by Howard Youth
Smithsonian scientists are working to understand how an Old World virus is spreading through the New World. Their efforts began at the National Zoo.
Crows gave Smithsonian National Zoo staff its first hint that West Nile virus had arrived in 2002. The birds literally dropped out of the sky, their dark bodies landing on the sidewalk at the feet of keepers and visitors.
In the first six months of 2002, only a smattering of dead crows turned up on Zoo grounds—one in May, then two in the last week of June. But things really heated up in July, when 86 dead crows were found on Zoo property and delivered to the Zoo’s pathology department. About half (44) were tested, and almost all of them (37) turned out to be positive for West Nile virus.
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| American crows (Corvus brachyrhynchos) were the first animals to fall prey to West Nile virus at the National Zoo. (Jessie Cohen/NZP) |
The crows were harbingers of a bigger problem, and Zoo staff scrambled to stay one step ahead of the virus. When a flamingo chick became the first Zoo animal to succumb to the disease on July 6, Zoo veterinarians quickly vaccinated zebras, tapirs, and rhinos—because animals closely related to horses may be most susceptible—and birds housed outside. They gave all these animals a horse vaccine because a West Nile vaccine for birds is still in development, although a specially formulated shot has already been administered to endangered California condors (Gymnogyps californianus).
During 2002, 23 dead birds from the Zoo's collection tested positive for West Nile virus, including six American flamingo chicks (Phoenicopterus ruber), various ducks, a black-necked swan (Cygnus melanocoryphus), a Micronesian kingfisher (Halcyon cinnamomina cinnamomina), Inca terns (Larosterna inca), a pheasant called the Impeyan or Himalayan monal (Lophophorus impejanus), and a bald eagle (Haliaeetus leucocephalus). West Nile virus was determined as the cause of death for all these birds except the two Inca terns and the eagle.
Most animals infected with West Nile virus don't die from it, so although some other Zoo animals tested positive, it does not mean that West Nile virus killed them. For example, an aged gray seal (Halichoerus grypus) died in early September 2002 of congestive heart failure. Its brain tested positive for the virus, but its heart did not, so West Nile was not necessarily fatal in that case. The seal did, however, display neurological signs of the virus, twitching its head and eyes.
Far West of the Nile
West Nile virus was first detected in the New World
in 1999 in Queens, New York City; since then, it has
spread to 47 states in the continental United States,
far into Canada, and is making inroads into the New
World tropics. In the U.S., it has been found in more
than 35 mosquito species; killed at least 513 people
and thousands of birds; and infected more than 20,000
horses, 27 other mammal species, and even alligators.
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| Asian tiger mosquitos spread West Nile virus in the United States. (James Gathany/CDC) |
West Nile virus belongs to the flavivirus family, a group of usually mosquito-spread and wide-ranging illnesses that includes St. Louis encephalitis, Murray Valley encephalitis, Japanese encephalitis, dengue fever, and yellow fever. West Nile’s sudden arrival and rapid dispersal recalls other news-grabbing viruses that animals may have transmitted to humans: SARS might have passed from civets to people, AIDS from chimpanzees to human hunters, and monkeypox from prairie dogs that may have been infected by Gambian giant pouched rats (Cricetomys gambianus).
Until recently, West Nile virus was not considered a threat to humans or even birds. "At present, of all the diseases you can get, West Nile virus doesn't rank high in terms of [an overall threat to] human health," says John Rappole, a senior research scientist at the National Zoo's Conservation and Research Center. Currently, West Nile virus threatens few people who contract it, primarily senior citizens with other existing health problems or compromised immune systems. Hundreds of thousands of people likely contract the virus each year and then develop antibodies to it without realizing that they ever had the illness. The same seems to be true for pet dogs and cats, which probably catch the virus from mosquitoes but rarely show signs of weakness from it.
Over the past few years, however, hundreds of people have died from West Nile encephalitis, severe swelling of the brain associated with far-blown cases of West Nile virus. In the wake of these deaths, and in light of the serious threat that West Nile poses to wildlife and livestock, doctors, researchers, and wildlife biologists are joining forces to understand—and control—the disease. Rappole is working with colleagues at the University of Massachusetts at Amherst to create models that illustrate how West Nile virus moves and how fast. "West Nile's arrival in the New World has been like a giant experiment, like a blank slate," he says. West Nile virus undoubtedly holds many surprises, but as scientists like Rappole step up their efforts to understand this invisible killer, some of the answers are falling into place.
Since West Nile virus was first identified in the blood of a sick Ugandan woman in 1937, it has also been found in much of Africa, the Middle East, large parts of Eurasia, and some areas of the Pacific; however, many experts believe it likely existed long before its detection in the 1930s. A historical review in the December 2003 issue of the journal Emerging Infectious Diseases even speculates that Alexander the Great may have died of the illness in June 323 BCE in Babylon, Mesopotamia, now present-day Baghdad, Iraq. According to authors John S. Marr and Charles H. Calisher, "Previous considerations [of Alexander the Great's death] omitted an event that was carefully recorded by Plutarch and which, before 1999, might have been considered irrelevant: the erratic behavior and observable deaths of numerous ravens outside the walls of Babylon."
