West Nile Virus


What is West Nile Virus?

West Nile Virus (WNV) is a virus that may cause encephalitis (inflammation of the brain) in humans, horses, and several other species of animals. WNV was first discovered in 1937 from the West Nile District of Uganda. The virus is transmitted by Culex mosquitoes that acquire it from infected birds. It was first seen in the United States (New York) in 1999.

The virus is present in the blood of infected birds, and when mosquitoes feed on infected birds they ingest the virus. Horses become infected with West Nile Virus after being bitten by an infected mosquito. Horses cannot transmit the West Nile Virus to other horses, birds, or people.

transmission cycle image

The virus infects the central nervous system and causes symptoms of encephalitis. Infected horses may or may not show clinical signs. Clinical symptoms include loss of appetite, depression, lethargy/drowsiness, mild low-grade fever, quivering muscles, ataxia, weakness of limbs (especially hind legs), or recumbency (inability to stand).

There is no specific treatment. Supportive veterinary care including IV fluids and anti-inflammatories is recommended.

Reduce the amount of standing water available for mosquito breeding:

  • Dispose of water-holding containers on your property.
  • Remove all discarded tires.
  • Drill holes in the bottom of recycling containers left outdoors.
  • Clean clogged roof gutters.
  • Turn over plastic wading pools when not in use.
  • Turn over wheelbarrows and don't let water stagnate in birdbaths.
  • Aerate ornamental pools or stock them with fish.
  • Clean and chlorinate swimming pools when not in use.
  • Use landscaping to eliminate standing water that collects on your property.

There are some easy steps you can take to prevent mosquitoes from affecting your horses:

  • House horses indoors during peak periods of mosquito activity (dusk and dawn).
  • Avoid turning on lights inside the stable during the evening and overnight.
  • Place incandescent bulbs around the perimeter of the stable to attract mosquitoes away from the horses.
  • Remove all birds, including chickens, that are in, or close to, the stable.
  • Topical preparations containing mosquito repellants are available for horses. Read the product label before using and follow all instructions.
  • Fogging of stable premises can be done in the evening to reduce mosquitoes; read directions carefully before using.

image of mosquito

Yes, there is an approved vaccine for horses. It must be purchased from a veterinarian. There is no specific vaccine or treatment for West Nile virus infection in humans.

Current AAEP guidelines for the different Core Vaccinations are listed below for Adults and Foals.
**ALL Vaccination Programs should be developed in consultation with a licensed veterinarian

  • Adults
  • Foals
  • Recommendations for Re-vaccination
    • Horses should receive a booster three to four weeks prior to vector season; and
    • If the vector season exceeds four months and viral load is high, they should probably receive a second booster.*
*No one knows for sure how long immunity lasts. Current thoughts are not past four to six months. Age, immune status, and residing in an endemic area may influence revaccination intervals.

It is recommended you contact your veterinary practitioner for more information on vaccination and testing.

The IgM Capture ELISA (equine, avian, canine, ovine, caprine)
Detects acute infection in animals six days post-infection and will continue to detect infection for two months. After two months infected animals will be IGM negative. Vaccinated animals will not test positive. IgM ELISA has some utility for detecting recent natural exposure and is useful in supporting a clinical diagnosis.

The IgG Capture ELISA
This ELISA detects infection in horses from approximately two weeks post infection and remains positive for one to two years and possibly longer. Vaccinated animals will test positive. IgG ELISA may be useful for evaluating immune status.

Real-Time PCR (RT-PCR)
RT-PCR can be attempted on Cerebrospinal Fluid (CSF) of clinically affected horses. It is best to contact your Veterinary Diagnostic Laboratory prior to acquiring this sample to receive specific guidance.

Diagnostic Testing for West Nile Virus available at:
Veterinary Diagnostic Center
4040 East Campus Loop North
University of Nebraska (UNL)
Lincoln, NE 68583-0907
Phone: 402-472-1434
Web site: http://vdc.unl.edu/