California animal health authorities confirmed the state’s sixth case of equine West Nile virus (WNV) for 2013 on Aug. 13, according to a statement on the California Department of Food and Agriculture (CDFA) website.

"On Aug. 13, 2013, the CDFA Animal Health Branch confirmed one additional case of West Nile virus (in) a two-and-a-half month old Quarter Horse filly in Stanislaus County," the statement read. "Both mare and foal were unvaccinated. The positive filly is recovering."

The statement noted that WNV-positive horses were located in Kern, Riverside, Sacramento (2), and Stanislaus (2) counties, and that four of the six positive horses were euthanized.

Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculations (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%