West Nile Virus

WNV in Montana

Mosquitoes belonging to members of the Culex species are the most common vectors of WNV in North America. In MT, Culex tarsalis is the most common WNV mosquito vector. Please remember that while WNV is a concern in Montana, Zika virus is not. In fact, the mosquitos that carry Zika virus have not been found in Montana or neighboring states.

The first confirmed case of WNV in MT was detected in a horse in July, 2002, followed shortly by the first human case in August, 2002. The highest rates of disease occurred in 2003 and 2007 (see CDC WNV).


Click on the year of interest below to learn more about WNV cases and distribution in MT.
WNV cases and distribution in MT
 Year Humans Deaths Horses
2021 2 0 2
2020 1 0 1
2019 4 0 2
2018 51 1 50
2017 13 0 8
2016 7 2 7
2015 3 0 4
2014 5 0 3

What You Need to Know About WNV

West Nile Virus (WNV) is most commonly transmitted to humans by mosquitoes. You can reduce your risk of being infected with WNV by using insect repellent and wearing protective clothing to prevent mosquito bites. There are no medications to treat or vaccines to prevent WNV infection. Fortunately, most people infected with WNV will have no symptoms. About 1 in 5 people who are infected will develop a fever with other symptoms. Less than 1% of infected people develop a serious, sometimes fatal, neurological illness (see CDC WNV: Symptoms & Treatment).

Steps you can take to minimize the probability of infection include remembering the 4 D’s of West Nile Virus prevention:

  1. DAWN & DUSK - When possible, avoid spending time outside at dawn and dusk.
  2. DRESS- Wear shoes, socks, long pants, and a long-sleeved shirt when outdoors for long periods of time or when mosquitoes are most active. Clothing should be light colored and made of tightly woven materials to keep mosquitoes away from the skin.
  3. DRAIN - Reduce the amount of standing water in or near your property by draining and/or removing it. Mosquitoes may lay eggs in areas with standing water.
  4. DEET - For additional protection from mosquitoes, use an insect repellent containing DEET (N,N-diethyl-m-toluamide) or picaridin (KBR 3023). Other insect repellents such as oil of lemon eucalyptus and IR3535 are also registered by the EPA but may be less effective than products containing DEET. It is important to follow the product guidelines when using insect repellant.

Many people who become infected with West Nile virus experience no symptoms. Some individuals develop a mild illness, called West Nile fever, which may last for three to six days. Generally, no treatment is needed. Other individuals, fewer than 1 out of 150, may become severely infected and develop symptoms of encephalitis or meningitis. These symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito. There is no WNV vaccine available for humans.

In MT, cases of equine WNV generally precede cases of human WNV. Numbers of cases range from as few as 1 in 2002 to 191 in 2003. A WNV vaccine for horses is now available. Horses vaccinated against Eastern, Western, and Venezuelan equine encephalitis are not protected against infection with West Nile Virus. Horses represent 96.9% of all reported non-human mammalian cases of WNV disease. The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33%. Data have supported that 40% of horses that survive the acute illness caused by WNV still exhibit residual effects, such as gait and behavioral abnormalities, 6-months post-diagnosis. Thus vaccination for West Nile virus is recommended as a core vaccine and is an essential standard of care for all horses in North America.

In nature, the virus is amplified during periods of adult mosquito blood-feeding and continuous viral transmission between mosquito vectors and bird reservoir hosts. Infectious mosquitoes carry virus particles in their salivary glands and infect susceptible bird species during blood-meal feeding. Bird reservoirs generally remain viremic (virus circulating in the bloodstream) for 1 to 4 days after exposure. Following a WNV infection, surviving birds develop life-long immunity. A sufficient number of mosquitoes must feed on an infectious host (a bird that has virus circulating in the bloodstream) to ensure that some mosquito vectors survive long enough to feed again on a susceptible reservoir host. This feeding pattern assures the maintenance of the virus in nature. People, horses, and most other mammals are generally considered "dead-end" or incidental-hosts. This means that mammals are not known to develop infectious-level viremias very often, and therefore do not help maintain the WNV in nature.