The west nile virus (WNV) is transmitted when a mosquito takes a blood meal from an infected bird, then feeds on a horse. During the process of taking a blood meal from the horse, the infected mosquito transmits the virus. Horses and humans can become clinically affected by WNV. Typical of numerous other viral infections, many horses experience no clinical illness following exposure to the virus for the first time. In horses infected with WNV, the virus may breach the blood-brain barrier and damage the brain and spinal cord.
While the clinical signs of WNV encephalomyelitis can vary in range and severity, those most frequently observed include incoordination or ataxia (especially of the hind limbs), twitching of the muzzle and lower lip, and twitching of the muscles in the neck, shoulders or pectoral region. Signs may be bilateral or unilateral. Also reported are behavioral abnormalities such as depression or heightened sensitivity to external stimuli, stumbling, toe dragging, leaning to one side, and in severe cases, paralysis of the hindquarters, recumbency, coma and death.
Other clinical signs that may be noted include fever, generalized weakness, impaired vision, inability to swallow, aimless wandering and convulsions. The nature and severity of clinical signs depend largely on the area(s) of the central nervous system affected by the virus and the extent of damage. The incidence of disease tends to be greater in older horses, where a favorable clinical outcome is less likely.