A 12-year-old mare in California has tested positive for contagious equine metritis (CEM), a highly contagious but treatable reproductive disease of horses.

The positive mare was bred via artificial insemination using shipped semen collected from a CEM-positive stallion. The mare remains under quarantine while being treated for the disease.

Watch the CEM testing and treatment protocol for mares and stallions.

CEM is a contagious bacterial infection spread between mares and stallions during mating or artificial insemination with infected semen. It can also be transmitted on contaminated breeding equipment. CEM is not known to infect other livestock or humans. Stallions infected with CEM do not exhibit any clinical signs, but infection in the mare can cause fertility problems. The disease is successfully treated with antibiotics.

As of February 17, 2009, a total of 11 stallions and three mares were confirmed infected in the United States. Three of the stallions are located in Indiana, four are in Kentucky, one is in Texas, and three are in Wisconsin. One mare is in Wisconsin, one in Illinois and one in California.

As of Feb. 25, the USDA’s Animal and Plant Health Inspection Service reported it had located 600 additional horses exposed to causative organism T. equigenitalis. The 614 horses are located in 45 states. There are 84 exposed or positive stallions in 16 states and 530 exposed or positive mares in 44 states. Another nine exposed horses, eight mares and one stallion, are still actively being traced.