Mosquitoes sampled from Blaine County have tested positive for West Nile Virus, prompting state health officials to urge Montanans to take extra care in avoiding bites this season.

There is an increased risk of West Nile Virus transmission to humans and horses, according to the Montana Department of Public Health and Human Services, which is expected to continue through October, or as long as mosquitoes are active in the state. No humans or horses have tested positive for West Nile Virus yet this season.

State health officials said there were no human West Nile Virus cases reported in 2022, but the state has seen more than 50 cases in a single summer in the past. Last year, two horses tested positive for the virus and positive mosquito pools were identified in four counties: Lewis and Clark, Blaine, Phillips and Dawson.

Most people infected with West Nile Virus go without symptoms, but one in five do experience a minor illness causing headache, rash, body aches, joint pains, vomiting and diarrhea. One in 150 infections result in severe disease, referred to as neuroinvasive West Nile.

When neuroinvasive, West Nile Virus can cause severe neurological symptoms including disorientation, stupor, coma, paralysis, vision loss and convulsions. Potentially fatal, the virus can also lead to long-term neurological complications. Similar effects can be found in horses who have the virus.

Environmental factors such as winter and summer temperatures as well as rainfall can influence mosquito populations and West Nile Virus activity.

“Mosquito surveillance this year has identified a large proportion of active Culex species mosquitoes, the type of mosquitoes that can carry and transmit West Nile Virus,” State Health Department Vectorborne Disease Epidemiologist Devon Cozart said in a statement.

Cozart said the best way to prevent mosquito-borne diseases is to protect against getting bit. 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.

According to the state health department, mosquitoes that carry West Nile Virus 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 up-to-date information on West Nile Virus in Montana, go to https://dphhs.mt.gov/publichealth/cdepi/diseases/westnilevirus.

Reporter Taylor Inman can be reached at [email protected].