Contagious equine metritis (CEM) is a highly communicable venereal disease of horses, caused by the bacterium Taylorella equigenitalis. This disease can spread widely from a single asymptomatic carrier, particularly a stallion. Infected horses do not become systemically ill or die, but reproductive success is reduced. Additional economic impacts include the cost of pre-breeding tests and treatment in endemic areas, as well as screening before importation into CEM-free countries.

Excluding T. equigenitalis from a country can be challenging. Control programs have significantly reduced the incidence of this disease in Thoroughbreds, which were severely affected by outbreaks in the 1970s; however, it also occurs in other breeds, and identifying carriers can be difficult. T. equigenitalis is fastidious and can be difficult to culture, and serological tests are useful only in mares and for short periods. In addition, some recent strains circulate with only mild clinical signs. In the U.S., an outbreak in 2008-2010 may have resulted from the importation of an infected horse 8 years earlier. Similarly, T. equigenitalis appears to have circulated for some time before the 2011 outbreak in South Africa, although the country was thought to be CEM-free. Thus, contagious equine metritis should be a diagnostic consideration even where this organism is thought to be absent.