FOR IMMEDIATE RELEASE
Date: July 29 2025
Contact: Jon Ebelt, Communications Director, DPHHS, (406) 444-0936, (406) 461-3757
jebelt@mt.gov
West Nile Virus Activity Confirmed in Montana
DPHHS urges Montanans to take increased caution against mosquito bites this summer
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. Severe, neurological cases of WNV also occur every year in Montana, which may lead to death or permanent disability like blindness or paralysis. Don’t take a chance; prioritize avoiding mosquito bites whenever possible.”
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, including 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.
Horses can also be infected with WNV. Horse owners may contact their veterinarian to obtain an annual vaccine to protect against WNV.
Preventing mosquito bites is important while spending time outdoors in the summer, especially during dusk and dawn, which are peak feeding activity times for female Culex mosquitoes. Permethrin is an insect repellent that can be utilized to treat clothing and gear, including tents. The Environmental Protection Agency also maintains lists of approved insect repellents that can be applied directly to the skin.
Emptying standing water at least once per week is important to keep mosquitoes away from homes. For items such as rain barrels, apply a screen to the opening to restrict mosquito access.
For other mosquito bite 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.
Contact your local health department or visit https://dphhs.mt.gov/publichealth/cdepi/diseases/WestNileVirus for more information about WNV protection and surveillance activities.