West Nile Virus Detected in Montana

State and local health officials are reporting the first West Nile Virus (WNV) detection of the year in mosquitoes sampled from Blaine County. No humans or horses have tested positive for WNV yet this season.

Infected mosquitoes can transmit WNV when they bite, and 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. 

While no human cases of WNV were reported in 2022, in previous seasons Montana has reported over 50 human cases in a single summer. In 2022, two horses tested positive for WNV and positive mosquito pools were identified in four counties (Lewis & Clark, Blaine, Phillips, and Dawson). Environmental factors such as winter and summer temperatures and rainfall can heavily influence mosquito populations and WNV activity.

"Mosquito surveillance this year has identified a large proportion of active Culex species mosquitoes, the type of mosquitoes that can carry and transmit WNV," said DPHHS Vectorborne Disease Epidemiologist Devon Cozart. "The best way to prevent mosquito-borne diseases, including WNV, is to protect yourself from mosquito bites."

This is especially important while spending time outdoors in the summer, and during peak feeding activity times for female Culex mosquitoes, which is dusk and dawn. Permethrin is an insect repellent that can be utilized to treat clothing and gear, including tents. For repellents that can be applied directly to the skin, here's a link to an Environmental Protection Agency search tool to find the appropriate EPA-approved insect repellent.

"It is also a good idea to check your insect repellent for a graphic that indicates the repellent works on mosquitoes and the length of time it repels them," Cozart said.

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 regularly empty standing water 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.

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, vomiting, and diarrhea. However, 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.

Currently, there is no vaccine, treatment, or other targeted medication for WNV in humans. A vaccine is available for horses, who should be vaccinated once a year. Montanans are encouraged to contact their local veterinarian for questions about horses and WNV.

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