The first case, a vaccinated 7-year-old pony mare from Stanislaus County, initially showed signs of WNV on Sept. 11. Signs consisted of fever, mild ataxia (incoordination), muscle fasciculations (twitching) in her head and neck, drooping lower lip, and hyper-reactivity to touch and movement.

The second case, an unvaccinated 3-year-old Quarter Horse mare from Amador County, first showed signs that included severe ataxia, muscle fasciculations, and hyper-reactivity to light on Sept. 15.

According to a CDFA statement, both horses are alive and recovering.

These cases mark the state’s ninth and 10th confirmed cases of WNV in equids. Other positive horses have resided in Fresno County (2), Kings County (1), Merced County (1), Sacramento County (2), and San Joaquin County (2). Six of the horses were unvaccinated, two were vaccinated, and two had unknown vaccination histories. Nine survived and one succumbed to the disease.

About West Nile Virus

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;
• 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);
• 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.