Researchers at the United States Geological Survey's 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 the transmission of the disease between birds. At this point, there is little or no speculation that mammals such as horses can spread the disease among themselves, writes Stephanie L. Church in the January 2001 edition of The Horse. West Nile virus made its first appearance in the Western Hemisphere in 1999. It caused illness and death in horses, birds, and humans that year and again in 2000. The mode of introduction of the disease to this country remains a mystery. Dr. Robert McLean was the principal investigator for the West Nile research involving birds. It is not known exactly how the transmission of West Nile from bird to bird occurs, but theories abound. An accumulation of the virus has been found in the kidneys of dead birds, which suggests that the virus is shed through feces. 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 a field setting. Similar transmission research also is being performed by the United States Department of Agriculture and the Centers for Disease Control, according to McLean.