State health officials have confirmed Montana's first human case of West Nile virus in 2026, with the infection traced to Cascade County. It is the first locally acquired human case in Cascade County since 2018, according to Cascade City-County public health officer Trisha Gardner, who said the evidence points to the virus being contracted within the county rather than during travel.
West Nile is transmitted by infected mosquitoes and circulates in Montana during late summer, when mosquito populations peak. Most people infected with the virus experience no symptoms, but a small percentage develop serious neurological illness. Health officials typically respond to confirmed human cases by urging residents to use insect repellent, wear long sleeves during dawn and dusk hours, and eliminate standing water around their homes.
The Cascade County case — which includes Great Falls — is the nearest confirmed human infection to the Helena area so far this season. Lewis and Clark County Public Health has not announced any confirmed human cases locally, but mosquito activity in the Helena Valley makes the risk real throughout the region during late summer months.
Residents with questions about West Nile risk or symptoms can contact the Lewis and Clark City-County Health Department. The Montana Department of Public Health and Human Services tracks West Nile activity statewide and updates its surveillance data throughout the season.