West Nile Virus


West Nile virus (WNV) is a mosquito-borne disease that was first isolated in 1937 from the blood of a woman in the West Nile district of Uganda. Since then, the WNV has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East. The WNV first appeared in the United States in 1999 in New York City and has continued to spread throughout the United States. 

West Nile virus belongs to a group of single-stranded RNA viruses called Flaviviridae whose members include Japanese encephalitis virus, St. Louis encephalitis virus, yellow fever virus and dengue fever virus. St. Louis encephalitis is not new to the United States. It first appeared in St. Louis, Missouri, in 1933, and is the most common mosquito-transmitted disease in the United States. 

Clinical outcomes in people infected with WNV depend upon the level of background immunity in the geographic location. For example, in Egypt the WNV infection is common in children, in which it causes a mild fever illness. Following infection, antibodies and "memory" white blood cells (T-lymphocytes) remain in the body to provide future protection from the virus. It is assumed that immunity will be lifelong; however, it may wane in later years. There is no scientific evidence indicating that people can be chronically infected with WNV.


Since it's discovery in New York City in 1999 -- West Nile Virus has established itself in the Western Hemisphere. 

In temperate climates (latitudes 23.5° and 66.5°) West Nile Virus transmission occurs primarily during the summer and early fall when mosquito activity is the highest. In southern climates, WNV can be transmitted year round. The virus has been documented to survive cold winters in northern regions to appear again the following year. How the virus survives the cold is not completely understood. 

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Montana's first case of West Nile virus in a horse was reported in Shepherd in August 2002.

States with Equine Cases


The first case of West Nile virus contracted by a person in Montana was confirmed in Yellowstone County, Fall 2002. The Billings resident's onset of illness was in mid-September. The resident required brief hospitalization for meningitis, but made a full recovery. 

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West Nile virus (WNV) persists in nature in a complex cycle of mosquito vectors and natural bird reservoir hosts. The virus has been detected in 43 species of mosquitoes and - at present - 138 species of North American birds. It is expected that the number of mosquito and bird species carrying the virus will increase as the virus spreads to new ecological habitats. Most birds infected with the virus survive and develop an immunity. However, for some birds the virus is deadly (crows, jays, hawks).

Both the mosquito (vector) and the bird (reservoir) are necessary to maintain the virus in nature. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit WNV to birds, humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.

Humans and animals that become infected are considered to be "dead-end hosts" in the transmission cycle. Neither is thought to become viremic enough to pass along sufficient virus to a bloodsucking mosquito.


Horses are affected by WNV much more often than any other domestic animals. It is important to remember that humans cannot contract the disease from horses and WNV is not a contagious disease from horse to horse, so no quarantine is not necessary for infected premises or animals.


Humans acquire the disease the same way horses do - through a mosquito bite of an infected mosquito. There is no evidence that a person can get the virus from handling live or dead infected animals. However, avoid bare-handed contact when handling any dead animals and birds. Use gloves or double plastic bags to place the carcass in a garbage can.

There is no evidence of transmission of the disease through eating meat of an animal infected with the virus.

The CDC has a web site that addresses issues of contracting WNV through blood transfusions, organ transplants, and breast milk. For more information on this topic, please visit the CDC web site at: http://www.cdc.gov/ncidod/dvbid/westnile/qa/transmission.htm


What about other Livestock?

The vast majority of infections have been identified in birds. The CDC reports that WNV has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The USDA reports that other livestock and poultry do not commonly show any illness if infected with WNV. Again, animals CANNOT transmit WNV to people.

Signs & Symptoms


  1. Horses are affected by West Nile virus much more often than any other domestic animal.
  2. The percent of infected horses that go on to develop symptoms and illness is not known.
  3. Of those horses that do present clinical signs, about one-third die or need to be euthanized. The clinical signs for horses include weakness or paralysis of hind limbs, muzzle twitching, impaired vision, incoordination, head pressing, aimless wandering, convulsions, inability to swallow, circling, hyperexcitability, or coma.
  4. The incubation period for horses is 5 to 15 days.


  1. About 80 percent of the infections are mild without any symptoms.
  2. About 20 percent of the infected people develop a mild flu-line illness. This is referred to as West Nile Fever. West Nile Fever is a mild human illness caused by the virus and includes flu-like symptoms such as fever, severe headache, sore throat, backache, mylagia, fatigue, stiffness of the neck, nausea, abdominal pain, diarrhea, rash. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.
  3. Less than 1 percent of West Nile virus infections lead to more serious illnesses - encephalitis, meningitis, or meningoencephalitis. These are more severe human illnesses caused by the virus. People with these illnesses often present with symptoms of fever, headache, stiff neck, mental confusion, disorientation, tremors, convulsions, muscle weakness, paralysis, and - in about 15 percent of these cases - coma.
  4. The incubation period for humans is 3 to 14 days.



