West Nile Virus
The most recent West Nile virus postive mosquitoes found in Montana were from two Culex tarsalis mosquito pools collected from the Medicine Lake area in Sheridan County. These were confirmed WNV positve on September 21 by the Montana Public Health Laboratory. As of October 4th, 2010, no human or equine cases were detected MT.
West Nile virus (WNV) can potentially cause serious illness in humans and is most often transmitted through the bite of an infected mosquito. West Nile virus first appeared in North America in 1999 causing encephalitis in humans and horses. Since then it has spread throughout the contiguous United States and typically causes illness during the summer and fall months.
WNV in Montana
Mosquitoes belonging to members of the Culex species are the most common vectors of WNV in the North America. In MT, Culex tarsalis is the most common WNV mosquito vector.
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. Since then, 507 human cases of WNV have been confirmed in MT. The highest rates of disease occurred in 2003 and 2007. No human WNV cases were reported for 2010.
In MT, cases of equine WNV generally precede cases of human WNV. Equine WNV has been confirmed each year from 2002 to 2009 in MT. Numbers of cases ranged from as few as 1 in 2002 to 191 in 2003. No equine WNV cases have been reported for 2010.
Click on the year of interest below to learn more about WNV cases and distribution in MT.
Year |
|||||||||||
Number of cases |
2002 |
2003 |
2004 |
2005 |
2006 |
2007 |
2008 |
2009 |
2010 |
||
Humans(deaths) |
2(0) |
228(4) |
6(0) |
26(0) |
34(0) |
202(5) |
5(0) |
5(0) |
0(0) |
||
Horses |
1 |
191 |
11 |
3 |
24 |
36 |
7 |
14 |
0 |
||
Average rates of human WNV infection by county 2002 - 2009
Average number of human WNV infections by county 2002 - 2009
How is WNV maintained in nature?
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.
What happens when humans become infected with WNV?
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.
WNV in horses
Cases of WNV disease in horses have been documented both by virus isolation and by detection of WNV antibodies in the blood. Approximately 40% of equine WNV cases results in the death of the horse. Horses most likely become infected with WNV in the same way humans become infected, by the bite of infectious mosquitoes.
In locations where WNV is circulating, horses should be protected from mosquito bites as much as possible. Equine vaccines that protect against eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE) do NOT protect horses against WNV infection. A WNV specific vaccine is available and recommended for horses that reside in WNV endemic areas. Data suggest that most horses recover from a WNV infection. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent.
WNV control and prevention
The easiest and best way to avoid WNV is to prevent mosquito bites.
- When you are outdoors, use insect repellent containing an EPA-registered active ingredient. Follow the directions on the package.
- Many mosquitoes are most active at dusk and dawn. Be sure to use insect repellent and wear long sleeves and pants at these times or consider staying indoors during these hours.
- Make sure you have good screens on your windows and doors to keep mosquitoes out.
- Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.
