The Montana Department of Public Health & Human Services (DPHHS) said on Tuesday that three mosquito pools in Lewis & Clark County have tested positive for West Nile Virus (WNV).
Last week, a mosquito pool in Cascade County also tested positive for WNV, the first confirmed case in Montana of 2025.
Detection of WNV in mosquitoes is an early indicator that activity is increasing within the state, elevating the risk of human illness. There have been no confirmed human cases in the state at this point.
In addition, the following counties in Montana have each reported one confirmed case of WNV in horses: Sheridan, Custer, and Daniels.
The agency said that 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.
Most people who become infected with WNV will not experience symptoms, but an estimated 1 in 5 do experience minor illness causing headache, rash, body aches, joint pains, fatigue, vomiting, and diarrhea. Fatigue and body aches from WNV may persist for months following infection.
About 1 in 150 WNV infections result in severe WNV disease, referred to as neuroinvasive West Nile. When neuroinvasive, WNV can cause severe neurological symptoms including disorientation, stupor, coma, paralysis, vision loss, and convulsions. WNV can be fatal or lead to long-term neurological complications. WNV can also cause severe neurological complications and death in horses.
The best defense against WNV is bite prevention. To protect yourself, use the 5 Ds:
- DEET - Apply repellent containing an EPA-registered active ingredient, such as DEET, and follow the directions on the package.
- DUSK and DAWN - This is when mosquitoes are most active. Try to avoid outdoor activities during these times.
- DRAIN STANDING WATER - Standing water is the perfect breeding ground for mosquitoes. Drain such areas around your home (gutters, pools, tires, buckets, water bowls, etc.).
- DRESS APPROPRIATELY - Wear long-sleeved shirts, long pants, and socks.
The severity and symptoms of WNV can vary widely. About 80% of persons infected experience no symptoms, but up to 20% of infected persons can develop a mild illness called West Nile fever. Fever generally resolves itself without treatment, but dangerous brain infections such as encephalitis or meningitis can develop in 1 out of 150 people.
Symptoms usually appear within 3 to 14 days after exposure and may include headache, rash, high fever, stiff neck, mental confusion, muscle weakness, tremors, convulsions, coma, and paralysis. Individuals who develop any of these symptoms should see their healthcare provider immediately.
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Currently, there is no vaccine, treatment, or other targeted medication for WNV in humans, aside from supportive care for cases. A vaccine is available for horses. The vaccine is typically administered in the spring to provide optimum protection during mosquito season. Horses cannot transmit the disease to people, but because of the severity of the disease in horses, the vaccine is a recommended core vaccine and should be given annually. Montanans are encouraged to contact their local veterinarian for questions about horses and WNV.
The mosquitoes that carry WNV rarely travel more than one mile from where they breed. To keep mosquitoes away from the home, it’s important to empty standing water at least once per week. For items such as rain barrels, a screen can be applied to the opening to restrict mosquito access. For other mosquito bite prevention tips, check out the 4 D’s of mosquito bite prevention below.
Click here to read more about WNV on the Centers for Disease Control (CDC) website.
