West Nile Virus has been confirmed in Cascade County.

It’s the first positive mosquito sample for 2026.

A WNV-positive sample of adult Culex tarsalis mosquitoes was collected southeast of the town of Simms, according to the Cascade City-County Health Department.

West Nile virus detected in Cascade County [2025]

The Montana Department of Public Health and Human Services in Helena on Aug. 20.

Treatment was conducted in the area where the sample was collected and county officials are continuing to monitor the area.

West Nile detected in Cascade County [2023]

No known human infections of WNV have been detected in Cascade County so far this year, according to CCHD.

Residents are strongly encouraged to take steps to protect themselves.

Human case of West Nile Virus reported in Cascade County [2018]

The single 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 percent of infected people experience no symptoms, but up to 20 percent 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 3-14 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.

Cascade County’s Weed and Mosquito Division manages mosquitoes by surveying and monitoring mosquito larvae, adult mosquitoes, West Nile virus presence, and conducting treatments when necessary.