Contagious Equine Metritis (CEM) is a venereal disease in horses caused by a bacterial organism (Taylorella equigenitalis), that lives only in the equine reproductive organs.

Infection is "silent" in stallions, meaning that their fertility and libido is unaffected and they infect mares when they are bred, or from artificial insemination with infected semen. Mares show signs of the disease between 2 - 12 days later, including severe vaginal discharge coming from massive inflammation of their entire reproductive tract and premature return to estrus.

Diagnosis requires a bacterial culture. The organism can be difficult to grow in the lab and takes longer than many bacterial species. Thus, special efforts must be taken to ensure the success of the diagnostic procedure, including the use of specialized bacterial transport media that usually must be specially ordered by your vet.

Treatment involves repeated topical antibiotic application to remove the organism from the external genitalia of mare and stallion. Repeated culture is required to ensure that the organism is gone.

CEM is a reportable disease, meaning that if a horse has or is suspected of having this disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other States and the USDA APHIS in preventing the spread of illness or disease on a national and international level.