West Nile Virus Update

Mosquito surveillance has identified 9 WNV positive mosquito pools this season. Last week, seven new mosquito pools tested positive for WNV in Cascade (1), Blaine (2), Phillips (2), Dawson (1), and Rosebud (1). Roosevelt County had two positive mosquito pools two weeks ago. 

This summer, over 13,000 Culex species mosquitoes have been collected across 30 jurisdictions. In Montana, no WNV cases in humans or horses have been identified this season.

Nationally, as of 08/18/2026, 222 cases of WNV have been confirmed across 35 states. Among cases, 156 have been classified as neuroinvasive and 13 deaths have been reported. An additional 62 asymptomatic blood donors have also been identified.

Detection in mosquitoes is an early indicator that activity is increasing within the state, putting humans at higher risk of illness. Historically, Montana identifies the first human and/or horse WNV cases of the year within about two weeks of detecting WNV in mosquitoes.

Infections can occur in humans after a bite from an infected Culex mosquito. Increased risk of transmission to humans is expected to continue through October – or as long as mosquitoes are active in the state.

Most people who become infected with WNV will not experience symptoms, but 1 in 5 experience minor illness causing headache, rash, body aches, joint pains, fatigue, vomiting, and diarrhea. Fatigue and body aches may persist for weeks following infection.

About 1 in 150 human infections result in severe disease, referred to as neuroinvasive West Nile. Neuroinvasive West Nile can cause neurological symptoms including disorientation, stupor, coma, paralysis, vision loss, and convulsions. WNV can be fatal or lead to long-term neurological complications such as paralysis.

Currently, there is no vaccine or targeted medication for WNV in humans, aside from supportive care.