West Nile Virus in Horses

Horses, like humans, can be infected with West Nile virus, an inflammation of the central nervous system resulting in encephalitis. Although a relatively small number of horses in the United States have been diagnosed with the disease in the approximately 15 years since it first appeared in this country, the numbers are on the rise. The death rate of horses infected with West Nile virus continues to be between 30 and 40 percent, so the illness is definitely cause for concern.

Transmission of West Nile Virus

Horses are infected with West Nile virus through the bite of a mosquito harboring the disease. The primary carriers of the virus are wild birds. When mosquitoes feed on the blood of an infected wild bird, the next animal they bite becomes infected. There is evidence that ticks can also be infected with the disease and may play a part in its transmission cycle, but the research on this has not yet been completed. In the Northeast and Mid-Atlantic states, West Nile occurs primarily during late summer or early fall.

Symptoms of West Nile Virus

It is possible for horses to be infected with West Nile virus without showing any clinical symptoms of the disease. The first signs of infection include general malaise and depression. Other symptoms of the illness include:

  • Difficulty swallowing
  • Excitability and twitching
  • Fever
  • Head pressing
  • Hypersensitivity to touching
  • Impaired vision
  • Seizures
  • Wandering or walking in circles
  • Weakness and/or paralysis of hind limbs

Even though more than 60 percent of affected horses may survive West Nile virus, some will develop chronic neurological symptoms. Because West Nile virus affects the brain, any sign of abnormal mental functioning requires evaluation by a veterinarian.

Diagnosis of West Nile Virus

In the presence of encephalitic symptoms, a specific blood test is used to diagnose West Nile virus by confirming the presence of antibodies in the blood serum of the affected horse. The test is known as the IgM capture ELISA and indicates that the horse has had contact with the virus within the last 3 months.

Further confirmation of the diagnosis of West Nile virus is made through the examination of cerebral spinal fluid.

Treatment of West Nile Virus

There is no specific treatment for West Nile virus in horses at this time. Approximately 60 percent of horse who become ill will recover with proper supportive veterinary care to keep them as comfortable as possible and to prevent secondary infections.

While there is no curative treatment for the illness, it is important to know that infected mosquitoes are in the area in order to take measures to protect other local animals.

Prevention of West Nile Virus

There is now an available vaccine to protect horses from West Nile virus. While the vaccine does not guarantee prevention of the illness, it does reduce the risk. The most effective method of preventing an outbreak of West Nile virus remains reduction of the local mosquito population. This involves careful removing of all standing water in the area such as that found in :

  • Empty containers
  • Pools and decorative ponds
  • Troughs and buckets
  • Clogged roof gutters
  • Birdbaths and puddles
  • Wheelbarrows

Because birds are the primary source of West Nile virus, it is helpful to reduce the nesting bird population in and around stables. Dead birds, especially crows, on the property are always cause for concern and should be reported to the Department of Environmental Protection (DEP). If necessary they should be picked up with gloved hands, placed in plastic bags and turned over to authorities for testing.

Topical mosquito repellents are also available for horses and should be used as recommended by veterinarians.

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