West Nile Virus Can Spread Without Mosquitoes' Help
RESTON, Virginia,October 26, 2000 (ENS) - For the first time, scientists have confirmed that the West Nile Virus can be transmitted from bird to bird, without a mosquito intermediary. The new finding suggests that controversial attempts to control the spread of the disease with pesticides may be ineffective.
The study also raises the specter of an epidemic of bird deaths across the continent.
Scientists from the U.S. Geological Survey (USGS) reported Wednesday that the West Nile Virus can be transmitted from bird to bird in a confined laboratory setting. It had been thought that the virus was only transmitted through mosquito bites.
The researchers from the USGS National Wildlife Health Center in Madison, Wisconsin, placed infected birds in the same biocontainment aviary as healthy birds. The infected birds died five to eight days later.
Most of the healthy birds, the researchers found, also became ill from the virus and died five to eight days after the first infected bird died.
"It confirms a suspicion that we had and wanted to verify," said Dr. Robert McLean, director of the USGS National Wildlife Health Center. "The setting was a very controlled scientific experiment and we're not sure if or how this relates to what is happening in the wild. Mosquitoes are the primary means of transmission of the virus between birds and to humans. But this certainly opens up a host of new questions."
Chief among the questions, McLean said, is exactly how the virus moves from bird to bird. He said he and other scientists are working on that question now.
"We know that crows are highly susceptible to the virus and that they are more likely than other bird species that live in close contact with one another to transmit the disease to other crows," he said. "We know that the virus attacks the crow's entire body and often affects all the major organs. So far we don't know how sensitive other bird species are to the West Nile virus."
McLean will report his findings at next week's meeting of American Society of Tropical Medicine and Hygiene in Houston.
In the experiment, 16 crows were housed in a 16 foot by 20 foot flight room with 12 foot ceilings. The birds shared food and water and sat on common perches. The room was cleaned daily.
Nine infected birds died within five to eight days. Four healthy or "control" birds died from the virus five to eight days later. Control bird five died eleven days after that, meaning the virus was transmitted from once healthy birds to another healthy bird.
The experiment was done in collaboration with the Wildlife Conservation Society who also helped fund the study.
"Now we're not sure how it moved: by mouth, by preening, did the birds shed the virus in their feces? We're not sure," said McLean. "But by keeping the infected and healthy birds together in close contact, we really maximized the potential that this bird to bird transmission could take place. Now we know it did and we want to figure out how."
McLean said that even though the research is significant, it means more to the wildlife community than the public health community as the threat of humans contracting the virus directly from birds is slim.
But he emphasized that anytime someone finds a dead animal, regardless of whether it is a dead bird or a neighborhood pet, they should avoid handling it, or use gloves or a plastic bag turned inside out to protect your hand.
The virus turned up in North America for the first time last year, when more than 30 people in the New York city became infected, and seven died. Most of those infected suffered relatively mild, flu like symptoms. Those that died contracted encephalitis, an inflammation of tissues around the brain that the virus can cause in people with depressed immune systems.
So far this year, the Centers for Disease Control have confirmed 18 human cases of West Nile Virus, with one death. Most of those cases - 13 - have been in New York state, with four occurring in New Jersey and one in Connecticut.
But the virus has already spread well beyond those states, with dead crows and other birds carrying the virus confirmed in Maryland, Massachusetts, New Hampshire, Pennsylvania, Rhode Island, Vermont, Virginia, the District of Columbia and late last week, North Carolina.
More than 3,600 birds have now tested positive for the virus in 11 states and the District of Columbia in the year 2000. At least 60 bird species have been affected, and while the majority have been the abundant American crow, other rarer species have also been found with the virus, including great horned owls, several hawk species and at least one bald eagle.Domestic and wild animals are also affected. Dozens of horses have tested positive, as well as bats, cats, raccoons, rabbits, squirrels, chipmunks and skunks.
Several states are now spraying aerial pesticides to control mosquito populations. This tactic has been criticized as dangerous to birds, other wildlife and humans, as it can leave pesticide residues on vegetation and in surface waters.
Aerial spraying also kills many non-target insects, reducing the available food for birds and other insect eaters.
Yet scientists acknowledge that aerial pesticides cannot completely destroy the mosquito populations blamed for carrying West Nile Virus. Now that biologists have confirmed bird to bird spread of the disease, conservationists charge that there is even less basis for supporting pesticide spraying as a means of battling the virus.
"The fact is this is a new virus, we didn't know what to do with it," said Linda Farley of the American Bird Conservancy. "All we're asking now is that we slow down and weigh the positives and negatives before we start spraying everywhere."
The finding also suggests the virus could spread across the continent faster than previously predicted. Although cold winter weather provides a temporary respite from mosquito bites, even winter will not keep the disease from expanding into new regions of North America.
That could prove deadly to huge numbers of American birds.
"When you expose a native species to an exotic virus," said Farley, "we have typically seen disastrous results. The birds cannot fight this virus immunilogically. It's possible that we could see dramatic decreases in numbers of lots of species because of this virus."
The finding could be particularly significant if it is true for English sparrows, one of the species suspected of helping to harbor the virus throughout the winter. These sparrows, which were introduced from Europe more than a century ago, appear to be immune to the virus, perhaps because they were exposed to the disease in their evolutionary past in the Old World.
But they can carry the virus, which can remain active in the sparrow's bloodstream for up to five days. During that time, a mosquito that bites an infected sparrow may pick up the virus and carry it to a new host.
If the sparrows, which tend to feed and roost in large flocks, can also spread the disease between themselves without the mosquitoes' help, they could harbor the virus indefinitely, Farley warned.
"If it were to spread in that manner in this species, that would be very significant," she said.
The American Bird Conservancy is urging the federal government to release funds to expand monitoring of wild birds for West Nile Virus, so that researchers can begin to determine whether bird populations are suffering because of the disease.
A new USGS West Nile Virus website with additional information is available at:http://www.umesc.usgs.gov/http_data/nwhc/news/westnil2.html