West Nile Virus on the Rise in Montana

State and local health officials are reporting increased West Nile virus (WNV) 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 [https://www.epa.gov/insect-repellents/find-repellent-right-you] 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 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."

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.

The 4 D's 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.

For more information about WNV protection and surveillance activities, contact the local health department or visit the DPHHS WNV website page: https://dphhs.mt.gov/publichealth/cdepi/diseases/westnilevirus