls, hawks and eagles. As the disease enters its fifth and perhaps most lethal year in the United States, "we're particularly worried about threatened and endangered species," says Saito. "The virus could set back species recovery efforts or even lead to the extinction of some birds."
Until 1999, when it suddenly turned up in New York City, West Nile had never been detected out-side Africa, Asia and Europe, where it is a well-established disease that's carried and transmitted by mosquitoes and lives primarily in birds. Yet in the pathogen's native range, it rarely kills its avian hosts, which is just one of many ways the virus is behaving differently, and unexpectedly, on this side of the Atlantic Ocean.
One surprise has been the extent and speed of its geographic spread. Confined initially to a small midatlantic zone, the virus had been identified in 44 states and 5 Canadian provinces by the end of 2002. This season, experts predict that the disease will reach all mainland U.S. states as well as Mexico and Central America.
The range of species that have contracted West Nile is also somewhat unusual. As in the Old World, the virus primarily infects birds, but it has struck down a Noah's Ark of other creatures, including horses, squirrels, bats, chipmunks, reindeer, skunks, alligators, domestic dogs and cats and even a harbor seal. Whether the disease—which belongs to a family of bird viruses—will ultimately harm any of these nonavian species is unknown, given the pathogen's unpredictability on the continent so far.
What is known is that West Nile virus severely affects North American birds. While the majority of victims have been corvids—blue jays, magpies, ravens and, especially, crows—a total of 152 species were known to have been infected by year's end, even as fresh specimens continued to trickle in from around the country. Midwestern raptors were hit particularly hard. "From southern Ohio north to Minnesota and south to St. Louis, it's possible that thousands of red-tailed hawks and great-horned owls were killed," says Pat Redig, director of the University of Minnesota Raptor Center in St. Paul.
Because the majority of dead raptors and other birds are never found—and because most states stop conducting necropsies once they've established the virus exists in an area—the exact number of victims will never be known. The impact the virus is having on bird populations as a whole is also uncertain. So far, most data have been anecdotal—individual bird lovers or bird-watching groups reporting local absences of what once were abundant species.
One research team, though, has been able to document declines in several eastern crow populations. Enlisting help from hundreds of volunteer birders, Andre Dhondt and Wesley Hochachka of the Cornell Lab of Ornithology compared crow numbers before and after the virus swept through sites in New York, New Jersey, Pennsylvania, New Hampshire and Vermont. "We discovered that the virus did have a major negative impact on bird abundance," says Dhondt, "but the effects were highly localized." While some crow populations on eastern Long Island seemed barely affected, for instance, others on the western side declined dramatically.
Why crows and other corvids are so susceptible is a mystery. Yet the fact that these birds inevitably die of the disease has conservationists worrying about endangered and threatened species that belong to the family, including the Hawaiian crow and the Florida scrub jay. Indeed, any avian species whose numbers have dwindled significantly is at risk, particularly birds whose last representatives are restricted to one or a few populations or live only in captivity.
Given the threat to such species, developing a West Nile vaccine is a high priority. To date, the only one commercially available was formulated for horses, and last summer, several U.S. zoos and raptor centers inoculated vulnerable birds with this vaccine. Although so far it seems to be safe, there is no evidence that it has provided the birds any protection.
Several experimental vaccines are in the works. The Raptor Center's Redig, for example, is working with the American Bird Conservancy and three U.S. universities to develop a formula specifically for birds. He says the group already has identified a promising virus isolate and is seeking funding to test its safety and efficacy. Redig hopes the vaccine will be commercially available later this year.
One of the more exciting prospects is a recombinant DNA vaccine being developed by the Centers for Disease Control and Prevention (CDC) with support from the American Bird Conservancy and other organizations. In trials conducted last fall, crows infected with West Nile suffered just 40 percent mortality if they had received the vaccine. By contrast, all unvaccinated crows, as well as all crows inoculated with the horse vaccine, died from the virus.
"Ideally, we'd like to see 100 percent protection," says CDC epidemiologist Mike Bunning, "but the results are encouraging because, of all birds identified so far, crows are the most sensitive to West Nile." He adds that the vaccine will probably not be available for another two years. Further down the road, Bunning hopes the team will develop an orally administered version of the injectable vaccine, which could potentially be added to birdseed to wipe the disease out of entire areas.
Meanwhile, results from the injectable vaccine trials were so promising that last winter it was administered to 67 endangered California condors living in captivity. Not only did the vaccine prove safe, initial serology studies suggest the condors developed high levels of antibodies against West Nile. If results continue to look good, all California condors in the wild will be captured and vaccinated this spring. "We've watched what this disease has done as it's moved across the country," explains Cynthia Stringfield, a Los Angeles Zoo veterinarian and veterinary coordinator of the California Condor Project. "We can't afford to just wait and see what happens."
Just how the disease is moving across the country is yet another unknown. Initially, scientists assumed that migratory birds were the primary culprits. In 1999, the CDC asked ornithologist John Rappole of the Smithsonian Conservation and Research Center to study correlations between bird movements and West Nile's spread. His results have been surprising. "It turns out the virus did not move anywhere near as fast as it might have if migratory birds were involved," says Rappole. "If migrants had been moving it, West Nile should have reached Florida within a month and spread throughout the hemisphere within a few months." Instead, the virus did not show up in Florida until late 2000 and was only detected in California last fall.
Rappole and other researchers are now beginning to think that a more sedentary species—the nonnative house sparrow—may play a major role spreading the virus. In laboratory studies, scientists have found that sparrows suffer lower mortality than most native birds tested so far, although they both carry and build up large amounts of the virus in their bodies. Ranging over vast geographic areas, house sparrows are also extremely abundant. And while the bird does not migrate, "it does move around through dispersal," says Rappole. "Each year, about a quarter of the population travels at least 18 miles, which may be enough to account for the virus's spread."
This spring, as the invasive virus heads into its fifth season, few things seem sure other than the fact that West Nile is here to stay. Most scientists believe that, over the longterm, the disease will kill fewer birds as the virus becomes less lethal and birds evolve resistance—a process they say will take somewhere between one and three decades. But in the intervening years, the continent's native birds, and especially its threatened and endangered species, could be in for a difficult time.
Laura Tangley is senior editor of this magazine.