First Confirmed Equine Case of West Nile Virus in Montana for 2018

Helena, Mont. – The Montana Department of Livestock has received the first reported cases of equine West Nile Virus in Montana for 2018 in Musselshell and Lake Counties. This follows the detection of the virus in mosquito surveillance pools from Cascade, Hill, and Lewis and Clark Counties. Montana typically sees cases of West Nile Virus through late summer and into fall.

West Nile Virus affects humans, equines, and birds. It is spread through the bites of infected mosquitos; horses cannot transmit the virus directly to people. Detections of the disease in horses and mosquitos in Montana serve as an important reminder for people to take steps to prevent West Nile Virus infection.

“There is no direct treatment for the virus in horses, but vaccination is highly effective in preventing disease. Horses that are vaccinated rarely die or are euthanized because of the disease,” said Dr. Tahnee Szymanski, Assistant State Veterinarian.  “Vaccination is typically administered in the spring but may offer some protection even this late in the season. Work with your veterinarian to determine if your horse could still benefit from vaccination.”

Horse owners should be aware of the typical signs of West Nile Virus which include:

  • Fever, loss of appetite and depression;
  • Incoordination or weakness of the hind limbs;
  • Muscle or muzzle twitching, drooling.

In the meantime, topical insecticides and eliminating standing water may help decrease your horse’s exposure to mosquitos. The mosquitos that carry West Nile Virus are most active at dawn and dusk, so consider keeping your horses off of irrigated pastures and away from water sources during those times of the day.

The mission of the DOL is to control and eradicate animal diseases, prevent the transmission of animal diseases to humans, and to protect the livestock industry from theft and predatory animals. For more information on the department, visit

Equine Owners Encouraged to Consult with Vets on West Nile Virus Vaccination

Montana Department of Livestock DOLMontana’s animal health officials are encouraging equine owners to consult with their veterinarians about vaccination for West Nile virus after a spike in the number of cases last year. “We had 32 cases last year, the most since 2007, and the third highest total in the nation,” said assistant state veterinarian Dr. Tahnee Szymanski. “That’s concerning because the disease is highly preventable.” Based on MDOL data, no equines that were current on vaccinations have ever contracted the disease in Montana. In contrast, one-third of the non-vaccinated equines that contracted the disease either died or had to be euthanized (161 of 492 since 2002).

The best time to vaccinate is before mosquito activity begins, as the vaccine takes a few weeks to offer full protection. Horse owners should consult with their veterinarians to develop a vaccination plan specific to their animal and situation.

Dr. Greg Johnson, professor of veterinary entomology at Montana State University’s Department of Animal & Range Sciences, says vaccination – which is recommended as a core vaccine by the American Association of Equine Practitioners – is prudent given WNV’s unpredictability. “It’s kind of like the flu season,” Johnson said. “We can look at the existing data and forecasts and make some guesses, but we can’t really predict what West Nile is going to do or how bad it’s going to be.” However, with snowpack running at 150 percent of normal throughout much of the state, environmental conditions could be right for WNV activity later this year. “With all of that snowpack, it looks like we’ll have lots of water, and that can mean more mosquitos,” he said. 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 statewide. Stressing the importance of vaccination, Szymanski said, is that there is no treatment for horses that contract the disease.

WNV is a reportable disease in Montana. Any confirmed or suspected case should be immediately reported to the Montana state veterinarian’s office at 406/444-2043.

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