Montana Department of Livestock

Here to serve the people of Montana and its livestock industry.

Animal Health Bureau

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Department of Livestock
Animal Health Bureau
PO Box 202001
Helena, MT 59620-2001
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West Nile Virus



Disease Summary

West Nile Virus (WNV) is a viral infection that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). WNV is spread by the bite of an infected mosquito (vector). Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite. WNV primarily affects birds, but can also infect bats, horses, cats, dogs, chipmunks, skunks, squirrels, domestic rabbits, alligators, and humans.

Historically in Montana, horses testing positive for WNV who were also showing clinical signs had not been vaccinated. The WNV vaccines available for horses have shown to be effective against the virus, protecting animals from the development of clinical signs. For this reason, the American Association of Equine Practitioners (AAEP) recommends administration of a WNV vaccine as part of their core vaccination guidelines.

Positive WNV cases are also reported to the Department of Public Health and Human Services (DPHHS) to support identification of positive mosquito pools within Montana and to help with messaging for WNV prevention in humans. While direct transmission of WNV between animals and people does not occur, a veterinarian’s diagnosis of WNV in an animal still warrants client education about the zoonotic potential of the virus. An infected mosquito is the route of transmission for both animals and humans, so a positive case of WNV in a horse, cat, dog, bird, or rabbit, suggests that the animal owner may have been exposed to the same infected mosquito population.


2023 Positive West Nile Cases 

MT LIV Animal Health WNV Positive Cases
Month County Species Vaccine History Outcome
August Teton Equine None Euthanasia
August Hill Equine None Euthanasia
August Rosebud Equine None Euthanasia
August Custer Equine None Euthanasia
August Park* Equine None Euthanasia
August Carbon Equine None Recovered
August Cascade Equine None Recovered
August Teton Equine None Recovered
August Glacier Equine None Recovered
August Carter Equine None Recovered
August Carbon Equine None Recovered
August Carbon Equine None Recovered
August Lake Equine Yes - 45 days prior Recovered
September Powder River Equine None Recovered
September Fegus Equine None Euthanized
September Treasure Equine Yes, first vax but no booster Recovered
September Beaverhead Equine None Recovered
September Yellowstone Equine None Recovered
September Yellowstone Equine None Recovered
September Lake Equine None Euthanized
September Park Equine None Recovered
September Broadwater Equine None Recovered
September Cascade Equine None Recovered

* Presumptive positive


The map below represents WNV positive equine, human, and mosquito pools for FY23 (7/1/2022 – 6/30/2023)

FY2023 WNV


Historical West Nile Data

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