West Nile Virus Confirmed In Mosquito Samples, Horse In Two Montana Counties

Montanans urged to take steps to avoid mosquito bites

State and local public health officials are reporting the season’s first West Nile virus (WNV) detections in positive mosquito samples in Dawson and Lewis and Clark counties.

The first horse infected with WNV this season was also confirmed earlier this week in Lewis and Clark County. To date, no human cases of WNV have been identified this season.

Increased risk of WNV infection in humans and horses is expected to continue through at least October.

The detection of WNV in mosquitoes and horses is an early warning that Montanans are at an increased risk of infection. It also serves as a reminder of the importance of avoiding mosquito bites, especially as Montanans enjoy summer and fall days outdoors.

In 2021, two human cases were reported (Rosebud and Lewis and Clark counties), and both were neuroinvasive.

Additionally in 2021, two horses tested positive for WNV (Rosebud and Phillips counties) along with mosquito pools from seven different counties. The number of human cases of WNV vary from season to season. For example, in 2018 there were 51 human and 50 equine cases reported.

The best way to prevent mosquito-borne diseases, including WNV, is to protect yourself from mosquito bites. While mosquitoes found in Montana are unable to transmit diseases like Zika virus, they can transmit WNV. The virus can also infect horses and birds, with birds serving as the source of infection for most mosquitoes who then pass the virus along to humans or horses when they bite to take a blood meal.

Most people who become infected with WNV experience no symptoms, but 1 in 5 develop a mild illness, with symptoms such as headache, rash, body aches, joint pains, vomiting, or diarrhea. Other individuals, fewer than 1 out of 150, may become severely ill with encephalitis or meningitis (inflammation of the brain or surrounding tissues).

Serious infection in or around the brain is called neuroinvasive disease and is indicated by symptoms such as disorientation, convulsions, vision loss, and numbness. If someone develops any of the mild or serious symptoms listed above, they should see their healthcare provider for evaluation and care.

There is no vaccine or targeted medication available for WNV infection other than supportive care. A WNV vaccine is available for horses, and vaccination is highly 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.”

DPHHS reminds Montanans to protect against WNV by following the 4 D’s of prevention.

The 4 D’s are:

1. DEET: Use insect repellent containing effective ingredients such as DEET or picaridin. You can verify the effectiveness of your insect repellent on the EPA’s website: https://www.epa.gov/insect-repellent

2. Drain: Drain standing water around your house to prevent mosquito breeding.

3. Dawn/Dusk: Mosquitoes are most active during dawn and dusk. Stay inside or take extra precautions to prevent mosquito bites during these times.

4. Dress: When possible, wear long sleeved shirts and pants to protect yourself from bites.

For more information about WNV protection, contact your local health department or visit the DPHHS WNV website page.


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