A horse in Klamath County, Ore., has tested positive for West Nile virus (WNV), according to an Aug. 22 statement from the Oregon Department of Agriculture (ODA). This is the first detection of the disease in Oregon horses for the 2012 season, the statement said.

"Horses become infected the same way humans become infected, by being bitten by infected mosquitoes," said State Veterinarian Brad LeaMaster, DVM, PHD, in the statement. "I would like take this opportunity to remind horse owners to contact their local veterinarian regarding proper WNV vaccination protocols during this time of increased risk. We want people to be aware of the recent diagnosis of the positive WNV horse in Klamath County and also the positive mosquito findings in other counties around the state so that people can take preventive measures for themselves and their horses."

Studies have shown that the WNV vaccine has a substantial effect on preventing disease. The American Association of Equine Practitioners (AAEP) recommends vaccinating all foals and horses against WNV. For horses residing in the northern United States veterinarians recommend vaccinating in the spring prior to peak mosquito levels. In the south, where mosquito populations are present year-round, horses might be vaccinated more frequently. In addition to geography, age and exposure play an important role in deciding how often to vaccinate horses. The AAEP’s complete vaccination guidelines are available (https://www.aaep.org/wnv.htm) online.

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%