A West Nile Virus vaccine was granted conditional license in 2001 from the USDA for use on horses. The vaccine is a killed virus product. The vaccine is available only through veterinarians; therefore, you need to contact your veterinarian to find out more about its use and availability in your area.

Dosage = Initially horses require two doses of the vaccine, administered three to six weeks apart (1 milliliter each dose, given intramuscular). The manufacturer (Fort Dodge Animal Health) recommends an annual booster. The booster should be given just prior to the start of the mosquito season in your area.

West Nile virus is not a contagious disease from horse to horse, so quarantine is not necessary for infected premises or animals.


Currently, there is no vaccine against West Nile encephalitis for humans. However, according to the CDC, "several companies are working towards developing a vaccine." In the meantime, preventive measures are recommended. See "prevention" section.



This site mentions the importance of protecting horses with the vaccine. Since West Nile virus is transmitted via mosquitoes - another key element to controlling future outbreaks of WNV among horses is to control mosquito populations.

  • Screened housing 
    Well-maintained insect screening for animal housing can be useful to reduce exposure to adult mosquitoes. However, it is important to first make sure mosquito populations inside the structure have been eliminated. Otherwise, such structures could lead to exposure. Mosquito adulticides can aid in this effort. Fans may be useful in reducing the potential of mosquitoes landing on horses.
  • Insect repellents 
    The use of insect repellents can play a part in decreasing exposure, but should not be used as the sole preventive measure. Insect repellents last for a limited amount of time and it is difficult to ensure complete coverage of the entire animal. Topical application of a product containing synthetic pyrethroid compounds (permethrin) may offer the best combination of safety and efficacy.
  • Outdoor exposure 
    Although some species of mosquitoes feed at dusk or dawn, others are daytime feeders or feed at any time of the day or night. As it is not yet clear which mosquitoes are responsible for the transmission of WNV to horses and other mammalian species, making recommendations as to when certain animals should avoid outdoor exposure may not be particularly useful at this time. However, a recently completed epidemiological study of WNV suggests that keeping horses in stalls at night may be helpful in reducing their risk of infection.


  • Screened housing 
    Installing and repairing window screens to prevent mosquitoes from entering the indoors is a good idea.
  • Insect repellents 
    Mosquitoes are attracted to a person's breath and chemicals released by a person's skin (perspiration, body odor, carbon dioxide, and lactic acid). The chemical DEET works by disrupting the ability of mosquitoes' antennae to detect the source of carbon dioxide). The higher the percentage of DEET in a repellent doesn't mean it provides a better barrier, it means it will last longer. Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product.
  • Clothing
    Wearing light-colored, long-sleeved shirts and long pants along with insect repellent can increase protection. Spray clothing with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing. Do not apply repellents containing permethrin directly to exposed skin. If you spray your clothing, there is no need to spray repellent containing DEET on the skin under your clothing
  • Outdoor exposure 
    Limiting outdoor exposure when mosquito activity is the highest in your area is a good idea. Typically, mosquito activity is the highest during dawn and dusk.

Controlling Mosquito Populations - Humans and Horses

Remove all man-made potential sources of stagnant water in which mosquitoes might breed. Mosquitoes can breed in any puddle that lasts for more than four days.

  • Dispose of any water-holding containers, including discarded tires, recycle bins, empty barrels, etc.
  • Drill holes in the bottom of containers that are left outdoors to allow water to drain.
  • Clean clogged roof gutters annually.
  • Turn over plastic wading pools or wheelbarrows when not in use and do not allow water to stagnate in bird baths.
  • Clean and chlorinate swimming pools that are not in use and be aware that mosquitoes can breed in the water that collects on swimming pool covers.
  • Aerate ornamental pools and use landscaping to eliminate standing water that collects on your property.
  • Thoroughly clean livestock watering troughs monthly.
  • Local mosquito control authorities may be able to help in assessing the mosquito breeding risks associated with a specific property.


The following resources were used in the development of this web site and may serve as additional information for you.

Prevention and Control of West Nile Virus Infection in Equine and Other Livestock or Poultry September 2002

CDC, division of vector-borne infectious diseases, West Nile virus questions and answers, November 2002.

CDC, Office of Communication, WNV current case count, November 2002

West Nile Virus, Spread of the Mosquitoborne Illness, Nurse Week, November 4, 2002, pages 22-23.

West Nile Virus Fact Sheet, Montana Department of Livestock, August 2002, p)

Montana West Nile Virus Surveillance Project, Montana Department of Public Health and Human Services