Madera County, California

The California Department of Food and Agriculture (CDFA) confirmed on Aug. 28 that an unvaccinated yearling Andalusian stallion presented with clinical signs of West Nile virus (WNV), including ataxia (incoordination), drooling, fever, and lethargy, on Aug. 18. The colt was tested using IgM-capture ELISA. By the confirmation date, the attending veterinarian reported the horse still displayed signs of ataxia but had received supportive care and improved.

This diagnosis marks the first confirmed case of equine West Nile virus in Madera County for 2019 and the fifth statewide, with other affected counties including Fresno, Kern, San Bernardino, and Stanislaus.

Seventy-five percent of the WNV cases confirmed in California are typically reported in August and September, with the first case being reported in late July or early August most years, said Katie Flynn, BVMS, MRCVS, equine staff veterinarian with the CDFA. In 2019 the first case was reported on Aug. 12.

“According to our records, the last confirmed equine case in Madera County was reported in 2014,” Flynn said. “It’s important to note, though, that not all horses displaying compatible clinical signs are tested and confirmed positive. Some horses receive supportive care and recover without a sample being collected and tested for WNV.”

Flynn added that California doesn’t quarantine equine WNV cases. “Our department receives positive laboratory reports for WNV in an equine and confirms with the veterinarian that the horse meets the case definition with compatible clinical signs (specifically neurologic signs),” she said. “Once confirmed, the Department of Public Health is notified so local mosquito control agencies can take appropriate action.

“CDFA continually monitors and investigates equine neurologic cases for WNV infection in horses and urges horse owners to minimize horse exposure to mosquitoes during the peak mosquito feeding periods at dawn and dusk,” she added.

In addition to applying mosquito repellant to your horses, Flynn advises the following control efforts to eliminate mosquito breeding sites:

  • Drain unnecessary standing water in wheelbarrows, tires, etc.
  • Clean water containers (bird baths, plant saucers, etc.) at least weekly.
  • Schedule pasture irrigation to minimize standing water.
  • Keep swimming pools optimally chlorinated, and drain water from pool covers.
  • Stock water tanks with fish that consume mosquito larvae (contact local mosquito control for assistance), or use mosquito “dunks” available at hardware stores.

West Nile Virus Facts

WNV transmission occurs when infected mosquitoes feed on animals, as well as humans, after having fed on infected birds.

Clinical signs of WNV in horses include:

  • Mild anorexia and depression;
  • Fine and coarse muscle and skin fasciculation (twitching);
  • Hyperesthesia (hypersensitivity to touch and sound);
  • Changes in mentation (mentality), when horses look like they’re daydreaming or “just not with it”;
  • Occasional drowsiness;
  • Propulsive walking (driving or pushing forward, often without control); and
  • Spinal signs, including asymmetrical weakness; and
  • Asymmetrical or symmetrical ataxia.

West Nile virus has no cure; however, some horses can recover with supportive care. Equine mortality rates can reach 30-40%. The American Association of Equine Practitioners includes WNV as one of the core diseases all horses should be vaccinated against at least annually.