West Nile Virus--Mosquito Not Required

Researchers at the United States Geological Survey's (USGS) National Wildlife Health Center in Madison, Wis., recently discovered that West Nile virus can be passed from bird to bird in a laboratory setting without the bite of a mosquito. The mosquito previously was regarded as necessary for transmission between birds. At this point, there is little or no speculation that mammals such as horses can do the same. West Nile virus made its first appearance in the Western hemisphere in the fall of 1999, and it caused illness and death in horses, birds, and humans in 1999 and 2000. The mode of introduction of the virus to this country is still unknown. (Read a veterinarian's first-hand encounter with West Nile on page 45.)

Robert McLean, PhD, was the principal investigator in the bird study, the second of a series of avian experiments in his quest to fully understand West Nile's epidemiology. The first study had crows confined in adjacent cages with no direct contact to confirm that the virus wasn't spread through the air.

The virus had a different environment during the next experiment. "We optimized the conditions of the virus to occur," said McLean. "We confined the birds so they couldn't get away from one another, we increased their contact (with one another), and we used a very susceptible species, the crow. We knew it wouldn't take much virus for this species to come down with the disease."

Sixteen birds--nine inoculated with WNV and seven healthy ones--were put in a laboratory's "free-flying" room. The first nine WNV-positive birds died in a similar pattern within four to seven days, as expected. Starting a few days later, five of the uninoculated birds died of clinical signs of the disease.

McLean now knows that the crows spread the virus through direct contact in the laboratory, and theories abound as to how the transmission occurs. An accumulation of the virus has been found in the kidneys of dead birds, which suggests that the virus is shed through the feces, even though the floor of the laboratory was cleaned daily. The birds might have spread the virus by preening, or possibly by contamination of their food source.

McLean is conducting further transmission studies with different species in the avian laboratory and in the field setting. He said that the United States Department of Agriculture (USDA) and the Centers for Disease Control are performing similar transmission studies, and that the USDA has even worked with domestic birds like chicken, geese, and turkeys.

The main question now is the exact means of transmission between these birds, and its impact on the wild bird population.

"Does this (means of transmission) occur in nature? We just don't know," McLean said. One thing is certain--no research suggests that this type of transmission can occur in mammals, and nothing negates the standing belief that the horse is a dead-end host for the virus and that other animals or horses cannot get the virus from a horse.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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