Veterinary Health Center

Equine Internal Medicine

West Nile Virus in Horses

West Nile virus (WNV) was first recognized in the United States in New York in 1999. WNV is considered an endemic (permanent) disease of the United States. West Nile virus is transmitted by mosquitoes, and produces clinical disease humans, birds, and horses. Crows are particularly susceptible to infection with WNV and experience high mortality. Many birds produce a large number of virus particles in circulating blood, which allows them to transmit virus to mosquitoes. Humans and horses are considered accidental, dead-end hosts and do not participate in the life-cycle. Many other mammals (cattle, dogs, cats, rabbits, deer) demonstrate evidence of exposure to WNV (seroconversion), however, clinical disease is not evident except in isolated cases associated with dysfunction of the immune system. If an infected mosquito obtains a blood meal from a WNV carrier bird, and takes a blood meal from a naïve horse several days later, the horse will become infected. Horses become ill 3 to 14 days after exposure to an infected mosquito (incubation period). Horses may be affected at any age, and there is no breed or gender predilection.

The clinical signs of WNV infection in horses are usually characteristic, recognizable signs that may wax-and-wane dramatically at the onset of disease. The most common early signs of infection include twitching of the muzzle and ears, frequent chewing, aggression, and fine muscle twitching. Horses then develop progressive incoordination, weakness, and listlessness. Severely affected horses may develop paralysis of the limbs, seizures, disorientation, coma, or death. Some horses may present as a single limb, non-weight bearing lameness. The most difficult diseases to differentiate from WNV on the bases of clinical signs are rabies, eastern/western encephalitis, and equine protozoal myeloencephalitis.

It is important to obtain a definitive diagnosis of WNV infection to document the number of cases in a region and to rule-out diseases with similar clinical signs. Diagnosis of WNV is determined by detection of antibody against WNV in serum. Vaccination does not appear to interfere with testing. False negative diagnosis may occur if the individual is tested early in the disease process or if the horse fails to mount a typical immune response to the virus. Retesting serum is recommended for horses with clinical signs typical of WNV, but negative test results.

Treatment of WNV is directed towards reducing edema and inflammation in the central nervous system. Medications are used that will decrease pain, reduce inflammation, and combat viral infection. The duration of medical therapy is dependent on the severity and duration of disease. In addition, protecting the horse from self-inflected trauma is crucial to the management of WNV encephalitis. Sedation, sling support, protective leg bandages, and a helmet are recommended for horses with moderate to severe signs of WNV.

Appoximately 30% of horses with WNV encephalitis will die from their infection or complications associated with encephalitis. However, the prognosis for an individual horse is dependent upon the severity of clinical signs. The survival rate in horses that remain standing is approximately 80 to 90%. The survival rate in horses that cannot be supported with the use of a sling is much lower. Horses cannot transmit WNV to humans through contact. There is no reason to destroy a horse infected with WNV based on concern for human disease.

The WNV vaccine is available for horses for prevention of infection. An initial vaccination is followed by a booster 3 to 6 weeks later. A single vaccination appears ineffective for prevention. Preliminary data indicates protection may begin 30 days after the booster vaccination. Vaccination is recommended once, twice, or three times per year, thereafter, dependent on geographic location. Foals from vaccinated mares should begin their vaccination program at 3 months of age, and should be followed with 2 booster vaccines. Foals from unvaccinated mares should be vaccinated at 1, 2, 3, and 6 months of age.

Control of mosquitoes is just as important in the prevention of disease for your horse as vaccination. To prevent exposure to adult mosquitoes, horses should be maintained away from wooded areas and standing water, particularly during times of mosquito feeding. Repellent should be applied frequently. The most effective method of controlling mosquito populations is targeting the larval stage (sources of standing water). Once mosquitoes become flying adults, control is more difficult and expensive. The following recommendations to prevent larval development are standard:

  • Eliminate unnecessary standing water (buckets, cans, used tires).
  • Empty livestock water sources once weekly.
  • Eliminate trapped water in plastic covers on boats, swimming pools.
  • Make sure rain gutters are clean and do not hold water.
  • Stock garden ponds with mosquito eating fish (e.g. minnows, goldfish).
  • Check water sources for developing mosquitoes. Obtain a water sample using a white plastic container (cottage cheese container), and observe for wigglers (larvae) and tumblers (pupae).
  • Chemical products based on the bacterium Bacillus thuringiensis are safe and effective, with selective pathogenicity against mosquito and blackfly larvae. Trade names: Vectobac, Gnatrol, Mosquito Dunks.
  • No matter how a used tire is oriented, it always collects water and is an ideal breeding site for several mosquito species. A handful of common rock salt will prevent mosquito replication in old tires.