West Nile Virus Detected In Mosquitoes

State and local health officials are reporting the first Montana West Nile virus (WNV) detections for summer 2024 in mosquitoes across four counties.

Six mosquito pools in Blaine, Dawson, Prairie, and Wibaux counties have tested positive for WNV. 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.

“Now is the time to take action against WNV if you haven’t already started thinking about mosquito bite prevention this season,” said DPHHS Vectorborne Disease Epidemiologist Devon Cozart. “Infection can result in minor or severe illness, including death. Even minor forms of infection can leave individuals feeling ill and fatigued for weeks.” 

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.

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 search tool is helpful for identifying the appropriate EPA-approved insect repellent that can be applied directly to the skin.

Mosquitoes that carry WNV rarely travel more than one mile from where they breed.

To keep mosquitoes away from homes, it’s important to regularly 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. Check out the 4 Ds of mosquito bite prevention below for other mosquito bite prevention tips.

The 4 Ds of Mosquito Bite Prevention:

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: https://www.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 a screen to keep mosquitos 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.