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

According to the California Department of Food and Agriculture (CDFA), 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.

The mare is part of a disease investigation involving 45 states to identify horses with CEM. Exposure primarily occurs through natural breeding or artificial insemination by a horse infected with CEM.

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 symptoms, but infection in the mare can cause fertility problems. The disease is successfully treated with antibiotics.

As of Feb. 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.

Nationwide, approximately 580 horses have been identified to date that are considered exposed to horses with confirmed infection. Due to the difficulty in detecting active CEM infection, all horses that may have been exposed to the disease are cultured and treated to prevent further spread. The source of the outbreak has not been determined.

In California, CDFA veterinarians are working with private practitioners to test 35 mares that have been quarantined due to exposure to an infected stallion. The mares will be released from quarantine following blood tests, cultures, and treatment. To date, only one of the 35 mares has been confirmed as CEM positive.

CEM is considered a foreign animal disease in the United States. The disease was previously detected in the United States in 1978, 1979, and 2006. In all instances, the limited outbreaks were quickly eradicated.