Eleven equine cases of West Nile virus (WNV) have been diagnosed in nine Colorado counties as of Sept. 14.

The WNV-positive horses are from Alamosa, Boulder, Broomfield, Conejos, Fremont, Jefferson, Larimer, Park, and Weld counties.

The incidence of WNV disease varies from year to year and depends on a number of factors, including the amount of mosquitos. West Nile virus can be carried by infected birds and then spread locally by mosquitoes that bite those birds. The mosquitoes can then pass the virus to humans and other animals, like horses. Horses are a “dead-end host” meaning they pose no threat to public health but they can be severely affected by physical symptoms.

In horses 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%