This viral disease is named for the West Nile district of Uganda, where it was first isolated from a woman in 1937. Birds were recognized as carriers in 1953, and since then we have learned that mosquitoes can transmit the virus to numerous species. The disease primarily affects birds, equine species and humans, although it can also be seen occasionally in goats, sheep, dogs, llamas, reptiles, and bears. In September of 1999, the first cases of West Nile virus (WNV) in the United States were identified in New York. Since that time, the disease has become a widespread issue.
Birds serve as an “amplifying host” for WNV. This means that the amount of circulating virus in an infected bird’s bloodstream becomes high enough that a mosquito will likely become infectious when it feeds on that bird. There are at least 36 species of mosquitoes known to transmit WNV. Mammals that become infected with WNV generally do not exhibit high levels of virus in their blood stream, and therefore mosquitoes cannot transmit the disease from a horse to another horse or a human.
WNV does not cause clinical signs in a large number of infected mammals, and causes only mild signs in many others. Unfortunately, when the virus breaches the blood-brain barrier damage can occur to the brain and spinal cord. Horses infected by WNV can exhibit a wide variety of clinical signs, dependant on the part(s) of the central nervous system affected.
Some of the more common signs that can be attributed to WNV infection include:
- Incoordination – especially of rear legs
- Twitching of muzzle and lower lip
- Twitching of neck, shoulder, or pectoral muscles
- Behavioral abnormalities – depression OR hypersensitivity
- Stumbling / Toe dragging
- Leaning to one side
Less common signs of WNV can include:
- General weakness
- Impaired vision
- Inability to swallow
- Aimless wandering
Diagnosis and Treatment
West Nile virus is diagnosed based on a combination of clinical signs and blood tests for viral antibodies. Quite a few other diseases can produce clinical signs that are very similar to those of WNV, including:
- Eastern equine encephalitis
- Equine protozoal myeloencephalitis
- Equine herpesvirus-1
Treatment of WNV is focused primarily on supportive care measures to control pain and inflammation. Unfortunately, there is no specific anti-viral medication for WNV available at this point in time. Depending on the severity of disease, an affected horse may need a combination of anti-inflammatory medications, IV fluids, sedatives, and nutritional support. Early detection is a vital part of lessening the severity of disease.
The best approach to dealing with WNV on your farm is by prevention. Vaccination for WNV is available for horses, and should be considered part of their annual wellness program. Discuss appropriate vaccination protocols with your veterinarian.
Additional measures you can take to prevent WNV in your horses are focused largely on mosquito control.
- Eliminate (or reduce) potential mosquito breeding sites – Areas where standing water can collect are prime sites for mosquito reproduction. Take the time to examine your horse’s area and try to remove as many of these sites (i.e. old containers, tires, etc.) as possible.
- Clean clogged roof gutters
- When not in use, turn over wheelbarrows, wading pools, buckets, etc.
- Thoroughly clean all water troughs at least once each month
- Consider careful use of appropriate larvicides where standing water cannot be eliminated (contacting local mosquito control authorities recommended)
- Keep horses indoors from dusk to dawn, when mosquitoes are most active
- Screen stalls or install fans over horses
- Place lights around perimeter of stable to draw mosquitoes away from horses at night, and avoid turning lights on in the stable at night
- Install lighted electric bug zappers
- Use insect repellants that contain pyrethrins
- Remove or limit bird access to stables
Remember, WNV affects horses AND people. Use protective measures for yourself as well as your horse!