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West Nile Virus activity increasing across Montana

West Nile Virus mosquito
Posted at 3:16 PM, Aug 17, 2023

The Montana Department of Public Health & Human Services is reporting increased West Nile virus (WNV) activity across the state, including the first three human cases of 2023, which have been identified in Dawson, Rosebud, and Yellowstone counties.

These three cases range in ages from the late 30s to early 70s - two men and one woman. All three were hospitalized for their illnesses.

West Nile Virus human cases in Dawson, Rosebud, and Yellowstone counties
West Nile Virus human cases in Dawson, Rosebud, and Yellowstone counties

Cascade County Weed & Mosquito Division earlier this week confirmed the detection of its first WNV-positive mosquito samples of the year.

In addition, two horses were also diagnosed this week with WNV infections in Hill and Pondera counties.

Mosquito pools also recently tested positive for WNV in Glacier, Lewis & Clark, and Toole counties.

Earlier this summer, mosquito pools tested positive in Blaine, Hill, and Phillips counties.

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

“With West Nile virus activity occurring in so many areas of the state right now, the best thing you can do to prevent infections is to protect yourself from mosquito bites,” said DPHHS vectorborne disease epidemiologist Devon Cozart in a DPHHS news release.

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

Unfortunately, about 1 in 150 WNV infections result in severe WNV disease, referred to as neuroinvasive West Nile. When neuroinvasive, WNV can cause severe neurological symptoms including disorientation, stupor, coma, paralysis, vision loss, and convulsions. WNV can be fatal or lead to long-term neurological complications. WNV can also cause severe neurological complications and death in horses.

“All three human WNV cases reported this year were hospitalized, which shows just how serious this disease can be,” said Cozart. “If you are concerned you have a West Nile virus infection, please see your doctor.”

The 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% of persons infected experience no symptoms, but up to 20% of infected persons 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 within 3 to 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.

2019: Great Falls woman dies from West Nile Virus

Currently, there is no vaccine, treatment, or other targeted medication for WNV in humans, aside from supportive care for cases. A vaccine is available for horses. The vaccine is typically administered in the spring to provide optimum protection during mosquito season. Horses cannot transmit the disease to people, but because of the severity of the disease in horses, the vaccine is a recommended core vaccine and should be given annually. Montanans are encouraged to contact their local veterinarian for questions about horses and WNV.

The mosquitoes that carry WNV rarely travel more than one mile from where they breed. So, to keep mosquitoes away from the home, it’s important to 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. For other mosquito bite prevention tips, check out the 4 D’s of mosquito bite prevention below.