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West Nile Virus

What is West Nile Virus?

West Nile is a virus that causes encephalitis in humans, horses, and several other species of animals. The virus is transmitted by mosquitoes that acquire it from infected birds. It was first seen in the U.S. in 1999.
How can a horse get West Nile 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.
West Nile Virus Transmission Cycle
transmission cycle image


What are the signs and symptoms of West Nile encephalitis in horses?
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, fever, quivering muscles, ataxia, weakness of limbs (especially hind legs), or recumbency.
Is there a specific treatment for West Nile encephalitis in horses?
No specific treatment. Supportive veterinary care including IV fluids and anti-inflammatories is recommended.
How can I reduce the number of mosquitoes around my home?
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.
How can I prevent mosquitoes from affecting my horses?
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

Is there a vaccine for West Nile Virus?
Yes, there is an approved vaccine for horses. It must be purchased from a veterinarian. There is no vaccine for humans.
What is the recommendation for vaccinating my horse?
Primary Vaccination Series

Adults
  • Primary vaccination series requires two doses of vaccine, three to six weeks apart, and it takes approximately four weeks after the second vaccination for optimal immunity.
Foals
  • Two doses of vaccine are required in the primary series.
  • If mare was previously vaccinated, first dose at three months and second dose three to six weeks later.
  • If mare was not vaccinated, first dose at two months and the second dose three to six weeks later.
  • Many veterinarians are currently recommending a third dose in all foals, similar to recommendations for Eastern and Western Equine Encephalitis.
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.
It is recommended you contact your veterinary practitioner for more information on vaccination and testing.
What tests are available for testing horses and other animals for West Nile Virus?
The IgM Capture ELISA (equine, avian, canine, ovine).
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 nottest 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.
What do we know about the 2002 outbreak in Nebraska?
  • There were 1,100 positive cases of West Nile Virus in horses.
  • Horses were infected in every county.
  • Range in age from 6 weeks to 34 years.
  • Median age was 7 years.
  • 53.7% of infected horses were male.
  • Overall fatality rate of 30%.
  • Fatality rate in recumbant horses > 60%.
What about WNV in other species in Nebraska?
  • Rocky Mountain goats and a sheep have also been confirmed with the virus in Nebraska.
  • Several dogs and cattle with clinical signs consistent with WNV have shown titers on serological tests.

Diagnostic Testing for West Nile Virus available at:
Veterinary Diagnostic Center
Fair Street and Campus Loop
University of Nebraska (UNL)
Lincoln, NE 68583-0907
Phone: 402-471-1434
Web site: http://vdc.unl.edu/



Additional Web Sites:

AAEP Guidelines for Vaccination
Centers for Disease Control and Prevention
Nebraska Health and Human Services
United States Department of Agriculture
West Nile Virus Guidelines for Horse Owners – University of Nebraska Extension