MISSISSIPPI STATE -- Vaccinate horses now against the West Nile Virus to reduce the risk of contracting the potentially deadly disease and fears of infection during the 2003 mosquito season.

The U.S. Department of Agriculture recently granted a full license to Fort Dodge Animal Health for its WNV vaccine, labeled West Nile - Innovator. The vaccine previously was available through a conditional license while studies were conducted.

"It's important to follow the protocol for giving the vaccination; the first dose should be given early in mosquito season, followed by a second dose three to four weeks later," said Dr. Stanley Robertson, a Mississippi State University Extension Service veterinarian. "Foals should receive three vaccinations during the same time interval."

Horses will become immune to the disease about three weeks after the second dose, and a booster is required at least annually thereafter. The full series should be given before the mosquito season begins, usually by April or May.

"Depending on the area you're in, some veterinarians recommend that if it seems to be a particularly bad season for West Nile Virus, horses should receive the booster every six months," Robertson said.

A Fort Dodge study shows the vaccine to be 95 percent effective in horses that received two doses of West Nile - Innovator one year earlier. In contrast, 85 percent of the non-vaccinated horses in the study did develop active WNV infection.

Three confirmed cases so far this year indicate the potential for more cases in the coming months. This would not be surprising considering that the virus has spread further westward each subsequent year since its 1999 introduction in the eastern United States.

In Mississippi, no cases were reported in horses in 2000, 27 cases were reported in 2001, and 311 cases in 2002. Only seven horses died or were euthanized in 2001, compared with 110 in 2002.

While the vaccine is highly recommended by veterinarians, owners should also take steps to prevent the transmission of WNV. The best way to keep mosquitoes -- a main carrier of the disease -- at bay is to discourage breeding activity by eliminating standing water and cleaning water tanks or other sources of drinking water at least weekly.

"Improving the overall cleanliness of the premises can also limit vector activity, so frequent removal of manure and removing or trimming weeds can be beneficial," Robertson said. "In addition, keeping horses indoors during peak mosquito feeding times, such as at dusk and dawn, can cut down on the chances of getting West Nile Virus. Turning off barn lights or using fluorescent lights can minimize the attraction of adult mosquitoes into barns."

Keep air moving in stabling areas by using fans, and apply insect repellents that contain permethrins to horses especially during peak mosquito feeding times.

The most common clinical sign associated with WNV infection is an altered gait, including reluctance to move, stumbling, perceived lameness, ataxia or weakness.

"Owners should be alert to identify whenever their horse's behavior changes, such as appearing lethargic, having a diminished appetite or developing a fever. Immediately consult a veterinarian if a horse develops any of these signs," Robertson said.

"Vaccinated horses that are infected with WNV are much less likely to become ill or die, making vaccination for the disease very beneficial and recommended as a preventive measure," he added.

Consult a licensed veterinarian to develop a disease prevention plan for WNV. For more information, contact Robertson at (662) 325-2283 or the local Extension office.