West Nile virus (WNV) has appeared in horses as far west as Wyoming and Colorado, according to reports from state officials and the United States Department of Agriculture. Equine cases have also been found in Michigan and Oklahoma for the first time. Confirmed or preliminary equine cases have been reported from at least 24 states in 2002.

Many horse owners are already familiar with WNV, as the neurological disease began causing illness and death in the fall of 1999 in birds, horses, and humans. The virus is spread by the bite of an infected mosquito and is harbored by birds, both of which have assisted in the disease’s spread over the past three years.

An equine WNV case was confirmed Aug. 19 in eastern Goshen County, Wyo., which is near the Wyoming-Nebraska border. Three  Colorado horses were confirmed as WNV-positive on Aug. 15 in Weld and Pueblo counties, Colorado’s first detection of WNV.

The Michigan Department of Agriculture announced three probable cases of WNV in horses in Washtenaw, Emmet and Gratiot counties last week, which are currently being treated. According to an Associated Press report from Oklahoma City, Ok., four horses in the western part of the state (in Harmon, Garfield, Texas, and Beaver counties) have tested positive for WNV and are expected to recover