Mosquito - Centers for Disease Control and Prevention
Centers for Disease Control and Prevention

MONTANA — Montana health officials confirmed the first West Nile virus activity of summer 2026 and urged people across the state to guard against mosquito bites.

Two mosquito pools tested positive in Roosevelt County, according to the Montana Department of Public Health and Human Services (DPHHS). No human or horse cases have been reported yet this summer.

Mosquito detections are an early sign that virus activity is increasing in Montana. People can get sick after a bite from an infected Culex mosquito.

“West Nile virus symptoms span a wide spectrum, with many people experiencing prolonged, flu-like fatigue and body aches, while others remain entirely asymptomatic,” said DPHHS Vectorborne Disease Epidemiologist Devon Cozart. “The risk of severe illness is a serious concern; every year, some Montanans experience neurological complications that can cause permanent disability or death. We urge everyone to take steps to mitigate these risks by making mosquito bite prevention a daily habit.”

There is no vaccine or targeted medication for humans beyond supportive care. Most infected people do not show symptoms, but one in five can have a milder illness such as headache, rash, body aches, joint pain, fatigue, vomiting and diarrhea.

The department said about one in 150 infections leads to severe disease, called neuroinvasive West Nile virus infection. That can include disorientation, stupor, coma, paralysis, vision loss and convulsions.

Horse owners are advised to talk with a veterinarian about an annual vaccine. Health officials also said mosquito bite prevention matters most during dawn and dusk, when female Culex mosquitoes are most active.

DPHHS urges people to use repellent with DEET or picaridin, drain standing water at least once a week and wear long sleeves and pants when possible. Screens on rain barrels, doors and windows are also recommended.