Southwest Montana News

State and local public health officials have confirmed the first detections of West Nile virus (WNV) for the season in mosquito samples from Dawson and Lewis and Clark counties, Montana. Additionally, a horse in Lewis and Clark County has tested positive for WNV. So far, no human cases have been reported this season.

Officials anticipate an increased risk of WNV infection for humans and horses through at least October. The presence of WNV in mosquitoes and horses serves as an early warning for Montanans to be cautious about mosquito bites during outdoor activities in summer and fall.

In 2021, two human cases were recorded in Rosebud and Lewis and Clark counties, both neuroinvasive. Two horses also tested positive in Rosebud and Phillips counties, with mosquito pools testing positive across seven different counties. Human cases of WNV vary each year; 2018 saw 51 human cases alongside 50 equine cases.

Preventing mosquito bites is crucial to avoid mosquito-borne diseases like WNV. While Montana mosquitoes cannot transmit diseases such as the Zika virus, they can spread WNV to humans, horses, and birds. Birds are often the source of infection for mosquitoes that then pass it on when biting humans or horses.

Most people infected with WNV do not show symptoms; however, 1 in 5 may experience mild symptoms like headache, rash, body aches, joint pains, vomiting, or diarrhea. Severe illness occurs in fewer than 1 out of 150 individuals with symptoms such as encephalitis or meningitis. Neuroinvasive disease involves serious brain-related symptoms like disorientation or vision loss.

There is no vaccine or specific treatment for WNV other than supportive care. However, a vaccine is available for horses and is strongly recommended.

“Mosquito season is West Nile season,” stated DPHHS epidemiologist Devon Cozart. “Detection of West Nile virus in mosquitoes means that it’s currently in Montana, so now is the time to make sure you’re taking precautions against mosquito bites.”

The Department of Public Health & Human Services (DPHHS) advises Montanans to follow the "4 D's" of prevention to protect against WNV.

For further information on protection measures against WNV, residents are encouraged to contact their local health department or visit the DPHHS website page dedicated to West Nile virus information.