West Nile virus, what you need to know

  • Published
  • By Malmstrom Public Health

West Nile virus was initially detected in North America in 1999, and has since spread across the continental United States and Canada. So far in 2016, a total of 301 cases of WNV have been reported across 43 states. The McCone County Health Department recently reported Montana’s first human death from WNV for 2016. This case brings the total number of WNV cases reported in Montana to six for 2016. Previously, the state of Montana has reported five cases of human infection – four in Dawson County and one in Garfield County. Four Montana counties have reported positive mosquito specimens including Lake, Prairie, Phillips and Valley counties. In addition, there have also been two reported equine cases in Petroleum and Dawson counties. In previous years in Montana, human infection cases have been as high as 415 in 2003 and 238 cases in 2007; however, in 2010 and 2011 there were very few human cases reported. Since the last spike in 2007, Montana has seen a steady decline with an average of only 16 cases reported annually.


The West Nile virus is most commonly transmitted to humans by mosquitoes, but additional routes of human infection have also been documented. These additional routes represent only a small proportion of total cases but include blood transfusions, organ transplants, exposure in laboratory settings, and transmissions from mother to baby during pregnancy, delivery and breastfeeding. WNV is not transmitted from person-to-person or from animal-to-person through casual contact, from handling live or dead infected birds, or through consuming infected birds or animals. The mosquito most commonly transmitting WNV in Montana is the Culex tarsalis species, although there have been 65 documented species to have transmitted WNV in the U.S. from 1999-2012. This species of mosquito is predominantly active from evening to morning, so it is wise to take precautions during these time periods.

West Nile virus illness occurs in only a small percentage, 20-30 percent, of those people who are infected. Individuals who have the illness typically develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea or rash. Most people with this type of West Nile virus disease recover completely, but fatigue and weakness can last for weeks or months. Less than 1 percent of people who are infected will develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues). There are no vaccines or specific antiviral treatments for WNV infection available. Over-the-counter pain relievers can be used to reduce fever and alleviate some symptoms. In severe cases, patients may need to be hospitalized to receive supportive care including intravenous fluids, pain medication and nursing assistance.

West Nile virus poses no significant risk with proper use of countermeasures to prevent mosquito bites. The primary method of prevention is using repellents containing DEET. To optimize safety and effectiveness, repellents should be used according to the label instructions. Other methods include wearing long sleeves, long pants and socks when outdoors, and taking extra care during peak mosquito biting hours (evening to morning). Staying in air-conditioned quarters with screened windows and doors, using mosquito bed nets if sleeping outdoors or in quarters without screened windows and doors, and reducing mosquito breeding areas by emptying outdoor containers that hold standing water can also help prevent mosquito bites.

The 341st Medical Group would like to remind all travelers visiting areas outside of the U.S. that Public Health is available to assist with health threat and vaccination recommendations based on the country or countries being visited.  Individuals can also visit the Center for Disease Control and Prevention website (www.cdc.gov) and its traveler’s health section to find country specific health threat information.  For questions regarding West Nile virus or other potential health threats associated with a travel location, contact Public Health at 731-4405.