The California Department of Food and Agriculture (CDFA) confirmed one additional case of equine West Nile virus (WNV) last week, according to a statement on the organization’s website.

"On Sept. 4, 2012, the CDFA Animal Health Branch confirmed one additional case of West Nile Virus in an unvaccinated 3-year-old filly located in Butte County," the statement read. "The horse was euthanized."

The Butte County filly is the tenth California horse to test positive for WNV in 2012. The nine other cases were identified in Fresno (2), Glenn, Merced, Sacramento, San Joaquin (2), Stanislaus, and Yolo counties. Three of the positive horses have been euthanized, the CDFA reports.

Clinical signs for WNV include flu-like 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 (incoordination on one or both sides, respectively). Equine mortality rate can be as high as 30-40%