E-Insight

September 2003

West Nile Virus Update: Diagnostic Laboratory Witnessing a Large Increase of WNV in Horses

The Colorado State University Veterinary Diagnostic Laboratory, designated as the state of Colorado's official diagnostic laboratory, reports a large increase in cases of West Nile virus in horses this year.

As of Aug. 7, the laboratory's serology technicians and virologist had tested 108 samples that were positive for WNV in equids. So far this year, one equine case tested positive in May, one case tested positive in June, 74 cases tested positive in July and 32 cases have tested positive to date in August. West Nile virus did not arrive in Colorado until August of last year and Colorado State's Diagnostic Laboratories did not receive equine samples to test for the virus until September 2002.

"With the worst of mosquito season still to come, we are already seeing a large number of tests coming back positive for West Nile virus in horses," said Dr. Barbara Powers, Director of Colorado State's Veterinary Diagnostic Laboratory. "We are seeing many cases from central Colorado as well as a good amount from eastern Colorado. At this point, horses in western Colorado do not seem to be infected with the virus."

To protect both horses and companion animals from WNV, Powers and other Colorado State veterinarians stress that owners should use methods of controlling mosquito populations on their property, such as eliminating standing pools of water where bugs can breed and keeping animals inside during the morning and evening when mosquitoes are most likely to be feeding.

A vaccine is available to help protect horses from contracting or dying from the disease. Colorado State University veterinarians are continuing to suggest to their clients that vaccinating equids according to the manufacturer's recommendations is one important step toward preventing WNV disease in horses.

"We only know the vaccination history of about one-quarter of the samples we receive," said Dr. Hana Van Campen, section head of virology at the Diagnostic Laboratory. "However, only one horse that we know that had the full recommended dose of vaccine has contracted the disease. This horse has survived the infection."

A study previously released by Colorado State University summarized the background characterization of last year's outbreak of West Nile virus in equids and suggested that vaccination is one very important step toward preventing the disease in horses. In the study, even horses that had not received both doses of vaccine several weeks prior to exposure to West Nile virus were more likely to survive than those that were not vaccinated at all.