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November 2000 Magazine
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November 2000 Magazine
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In our June issue we published an overview article on the West Nile Virus, An Overview of the West Nile Virus. Since that time it has continued to spread. More information on its transmission has been discovered and a list of species that have been affected has been published.West Nile encephalitis is an infection of the brain caused by the West Nile virus (WNV), a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States.
This virus is of special concern to bird lovers because birds are not only susceptible to the WNV, but also act as hosts, thus participating in the spread of the disease.
The National Wildlife Health Center at the U.S. Geological Survey site and the Center for Disease Control and other related websites are the source for most of the information in this article. For further information, visit the sites listed at the end of this article.
Transmission Cycle
USGS' National Wildlife Health Center reports that West Nile virus (WNV) is transmitted in the US by multiple species of mosquitoes, in addition to the Culex species. The mosquitoes found positive include species that feed on both avian and mammalian hosts.
Mosquito Species Tested Positive
for West Nile VirusCulex pipiens
Culex restuans
Culex salinarius
Aedes japonicus
Aedes triseriatus
Aedes vexans
Anopheles punctipennis
Psorophora feroxThe following press release of October 25, 2000, notes that WNV has been transmitted from bird-to-bird in the labs, presumably without the intervention of mosquitoes and raises new questions about methods of trasnmission as well as incubation times of the disease.
USGS Researchers: West Nile Moves Bird-to-Bird in LabScientists from the U.S. Geological Survey said today 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.
Scientists from the USGS National Wildlife Health Center in Madison, Wisc., placed infected birds in the same biocontainment (BL3) 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.
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. 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.
In the experiment, 16 crows were housed in a 16-foot by 20-foot flight room with 12-foot ceilings. There, they 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.
An earlier test where infected and healthy birds were housed in separate cages placed side-by-side showed no evidence of direct transmission of the virus leading McLean to believe that the virus is not transmitted through the air.
"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," he said. "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."
A new USGS West Nile Virus website with additional information is available at: http://www.umesc.usgs.gov/http_data/nwhc/news/westnil2.html .
As the nation's largest water, earth and biological science and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation, economic and physical development of the nation's natural resources, and enhance the quality of life by monitoring water, biological, energy and mineral resources.
***USGS*** This press release and in-depth information about USGS programs may be found on the USGS home page: http://www.usgs.gov .
There has been discussion on some of the internet mailing lists as to what precautions in the above tests were taken to rule out parasites. The following conversation was reported on the RARS mailing list.
West Nile virus transmission: Exclusion of Ectoparasites In regards to the questions about the presence of ectoparasites on the crows in the experimental study by Bob McClean, et. al., I had a telephone discussion with Dr. McClean on Friday (27 Oct 2000) about the study and he informed me that the birds were examined for ectoparasites and treated with Ivermectin prior to the study which took place in a BSL-3 facility.
Richard H. Evans, DVM, MS
Research Associate
Orange County Vector Control
Infected AnimalsUSGS' National Wildlife Health Center also reports:
Wildlife involvement in the WNV outbreak continues to expand with the recent finding of 19 free-living mammals positive for WNV including three species of bats, a raccoon, an eastern chipmunk, and tree squirrels in New York. Seven domestic and pet animals have been positive as well (rabbits, chicken, macaw, parakeet, and cockatoo). In NJ an infected cockatiel was found dead in the wild. This year, the virus has again been detected in 8 horses from 5 states.
West Nile virus has been isolated from over 63 species of birds, including 53 free-ranging species from 7 states. The species that appears to be the most susceptible and has been found in the greatest numbers is the American crow. There is also high mortality in blue jays, another member of the Corvidae family.
Wildlife health biologists are concerned that the fall migration of millions of birds from and through the 400 mile wide infected region in the northeastern U. S. may move West Nile virus southward along the Atlantic and Gulf coast states. On September 21, 2000, Maryland officials reported their first two WNV positive birds this year and Pennsylvania reported their first two WNV positive crows on September 26, 2000.
SPECIES FOUND POSITIVE FOR WNV
Free-Ranging Native North American Bird Species
Tested Positive for West Nile VirusBittern, Least
Blackbird, Red-winged
Bluebird, Eastern
Cardinal, Northern
Catbird, Gray
Chickadee, Black-capped
Cormorant, Double-crested
Crow, American
Crow, Fish
Dove, Mourning
Duck, Mallard
Finch, House
Goldfinch, American
Goose, Canada
Gull, Great Black-backed
Gull, Herring
Gull, Ring-billed
Grackle, Common
Grouse, Ruffed
Hawk, Broad-winged
Hawk, Cooper's
Hawk, Red-tailed
Hawk, Sharp-shinned
Heron, Great Blue
Heron, Green
Hummingbird, Ruby-throated
Jay, Blue
Kestrel, American
Killdeer
Kingfisher, Belted
Merlin
Mockingbird, Northern
Nighthawk, Common
Ovenbird
Owl, Great Horned
Raven, Common
Robin, American
Skimmer, Black
Sparrow, Song
Titmouse, Tufted
Thrush, Wood
Turkey, Wild
Turnstone, Ruddy
Veery
Warbler, Black-throated Blue
Warbler, Canada
Warbler, Yellow-rumped
Waxwing, Cedar
Captive North American Bird Species
Tested Positive for West Nile VirusCrane, Sandhill
Eagle, Bald
Gull, Laughing
Magpie, Black-billed
Night-Heron, Black-crowned
Owl, Snowy
Other Free-Ranging Bird Species Tested
Positive for West Nile VirusDove, Rock (pigeon)
Pheasant, Ring-necked
Sparrow, House
Starling, European
Swan, Mute
Free-Ranging Mammal Species Tested
Positive for West Nile VirusBat, Big brown
Bat, Keen's
Bat, Little brown
Chipmunk, Eastern
Raccoon
ReferencesWest Nile Virus - U.S. Geological Survey website.
West Nile Virus - Center for Disease Control website.
Mosquitoes and mosquito repellents: A clinician's guide. - American College of Physicians website
Animal and Plant Inspection Service (APHIS) - USDA website
West Nile Virus Maps (updated weekly) - National Atlas website
Links to City and State Web Sites - Center for Disease Control website.
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