West Nile Virus - a Threat to Horses

West Nile virus (WNV) is the leading cause of arbovirus encephalitis in horses and humans in the United States.
West Nile Virus - a Threat to Horses - Articles


West Nile Virus Signs In Horses

  • The most common signs of WNV infection in horses include stumbling, in-coordination, weak limbs, partial paralysis, muscle twitching and in some cases, death. Fever has occurred in less than one fourth of all confirmed equine cases.
  • Each horse may exhibit a combination of symptoms or not exhibit any symptoms.
  • Once a horse has been bitten, it may take only 5 to 15 days for signs of West Nile virus to appear. Horse to horse transmission does not occur. The virus is most prevalent from May to October when mosquitoes are most abundant.


  • Treatment is vital for any horse with WNV. Since there is not any specific antibody to counter attack the virus, it is important to consult your veterinarian and provide supportive therapy. Depending upon the affect the virus has on each individual horse will determine if home or clinical care is warranted. Each animal is assessed according to it's age and health and all treatments should be under the direction of a veterinarian.
  • Recovery times depend upon the health and age of the affected horse. Many horses will improve within 5 to 7 days of displaying clinical signs, however about 20-30% can exhibit severe neurological deficits for several weeks.
  • In cases of WNV, 33% will die, 50% will fully recover and 17% will have relapse or incomplete recovery.
  • Treatment includes treating a fever if present.
  • Ensure horse receives sufficient fluids, possibly through intravenous treatment if the horse is unable to drink on its own. Oral or intravenous feeding may also be necessary for horses unwilling to eat.
  • For horses unable to rise slinging is recommended 2 to 3 times per day to aid in circulation and to try to prevent pressure point sores. (bed sores).
  • Head and leg protection is also frequently needed.
  • Horses with WNV sometimes develop other problems because they were weakened by WNV. Joint and tendon infections, sheath infections, pneumonia, and diarrhea can all occur as secondary events.


  • Horse owners should consult their veterinarians regarding vaccination. The vaccine shots are of no value if they aren't given prior to exposure to the disease. If the horse develops WNV it is too late for the shot.
  • The vaccines require two doses, administered three to six weeks apart, and full protection doesn't develop until four to six weeks after the second dose. Sometimes a third does is recommended. Boosters are recommended, but recommendations vary and depend upon mosquito infestation where you live. It can take from 7 to 12 weeks for the horse to develop maximum resistance to infection.
  • To eliminate mosquito breeding habitats: Eliminate any unnecessary standing water on your property (tires, wheelbarrows, old buckets, etc.), make certain roof gutters drain properly and remove any standing water, especially from flat roofs, keep swimming pools clean and free of water on covers.
  • Stable horses inside during active mosquito feeding times (dawn/dusk), utilize fans, barrier cloths, screens, flysheets, repellent sprays (permethrin, DEET), and insecticide misting systems, turn off lights that attract mosquitoes at night, or use fluorescent lights, which do not attract mosquitoes.


  • West Nile Virus (WNV) is a mosquito-borne virus that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord) in humans and horses. Mosquitoes acquire the virus from infected birds and transmit it.
  • Since 1999, over 25,000 cases of WNV encephalitis have been reported in U.S. horses. Horses represent 96.9% of all reported non-human mammalian cases of WNV disease.
  • In 2000, West Nile virus appeared for the first time in Pennsylvania in birds, mosquitoes and a horse.
  • To combat the spread of West Nile virus, which is transmitted by mosquitoes, Pennsylvania has developed a comprehensive network. This network, which covers 40 counties, includes trapping mosquitoes, collecting dead birds and monitoring horses, people and, in past years, sentinel chickens.

Mosquito Control

State DEP and county mosquito control professionals have been using Bti, a naturally occurring bacteria, to kill mosquito larvae for years.

  • This material is now becoming widely available for you to buy and use yourself at home.
  • Bti can be purchased in small, donut-shaped form, often called "mosquito dunks", which are useful in small areas of standing water, such as a birdbath or small puddle of water that may gather in a low spot on your property. A granular form of Bti is available, and effective for larger areas, such as backyard ponds.
  • Bti can be purchased in many lawn and garden, outdoor supply, and home improvement stores. The great thing about this bacteria is that it kills only mosquito and black fly larvae. It is not harmful to people, pets, aquatic life (such as fish) or plants.
  • The best way to control mosquitoes is still to get rid of standing water on your property. And, make sure you follow all label instructions carefully if you use Bti at home.

American Association of Equine Practitioners information on West Nile

  • This virus has been identified in all of the continental United States, most of Canada and Mexico. Several Central and South American countries have also identified WNV within their borders.
  • The virus is transmitted from avian reservoir hosts by mosquitoes (and infrequently by other bloodsucking insects) to horses, humans and a number of other mammals.
  • Horses and humans are considered to be dead-end hosts for WNV; the virus is not directly contagious from horse to horse or horse to human.
  • 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.