West Nile Encephalomyelitis in the Horse

Filed Under: Horses, Diseases, Parasites

West Nile virus (WNV) is a flavivirus that was first identified in Africa during 1937. WNV is considered to be endemic in Africa, Asia, Southern Europe and North America. The virus first appeared in North America around the New York City area in 1999, when wild and zoo birds, horses and humans died of meningoencephalitis. WNV has been found in 225 different wild and captive bird species. Birds are the natural host and reservoir of this virus. The mosquito is the primary route of disease transmission; however, carnivores may occasionally be infected through the consumption of infected birds. Most outbreaks occur between July and October—the time of year when mosquito numbers are at their highest.

Birds tend to show neurologic clinical signs. The affected bird may be totally recumbent (unable to stand), have head tremors or show leg and/or wing paralysis. Affected birds are either reluctant or unable to move when disturbed. Affected birds will be uncoordinated and some will actually flip over while attempting to stand. Mortality may be as high as 60% to 100%.

Already there have been notable population decreases in many bird species. In some areas, entire populations of certain birds have disappeared. As of today, there is no approved vaccine for the use in birds. Some zoos and wildlife centers are using the Equine vaccine to protect their birds, but the efficacy of using Equine vaccine in birds is unknown.

Several mammal species have been found to be affected by WNV and they include cattle, bears, skunks, deer, rabbits, squirrel, horses and even man. In an unusual twist for a virus, reptile species have also been shown to be susceptible. Captive alligators have been infected by the virus.

The incubation period for WNV in the horse appears to be 3 to 15 days. Most horse infections are symptom free. If a horse exhibits clinical infection, they typically present with neurologic signs. Neurologic clinical signs consist of stumbling, ataxia (unbalanced), blindness, intermittent lameness; they may be recumbent (unable to stand), have convulsions or suddenly die. Most horses do not have an elevated temperature. The course of the disease may be from 24 hours to several weeks. The mortality rate for horses exhibiting clinical signs of WNV is 33%. Up to 40% of horses recovering from acute illness will exhibit residual gait abnormalities 6 months following the onset of clinical signs.

WNV is diagnosed through the use of paired (two) serum samples taken weeks apart. Results may be confused between vaccination and recent exposure. A fourfold increase in titer between the two serum samples indicates infection. An IgM-capture ELISA test is currently the most reliable test for confirmation of recent exposure to WNV in a horse exhibiting acute disease. Limited quantities of IgM are demonstrated in horses that are vaccinated. Therefore, measurement of IgM is more indicative of acute disease.

Prognosis seems to correlate with the severity of the clinical signs. The worse the neurologic signs the worse the prognosis.

Treatment for viral infections is limited and consists primarily of supportive care in the horse.

To prevent WNV in horses, all horses in North America should be immunized. Parts of the country having the most insect vectors tend to be the areas most severely affected by WNV. To date, the State of Florida has reported the most cases of WNV in horses. Label recommendations for each vaccine should be followed. In general, vaccination of previously unvaccinated horses involves administration of 2 doses of vaccine, 3 to 6 weeks apart. Annual revaccination is suggested just prior to the mosquito season. In endemic areas it may be warranted to vaccinate even more frequently. No currently licensed vaccine is approved for use in pregnant mares. Mares should be vaccinated prior to breeding. Thousands of pregnant mares have been vaccinated when the risk of disease outweighed the possibility of adverse reactions from vaccination. Booster vaccination of mares 4 to 6 weeks before foaling provides augmented passive colostral protection for up 4 months. Foals should be vaccinated at 3 to 4 months of age with a series of 3 vaccines at 4 to 6 week intervals. When a mare is non-vaccinated the foal may be vaccinated at an earlier age. Vaccines are available from Fort Dodge Animal Health called West Nile-Innovator®, Intervet’s PreveNile®, and Merial manufactures Recombitek Equine West Nile Virus® vaccine.

Effective mosquito control is a must! Mosquito control is currently the only way to prevent infection for those species for which a vaccine has not yet been developed. Topical and environmental insecticides will decrease the possibility of exposure to both humans and animals.

Humans are susceptible to WNE but can only be infected through mosquitoes that are harboring the virus and not by direct exposure to an affected horse. Horses and humans are considered to be dead-end hosts of the virus and do not contribute to the transmission cycle because these species do not experience a significant viremia (produce enough virus to allow a mosquito to pick up and transmit the virus).

References:
AAEP “West Nile Virus Vaccination Guidelines” developed by the American Association of Equine Practitioners. 2005.
West Nile Virus Information – Centers for Disease Control and Prevention
Brown, Christopher and Bertone, Joseph. The 5-minute Veterinary Consult Equine. Blackwell publishing. 2005. pp1132-1133.
The Merck Veterinary Manual, Ninth Edition. 2005 pp. 1077-1079 and 2289-2290.

Topics: birds, mosquitoes

Symptoms: blindness, lameness, stumbling

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