Contagious Equine Metritis

Disease Background

Contagious Equine Metritis (CEM) is a sexually-transmitted disease of horses caused by the bacteria, Taylorella equigenitalis. Clinical signs in mares may include a mucopurulent vaginal discharge in up to 40% of affected mares, abortion and infertility. Stallions typically show no clinical signs. Both mares and stallions can become chronic CEM carriers and sources of infection for future outbreaks. The transmission rate is high with live cover breeding, but contaminated instruments and equipment may be an indirect means of infecting mares and stallions. The bacteria can also be spread in semen collected for artificial insemination.

Most CEM cases involve non-clinical mares with mild uterine inflammation. Asymptomatic mares can be infectious and remain carriers for several months. An acute infection can cause active inflammation of the endometrium that results in a mucoid, vulvar discharge 10-14 days post-breeding. Abortions due to CEM are rare, but positive mares that carry a foal to term can produce subclinical carrier foals. Infected stallions exhibit no clinical signs but can carry the organism on their external genitalia for an extended period of time.

Carrier mares and stallions act as a reservoir of T. equigenitalis. Undiagnosed mares and stallions are the source of infection in disease outbreaks. Detection of the carrier state relies on isolation of the bacteria from urogenital swabs. Due to the fastidious nature and slow growth characteristics of the organism, it is difficult to culture and thus, requires obtaining multiple culture samples over a period of one week.

The disease is not known to affect humans or other species of livestock.


Currently, the US is considered free of Contagious Equine Metritis. To ensure this free status, it is required that horses over two years of age being imported from CEM affected countries are quarantined and screened for T. equigenitalis

California CEM Import Quarantine FAQ