From thehorse.com :
In 2011, 30 U.S. States reported 87 cases of equine WNV, according to the USDA Animal and Plant Health Inspection Service National Animal Health Surveillance System.
So far in 2012, according to the United States Geological Survey's Disease Maps, 77 cases of equine WNV have been reported in the following states, as of Aug. 14: Arizona, California, Colorado, Indiana, Iowa, Kentucky, Louisiana, Michigan, Mississippi, Missouri, New Jersey, New Mexico, North Dakota, Oklahoma, Pennsylvania, South Carolina, South Dakota, Texas, and Wyoming.
Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation (twitching); hyperesthesia (hypersensitivity to touch and sound); changes in mentation (mentality), when horses look like they are daydreaming or "just not with it"; occasional somnolence (drowsiness); propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia. Equine mortality rate can be as high as 30-40%.