Vaccination can prevent West Nile Virus in horses |

Vaccination can prevent West Nile Virus in horses

Montana’s animal health officials are encouraging horse owners to vaccinate their animals against West Nile Virus (WNV).

“Montana has a problem,” said Dr. Jeanne Rankin, assistant state veterinarian with the Montana Department of Livestock. “For whatever reason, most people are not including West Nile in vaccination programs for their horses.”

During the past six years, 94 percent of the state’s reported equine WNV cases involved unvaccinated horses, Rankin said, and one-third of the reported cases resulted in the death or euthanasia of the infected animal. The state was second in the nation last year for the number of reported equine WNV cases.

“It’s really unfortunate, as the disease is highly preventable,” Rankin said. “Using a vaccine, or better yet, a vaccine in combination with measures to control the vector, will virtually eliminate the potential for horses to contract the disease.”

A mosquito-borne disease, WNV was first found on the east coast of the U.S. in 1999. Since then, the disease has spread westward, arriving in Montana in 2002. The disease knows no climactic or geographic boundaries in Montana, and has been found in 34 counties.

Clinical signs of the disease in horses include loss of appetite and depression, weakness or paralysis of hind limbs, muzzle twitching, impaired vision, loss of coordination, head pressing, aimless wandering, convulsions, inability to swallow, hyper-excitability and coma. WNV mimics other serious neurological diseases like sleeping sickness, equine encephalitis and rabies, and should be immediately reported so that a licensed veterinarian can make a diagnosis.

Three WNV vaccines are currently approved for equine use: Two that require two shots three to six weeks apart, and a newer vaccine that requires just one shot. Horse owners should consult with their veterinarian to determine which vaccine best suits their needs.

Rankin said it’s not too late to have horses vaccinated or get booster shots for previous vaccinations.

“Historically, this is about the time of year when the first cases of WNV appear,” Rankin said. “The WNV season can run as long as late October.”

Effective mosquito control also helps decrease the potential for spreading the disease. Watering troughs should be cleaned thoroughly and regularly, and standing water where mosquitoes can breed should be managed if possible. A variety of water treatment solutions that kill fly and mosquito larvae but are nontoxic to animals are commercially available. For additional information on controlling mosquitoes to protect livestock, contact Greg Johnson, veterinary entomology for the Department of Animal and Range Sciences at Montana State University.

WNV has not yet been found in Montana this year, but 11 cases have been reported nationally.

West Nile Virus is a reportable disease. Any confirmed or suspected case should be immediately reported to the Montana state veterinarian at 406/444-2043 and/or USDA-APHIS Veterinary Services 406/449-2220.

Additional information about WNV can be found at MDOL’s web site,, and the USDA-APHIS web site, (includes a 2-page brochure, fact sheet and FAQ).

Additional information about human health aspects of WNV can be found on the Montana Department of Human Health & Public Services web site,, and the Centers for Disease Control web site,


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