Contagious equine metritis (CEM) is a highly contagious venereal disease of horses that is characterized by endometritis, transient infertility, or rarely abortion. The disease is caused by Taylorella equigenitalis, a gram-negative coccobacillus bacterium. CEM is an OIE-listed disease. In the United States, CEM is classified as a reported disease and rigorous testing, treatment, quarantine procedures, and surveillance of infected and exposed horses has taken place to minimize outbreaks. CEM has the potential to cause substantial economic damage in valuable equine breeding operations. The disease has the potential to cause widespread outbreaks of transient infertility in mares, due to the increased international movement of stallions between countries.
Clinical signs
Horses that become infected with CEM may develop large amounts of mucopurulent vaginal and cervical discharge within 48 hours of exposure, lasting 2-3 weeks. The hallmark sign of CEM is transient infertility.

Transmitted
Taylorella equigenitalis is spread by asymptomatic carrier stallions (stallions that are infected with CEM but show no outward clinical signs of disease) and infected mares during mating or indirect genital contact with contaminated fomites (breeding equipment, wash buckets, mounting blocks, etc.). It can also be spread during the semen collection process.

Incubation period
The incubation period is 2 to 14 days; most infections become apparent 10 to 14 days after breeding.

Treatment
Both mares and stallions can be successfully treated using disinfectant scrubs and antimicrobials. Most importantly, after treatment, fertility appears to return to normal in recovered horses. The United States has detected a small number of CEM cases in post-arrival quarantine.