The California Department of Food and Agriculture recently confirmed that a 12-year-old mare tested positive for contagious equine metritis.
The mare, which remains under quarantine while being treated for the reproductive disease, was bred via artificial insemination using shipped semen collected from a CEM-positive stallion, according to the department.
The mare is part of an investigation involving 45 states that aims to identify horses with CEM.
Eleven stallions and three mares have been confirmed infected. They include three stallions in Indiana, four in Kentucky, one in Texas and three in Wisconsin, as well as the California mare and single mares in Wisconsin and Illinois.
Nationwide, about 580 horses have been considered exposed to infected horses. All horses that may have been exposed to the disease are cultured and treated to prevent the infection from spreading.
The source of the outbreak has not been determined.