State and local health officials confirmed the first detections of West Nile virus (WNV) in Montana for summer 2025. WNV activity appears to be widespread across the state, and the Department of Public Health and Human Services (DPHHS) is urging all Montanans to take increased precautions against mosquito bites.
Four mosquito pools have tested positive for WNV, including three in Lewis and Clark County and one in Cascade County. Detection of WNV in mosquitoes is an early indicator that activity is increasing within the state, elevating the risk of human illness. Additionally, Sheridan, Custer, and Daniels Counties have each reported one confirmed case of WNV in horses, totaling three positive horses in Montana so far this season.
WNV can cause illness in humans after a bite from an infected Culex mosquito. The increased risk of transmission to humans is expected to continue through October, or as long as mosquitoes remain active in the state.
“Illness caused by WNV can vary significantly in severity,” said DPHHS Vectorborne Disease Epidemiologist Devon Cozart. “Although some people may not experience any symptoms at all, many people experience the onset of flu-like symptoms such as body aches and fatigue that may persist for weeks to months.
Currently, there is no vaccine or targeted medication for WNV in humans, aside from supportive care. Most people who become infected with WNV will be asymptomatic, but 1 in 5 experience mild symptoms. About 1 in 150 human infections result in severe disease, referred to as neuroinvasive West Nile.
West Nile is a preventable disease in horses. Vaccination works and is inexpensive, especially compared to the cost of treatment and/or loss of an animal.
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. 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.
Once a horse has clinical signs of WNV infection, treatment is not 100 percent effective in many cases, and permanent disability or death may occur even with aggressive treatment.
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 day
For other WNV prevention tips, check out the 4 Ds of mosquito bite prevention below.
The 4 Ds of Mosquito Bite Prevention include:
- DEET: Use insect repellent containing effective ingredients such as DEET or picaridin. To verify the effectiveness of insect repellent, go to the EPA’s website: epa.gov/insect-repellents.
- Drain: Drain standing water around the house to prevent mosquito breeding.
- Dawn/Dusk: Mosquitoes are most active during dawn and dusk. Stay inside or take extra precautions to prevent mosquito bites during these times. If doors or windows are left open in the summer, make sure they’re fitted with screens to keep mosquitoes out of the house.
- Dress: When possible, wear long-sleeved shirts and pants to protect against bites.
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MT DPHHS
