West Nile Virus in Horses

Horses October 14, 2014 Print Friendly and PDF

West Nile Virus (WNV) is a mosquito-born 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 that acquire it from infected birds transmit the virus. This virus was first discovered in the United States in New York in 1999 and has quickly spread throughout the U.S. The virus was found in dead birds as far west as Missouri and eastern Iowa in the fall 2001. Previously, West Nile Virus had only been seen in Africa, Asia and southern Europe

Transmission: The virus is transmitted when a mosquito acquires it from an infected bird, and then the mosquito feeds on a horse, human or other mammal. Many birds can be infected with WNV and not exhibit clinic signs, but crows and blue jays are most likely to die from infection. The reason why these two species die of WNV is unknown. Horses and people are a “dead end host” meaning there is no horse-to-horse, horse-to-person or person-to-person transmission of the virus. In other words, a horse cannot infect another horse and a horse cannot infect a person. A bite from an infected mosquito will not always make a person or animal sick, most people with WNV either have no symptoms or experience mild illness. Incubation period for humans is 3 to 15 days. All people in areas where WNV activity has been identified are at risk, however, persons older than 50 years have been shown to have the highest risk of severe disease. Most human infections are mild and symptoms include fever, headache, and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and rarely death.

Symptoms in Horses: West Nile virus causes encephalitis – or swelling of the brain and spinal cord. Once a horse has been bitten, it may take 5 to 15 days for the signs of West Nile virus to appear Horses that become clinically ill, can suffer loss of appetite, depression, weakness of the hind limbs, partial paralysis, fever, impaired vision, ataxia, head pressing, aimless wandering, convulsion, inability to swallow, circling, hyper excitability, coma and death. Many of these symptoms are similar to horses with rabies, Equine Protozoal Meylities (EPM) equine encephalitis and other serious neurological diseases. If symptoms are present, a veterinarian should be contacted immediately.

Prevention – Vaccination: By the end of 2001, there were a reported 149 human cases in 9 states with 18 fatal. There were 731 equine cases from 20 states with a 25% mortality rate. Florida alone reported 492 equine cases. In 2001, a vaccine was conditionally approved for horses, which must be administered by a veterinarian. The initial vaccine is a two injection series given 3 weeks apart. Both injections must be given to provide protection from WNV. Other equine “encephalitic” diseases (sleeping sickness: eastern equine encephalitis, western equine encephalitis, and Venezuelan equine encephalitis) belong to another family of viruses for which there is not cross-protection. Therefore horses should be vaccinated specifically for WNV. There is no vaccine for humans.

There is no specific treatment for WNV other than supportive veterinary care standard for animal infected with a viral agent. Data suggest that most horses do recover from the infection.

Prevention – Mosquito control: Limiting exposure to mosquitoes is fundamental in helping to prevent the spread of WNV. Reducing the amount of standing water available for mosquito breeding is a key element in mosquito control. Other mosquito control guidelines include:

  • Dispose of tin cans, plastic containers, ceramic pots or similar water holding containers.
  • 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.

Listed below are steps that can be taken to prevent mosquitos from affecting horses:

  • House horses indoors during peak period 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
  • Eliminate areas of standing water.
  • 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.

Dead bird submission: As part of Nebraska’s surveillance program for WNV, the Nebraska Department of Health and Human Services (NDHHS) and the Nebraska Department of Agriculture are monitoring WNV infections in crow, magpies, blue jays and raptor (hawks and owls). In collaboration with the University of Nebraska Veterinary Diagnostic Center as the WNV testing center, any dead bird that shows now sign of injury or reason for death should be treated as suspect of WNV infection and should be submitted for testing. Bird deaths have preceded outbreaks of this disease in other cities where WNV infection has been diagnosed. The toll free number for dead bird submission and WNV information is 877/220-1237. Birds must be in good condition and kept cold. If a dead crow, blue jay, magpie, or raptor is found, rubber gloves should be worn when placing the bird in a plastic bag. Tie the bag shut and then place inside a second bag and tie shut. Contact your local health department or the phone number listed above for instructions regarding the collection and shipment of animal carcasses for evaluation.

Web sites for more information:

Nebraska Department of Health and Human Services (NDHHS) US Department of Agriculture, APHIS CDC (Center for Disease Control and Prevention

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.