If you've had equine cases of West Nile virus (WNV) pop up in your county, most of the equids in the area probably were exposed to the disease or even infected by the time those infections occurred. That is why it's extremely important to vaccinate horses at least two months before the WNV season, according to Maureen Long, DVM, PhD, Dipl. ACVIM, assistant professor of large animal veterinary medicine at the University of Florida. She presented rough data regarding 2001 Florida exposure rates at the 2003 American Association of Equine Practitioners' Convention.

"Even on a very small geographic level, there is no prediction on where you're going to see infection," said Long.

"Little is known about the occurrence of subclinical exposure in horses (how many horses have been infected, but don't show clinical disease)," she and colleagues wrote in the study. "Without this data, little inference can be drawn regarding the asymptomatic to symptomatic ratio and the need for vaccination once disease has occurred in the herd."

Long wanted to determine the rate of subclinical exposure in horses in the face of the 2001 outbreak of WNV in Florida. That year, 492 cases of WNV clinical illness were confirmed, with a 33% mortality rate. Also, researchers wanted to measure the IgM response in horses receiving the killed virus vaccine. IgM is a protective protein manufactured by lymphocytes (types of white blood cells). This response can also be used to detect recent exposure to a disease