Infection with West Nile Virus does not always lead to signs of illness in people or animals. Horses are affected by WNV more often than other domestic animals. Many horses infected with WNV do not develop any illness, but of the almost 15,000 equine cases reported in 2002, about one-third died or were euthanized. Other livestock and poultry do not commonly show illness if infected with WNV. In horses that do become clinically ill, the virus infects the central nervous system and may cause symptoms of encephalitis. Clinical signs of encephalitis in horses may include a general loss of appetite and depression, in addition to any combination of the following signs:
- Ataxia (stumbling, staggering, wobbly gait or incoordination)
- weakness or paralysis of limbs
- inability to stand
- circling
- fever (in 25% of affected horses)
- muscle fasciculation
- paralysis
- head tilt
- knuckling over
- muscle twitches or tremors
- apparent blindness
It is important to note that not all horses with clinical signs of encephalitis have West Nile encephalitis. There are other diseases that can cause a horse to have symptoms similar to those resulting from infection with West Nile Virus. If you are concerned that your horse may be exhibiting signs of encephalitis, please contact your veterinarian. Laboratory tests are necessary to confirm a diagnosis.