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West Nile Virus in Montana


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On August 2, state officials confirmed the first case of West Nile virus (WNV) in Montana for 2006. A 2-year-old horse from Flathead County died of the disease July 22. It had not been vaccinated, according to Dr. Jeanne Rankin, assistant state veterinarian with the Montana Department of Livestock (DOL). The case marked the first evidence this summer that West Nile virus is still active in Montana.

WNV is carried primarily by birds but can be transmitted by mosquitoes to humans, horses, and some other animals. The first documented case of the disease in the United States occurred in New York in 1999, and it has since spread westward into nearly every state.

In Montana, the first confirmed case of the disease was found in a horse in 2002. Since then, the state has had a total of 262 confirmed cases of WNV in humans, resulting in four deaths, and 349 cases in horses, with 98 deaths. For more details, see 2002-2005 West Nile Cases.

In many states, the number of West Nile cases has gradually declined over time as people and animals develop immunity from infection. Whether Montana has seen the worst of WNV is difficult to predict, state health officials say. They continue to monitor cases closely and advise people to take precautions to avoid the disease.

Symptoms, Prevention, Treatment

Although WNV is a potentially serious illness, about 80 percent of those infected notice no symptoms and develop immunity. About 20 percent experience mild flu-like symptoms. About 1 in 150 people infected with WNV will develop severe illness, and about one in 1,000 cases is fatal. People over the age of 50 are at greatest risk of serious illness.

In Montana, victims of the virus have ranged in age from 3 to 91. However, fatalities have involved individuals aged 65 to 84.

For more information about WNV, including symptoms, prevention, and treatment, visit the U.S. Centers for Disease Control and Prevention.

West Nile Virus in Horses

WNV can impact horses more severely than humans, but equine vaccines are available. To learn more about preventing and treating WNV in horses, visit the Montana Department of Livestock.

Mosquito Control, Pesticides

If WNV surveillance activities reveal that the virus is present in your community, local officials may decide to spray pesticides to control infected mosquitoes. For more information about pesticides used in mosquito control, visit the U.S. Environmental Protection Agency or the National Pesticide Information Center.

Because pesticides would be sprayed at extremely low volumes, most people should not experience any adverse health effects. However, sensitive individuals and people with pre-existing medical conditions--including asthma, allergies and emphysema--may experience temporary skin and eye irritations, congested or runny nose, sore or scratchy throat, and moderate respiratory irritations. For these reasons, health officials recommend taking the following precautions if spraying should occur in your area:

  • If possible, stay indoors during spraying.
  • Have children stay indoors until 1-2 hours after spraying.
  • Bring pets indoors and cover fish ponds and beehives.
  • Close windows and turn off fans and air conditioners that are exposed to outdoor air.
  • Remove children's toys, outdoor equipment and clothes from outdoor areas.
  • If toys, play equipment or furniture are left outside, wash with soap and water before using them again.
  • If you must stay outdoors during spraying, avoid eye and skin contact with the spray. If contact should occur, rinse eyes with water or saline solution and wash skin and clothing with soap and water
  • Wash fruits and vegetables that are exposed to the spray before eating them.

If you believe you are experiencing symptoms associated with spraying, contact your health care provider.

WNV Education Materials

Page last updated 08/11/2006