The strain found in North America appears to be a strong one first identified in Israel in 1998 after scientists there collected and analyzed sick and dead birds, including white storks (Ciconia ciconia) and farmyard geese (Anser "domesticus"). "In the Old World," says Rappole, "most birds didn't seem to get sick. The finding of sick storks at Eilat, Israel, was sort of an anomaly." After the 1999 New York outbreak, Israeli scientists sent U.S. scientists samples of their strain, which experts now agree is virtually identical to the one now present in North America. There was another outbreak in Israel in 2000, in which 230 people were hospitalized and more than two dozen people died.
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| Gray wolves are one of 27 mammal species to contract West Nile virus. (Jessie Cohen/NZP) |
West Nile virus—or at least the strain present in Israel and North America—has been found in at least 200 species of birds, which are, at present, the only vertebrate class known to carry enough of the virus to spread it to the guts of dining mosquitoes, which in turn spread it to their next victims. However, research is ongoing, and the virus, or antibodies to it, have been found in a growing number of mammal species, including domesticated cattle (Bos taurus), rabbits (Oryctolagus cuniculus), and sheep (Ovis aries); and wild animals such as white-tailed deer (Odocoileus virginianus), striped skunks (Mephitis mephitis), little brown bats (Myotis lucifugus), gray squirrels (Sciurus carolinensis), and eastern chipmunks (Tamias striatus). Mammals infected at zoos include Asian elephants (Elephas maximus), ring-tailed lemurs (Lemur catta), greater one-horned Asian rhinoceroses (Rhinoceros unicornis), and gray wolves (Canis lupus). Two reptile species living in captivity—American alligators (Alligator mississippiensis) and crocodile monitors (Varanus salvadorii)—have also contracted West Nile.
At the Zoo, and Beyond
In 2002, Smithsonian Environmental Research Center animal ecologist Peter Marra and his research team tested the National Zoo's wild birds as part of an ongoing effort to determine whether birds in the Baltimore–Washington corridor carry the virus or antibodies to it. "We found 50 percent antibody prevalence at the Zoo. That's high. We don't know how many birds died, but it means that the Zoo is a hotspot," he says. House sparrows (Passer domesticus) were the most common wild Zoo bird to carry antibodies for the virus.
The summer of 2003 proved far less dramatic than the previous year. Despite continuous sampling, Zoo pathologists found no Zoo animals and only a few wild birds that tested positive. The initial spike in animal deaths after the virus' National Zoo debut in 2002, followed by a relatively small presence the next year, mirrors similar experiences at the Bronx Zoo, the Philadelphia Zoo, and other zoos. Was it the weather? Did the vaccinations and booster vaccinations pay off? Were the weakest animals weeded out the year before in a case of Darwinian survival of the fittest? Did those that survived develop immunity? No one knows for sure, although ongoing research may soon answer such questions.
It is certain that however West Nile virus spreads, it does so steadily and rather quickly. In spring 2002, Marra and his team found that members of 11 resident (non-migratory) Jamaican bird species carried antibodies for the virus in their blood—the earliest evidence of the virus' introduction to the New World tropics. In addition, the team found migrating warblers in Mexico and Puerto Rico that also carried West Nile antibodies. In the July 2003 issue of the journal Emerging Infectious Diseases, Marra and his colleagues Alan P. Dupuis II and Laura D. Kramer, both from the New York State Department of Health, wrote that "individual birds from at least three species of Neotropical migratory birds have survived WNV infection and may serve as hosts for spreading the virus." All three species—northern waterthrush (Seiurus noveboracensis), yellow warbler (Dendroica petechia), and black-and-white warbler (Mniotilta varia)—breed in North America but winter south of the U.S.
Since then, scientists have tested for and found the virus in birds on Mexico's Yucatan Peninsula, in El Salvador, and in the Dominican Republic. Conservationists fear that West Nile virus will cause irreparable harm to endangered bird populations in North America, the Caribbean islands, and Central America, and in South America, where many scientists expect it to turn up any day now. "In the tropics, where species are already hit by habitat destruction—this is where my fears lie," says Marra. So far, though, no die-offs in tropical birds have been documented. "Even though we're baffled by this, we're happy that's the case," says Marra.
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| At the Smithsonian Environmental Research Center, graduate student Pauline Roberts tests an ovenbird for West Nile virus. (Peter Marra) |
West Nile is spreading west as well and will likely head beyond California, where it arrived in 2003. "A particular concern would be the Hawaiian Islands, where many bird populations sit right on the edge of extinction," says Rappole. Most of the islands' endemic forest birds hang on in small populations already endangered by introduced avian malaria and pox, invasive species, and habitat destruction. "Could West Nile virus push them over the edge? That seems possible," says Rappole.
