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West Nile Virus On Rise In Montana

State and local health officials are reporting increased West Nile virus activity in several counties across the state this week, including the first three human cases for 2023, which have been identified in Dawson, Rosebud and Yellowstone counties.

These three cases range in ages from the late 30s to early 70s, including two males and one female. All cases were hospitalized for their illnesses.

Additionally, this week, two horses were diagnosed with WNV infections in Hill and Pondera counties. Mosquito pools also tested positive for WNV in Glacier, Lewis and 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.

Preventing mosquito bites is especially important while spending time outdoors in the summer, and during peak feeding activity times for female Culex mosquitoes, which are dusk and dawn. Permethrin is an insect repellent that can be utilized to treat clothing and gear, including tents.

The Environmental Protection Agency search tool offers EPA-registered insect repellents that can be applied to the skin.

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 longterm 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.”

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.

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