Horses have proven uniquely susceptible to WNV infection, comprising more than 99 percent of veterinary (i.e. non-human) mammalian cases. Clinical signs in affected horses include-in order of reported frequency-weakness, ataxia, abnormal mentation, fever, muscle twitching (especially of the face and muzzle), and cranial nerve deficits. The course of illness varies from a few days to a few weeks, and may be rapidly or slowly progressive. Horses that become recumbent have a poor prognosis for recovery, and most horses that reach this stage are euthanized or die. The reported mortality rate for symptomatic WNV-infected horses is 30-40 percent. Most horses, then, recover from the infection, and in most cases, recovery seems to be complete.

As with most viral infections, there is currently no specific treatment for disease caused by WNV. Treatment is essentially supportive, and may include anti-inflammatory medications, intravenous fluid administration, parenteral nutrition, and physical support if the horse is extremely weak or prone to recumbency.

Follow this link to a Colorado State University study on West Nile virus survival rates in horses.
Data from two-state project looks at non-vaccinate survival patterns.

USDA APHIS 2003 Equine WNV Outlook for the United States
USDA 2003 WNV Equine Program Information