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Conservation > Fish & Wildlife Diseases West Nile Virus

For questions/concerns about this disease in humans, please call your doctor or the Montana Department of Public Health and Human Services  (DPHHS).


For questions about this disease/parasite in wildlife, please call the FWP Wildlife Health Lab at 406-577-7882.


West Nile virus (WNV) is a mosquito-borne virus (family Flaviviridae) that can cause encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord) (Michigan, 2010).


WNV was first isolated in 1937 in the West Nile district of Uganda, Africa. Outbreaks of WNV have occurred in several countries including Egypt, Israel, South Africa, and countries in parts of Asia and Europe. Recent outbreaks outside of North America have occurred in France and Israel in 2000, Russia in 1999, and Romania in 1996-97. WNV was first detected in the United States in September 1999 during the investigation of an outbreak of encephalitis in humans in New York City and has since spread west across the U.S. to the Pacific Ocean.

The first case of WNV in Montana was in a horse in July 2002. Shortly after the horse case, Montana had its first human case of the disease. 

Species affected

All North American bird species that have been experimentally challenged are susceptible to infection with WNV (none is immune). WNV infection has been identified in over 170 other avian species across North America (Michigan, 2010), and has caused death in many bird species including grebes, bald eagles, great horned owls, red tailed hawks, goshawks, sage grouse, house sparrows and house finches. Corvids tend to be most susceptible and have the highest mortaility rates (University of Pennsylvania). 

Horses and humans appear more likely to develop clinical illness than other mammalian species. Wild mammals in which the virus has been dected in the absense of clinical disease include white-tailed deer, bears, foxes, mice, rats, opossums, racoons, caribou, chipmunks, squirrels, skunks, and wolves (University of Pennsylvania). 


WNV is primarily spread between birds through the bite of an infected mosquito. When the level of virus transmission among birds and mosquitoes becomes high, horses and humans can be incidentally infected, also through the bite of a mosquito. Mosquitoes become infected when they bite, or take a blood meal from a bird carrying WNV. The infected mosquito then transmits the virus to another host through a second bite. Horses and humans are considered dead-end hosts because they do not produce enough virus in their blood to infect biting mosquitoes. There is no evidence that infected horses, people, or other mammals can transmit the virus through normal contact (Western, 2009; Michigan, 2010).

Research has shown that it is possible for bird-to-bird transmission to occur as a result of oral and cloacal secretions contaminating water and food. Research has also shown that the virus may be passed to raptors by eating infected birds. However, the most likely route of transmission is still through a mosquito bite (Michigan, 2010).

Migrating birds carrying WNV in their blood have had a significant impact on the spread of WNV across the U.S. Because corvids have proven to be especially susceptible to WNV they have been the focus of WNV surveillance efforts across the U.S. and Canada.

In 2002, the CDC confirmed additional routes of transmission which include infection through transplanted organs, transplacental (mother-to-child) infection, transmission through breast milk, and blood transfusion-associated transmission. Blood banks now screen donor blood units for WNV (Michigan, 2010).


Affected birds are often found dead without symptoms being noticed. Most corvids infected with WNV die within 3 weeks of infection. Clinical signs prior to death may include uncoordinated walking, weakness, lethargy, tremors, and abnormal head posture. Clinical signs and pathologic findings of WNV have not been thoroughly described in most mammalian species. In cases where a description is given, both clinical illness and pathology involve the nervous system. Therefore, it seems that WNV associated disease in both horses and other mammals is primarily a neurological disease (Michigan, 2010).

Most infected humans will not develop symptoms; however, a small proportion of infected people develop fever and other flu-like symptoms. Fewer than 1% of infected people will develop a severe, potentially fatal neurologic disease (DPHHS).

Public Health Concerns

Most infected humans will not develop symptoms; however, a small proportion of infected people will develop fever and other flu-like symptoms. Fewer than 1% of infected people will develop a severe, potentially fatal neurologic disease (DPHHS).

People over the age of 50 are found to be at the highest risk of developing severe disease. In addition, immune-compromised persons or people with underlying health conditions have an increased risk of developing more severe forms of WNV disease (Michigan, 2010). Case fatality rates in the U.S. have been approximately 7% (Western, 2009).

Since 2002, documented cases of WNV have occured in humans in Montana every year except 2010. The number of human cases varies by year, with 2003 and 2007 having the highest total number of human cases to date (DPHHS). 

Reducing exposure to mosquitoes is the best method of control. This can be accomplished by reducing time outdoors at dusk and dawn, wearing long sleeves and pants when outdoors, applying insect repellent containing DEET (follow manufacturer's directions), and draining standing water where mosquitoes may breed (Michigan, 2010).


Michigan Wildlife Disease Manual. 2010. Michigan Department of Natural Resources. Website.

Western Wildlife Disease Workshop Notebook. 2009. Southeastern Cooperative Wildlife Disease Study. College of Veterinary Medicine, University of Georgia. Website.