Trailing the Virus, One Bird
at a Time
"We're trying to understand how West Nile virus
affects populations of birds," says Marra. "We're
seeing that there are some local declines but no large-scale
regional declines." Marra and colleagues at the
U.S. Department of Agriculture have begun monitoring,
via radio-transmitters, American robins (Turdus
migratorius) in Fort Collins, Colorado, and the
Washington, D.C., area to see how well breeding adults
fare when they carry the virus. They are also studying
how vulnerable nestlings are to developing and spreading
the virus. "Nestlings are sitting there pumping
out CO2," says Marra. "They're potentially
very susceptible to being bitten by mosquitoes [which
are attracted to exhaled carbon dioxide] and carrying
and dispersing West Nile virus."
Current data suggest that early on, adults keep their young covered and thus protected from mosquitoes. "But as they get older, adults brood less and less," says Marra. "Nestlings then get bitten by tons of mosquitoes." It still remains to be seen how often nestlings succumb to the virus. Marra points out that nestlings already suffer high mortality rates, primarily due to predation.
Scientists are also preoccupied with finding out which bird species most readily spread the virus to new areas, or help amplify and strengthen the virus' presence at new sites. American crows (Corvus brachyrhynchos) are no longer the biggest suspects: Although they catch West Nile at a high rate, they also seem to die soon after they contract it, probably limiting their ability to spread the virus.
Currently, house sparrows are being eyed as potential culprits, based on limited studies. These European songbirds spread quickly from their release site in New York City in 1851, and are now prolific throughout North America and as far south as Nicaragua, often nesting close to or on houses in cities and towns. They may play a major role in the dispersal of West Nile for three reasons: The majority of house sparrows that contract West Nile do not die from it, house sparrows are one of the few species known to carry the virus for up to five days while maintaining normal activity, and they are abundant and tend to move around in their lifetimes.
The question of how the virus arrived in the U.S. is far more difficult to answer. "There are now many funded studies going on where people are trapping birds and testing if they've been exposed to the virus," says Rappole. "This will help answer how fast and far it's spreading, but will not answer how it arrived."
Many people assume that West Nile was brought over the ocean by a migratory bird. But most long-distance migrants, except strictly oceanic birds, migrate north and south, not east and west across the Atlantic Ocean. Others speculate that an imported captive bird or a sick person carried it over. Perhaps infected mosquitoes were ferried over in containers tucked in aircraft cargo holds. Bioterrorism has even been suggested. "Sometimes, people get frustrated with scientists because they say, 'Well we don't know for sure,' but being frustrated with them is better than being really, really mad at them for giving you the wrong answer," Rappole says.
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| West Nile virus poses a threat to livestock. (Keith Weller/ARS USDA) |
As more data accumulate, old theories are debunked in light of new revelations. "Old World literature said mammals were a dead end for West Nile virus, that most mammals wouldn't even contract the disease. Now there's quite a bit of information and quite a few mammals that get it. You can't dismiss an entire class of organisms and say they won't get it. Just like you can't say that an entire class—birds—will have it. There's still a lot we're learning," says Rappole.
Scientists previously believed that transmission of West Nile requires a mosquito or other vector to pass from one animal to the next, but a recent study suggests that ring-billed gulls (Larus delawarensis), blue jays (Cyanocitta cristata), black-billed magpies (Pica hudsonia), and American crows—the last three all members of the crow family—caught the virus from sick cage mates. The authors of this study stress that it had small sample sizes and that researchers do not know how the transmission occurred, so further investigation is needed to reach a firm conclusion.
Along those lines, researchers are exploring the possibility that birds catch West Nile from their prey. A lab study conducted by scientists at the Centers for Disease Control and Prevention, Colorado State University, and the Office of the Surgeon General shed some light on this subject. Published in the March 2003 issue of Emerging Infectious Diseases, the study mentions that a great horned owl (Bubo virginianus) caught the virus after eating an infected mouse; that American crows contracted it after eating infected house sparrow carcasses; and that a house finch (Carpodacus mexicanus) likely caught the virus by eating an infected mosquito.
And for years, researchers assumed that migratory birds brought West Nile back each year. But virologists now believe that West Nile virus persists in places like New York City because of vertical transmission between mosquitoes, with a mother mosquito passing the disease to her young.
With West Nile virus seemingly here to stay, some research has been focused on preventative medicine. Several laboratories report progress in developing a vaccine for humans for West Nile, although it might not hit the market for a few more years. Even then, unless the virus becomes more of a threat to children or other age groups, only elderly people or those with lowered immunities are likely to get the shot.
Few North Americans knew about West Nile virus before it arrived in 1999. Now it resides in the guts of mosquitoes buzzing around our neighborhoods. This widespread, fast-paced, and potentially fast-changing illness has brought with it alarming environmental, health, and conservation concerns, but also sparked a great deal of new scientific investigation—activity that helps us better understand how animals and pathogens interact on our ever-changing planet.
—Howard Youth is a contributing
editor to ZooGoer.
ZooGoer 33(2)
2004. Copyright 2004 Friends of the National Zoo.
All rights reserved.
