Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annually—one in the spring and another in the fall—in areas with prolonged mosquito seasons. | Photo: Erica Larson/The Horse

California public health officials are encouraging owners to vaccinated horses against West Nile virus (WNV), a mosquito-borne illness transmitted to animals and people via a bite from an infected mosquito. In 2018, there were 11 confirmed cases of WNV in California horses. Six of the affected horses (54.5%) died or were euthanized.

Butte County Public Health and the Butte County Mosquito and Vector Control District announced May 29 that they’re concerned about an increase in the number of mosquitoes capable of transmitting WNV in Butte County due to late-season rainstorms and more mosquito breeding sites in the Camp Fire burn zone.

In Butte County, WNV season runs June through October, the groups said