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