Contagious equine metritis (CEM)

Veterinary advice should be sought before applying any treatment or vaccine.

Contagious Equine Metritis (CEM)

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.

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.

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.


Copious mucopurulent vaginal discharge
Dried vaginal discharge on the inside of the thighs
Temporary infertility
Early abortion


  • Bacterial culture - Considered to be the "gold standard".
  • PCR
  • Complement fixation test (CFT) - is valuable for detecting infections in acutely infected mares, but is not reliable for identifying chronically infected carrier mares.



Report diseaseCEM is a reportable disease, meaning that if you suspect that your horse has this disease, by law you need to report it to your veterinarian, or a state or federal veterinarian.
AntibioticsTrimethoprim-sulfamethoxazole (30 mg/kg) administered orally, twice dayM Kristula
For infected maresClitoral sinus flushing, disinfection, and thorough cleaningM Kristula


  • Perform prebreeding bacterial cultures for CEM in horses prior to breeding.
  • Always clean and disinfect semen collection equipment between stallions.
  • Keep records on horse movements and semen shipments.

Scientific Research

General Overviews

Risk Factors

  • Stallions and mares used for breeding purposes.
  • Not performing prebreeding bacterial cultures for CEM in horses prior to breeding.