Contagious Equine Metritis

Learn about this disease and its effects on horse breeding.

Contagious equine metritis was first diagnosed in Thoroughbred breeding horses in England and Ireland. In the years that followed, the disease was subsequently diagnosed in most of the European countries, as well as Australia and Japan. The United States banned importation of breeding stock from the affected countries. Despite the ban, CEM showed up in Kentucky in 1978 in a mare that had been bred to an imported stallion from France. By the time the outbreak was brought under control that year, nearly 500 breeding horses had been quarantined and the horse-breeding season thoroughly disrupted. The

largest U.S. outbreak occurred in 2008 when 28 positive horses and 977 exposed horses were found in seven states. An epidemiological study of the outbreak determined that a likely source of the outbreak was a single stallion imported from Denmark in 2000. CEM is a true venereal disease and is spread primarily through breeding or artificial insemination, but the disease can also be spread by contaminated barn equipment, insemination equipment, vaginal speculums or by handlers who fail to maintain proper hygiene when handling mares and stallions during breeding.

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Mares are usually infected by stallions harboring the bacteria and vice versa. The bacteria can cross the placenta to infect the fetus but seldom causes abortion. Foals can also be infected by the mare during foaling as they pass through the birth canal and can retain the infection until breeding age. The incubation period (time from exposure until clinical signs appear) for the disease is two to 12 days. Infected mares typically develop a copious, odorless vaginal discharge that usually lasts approximately two weeks. Infertility in the affected mare is usually temporary, but the bacteria can remain in the mare’s external reproductive tract indefinitely as a source of infection to a stallion. Infected stallions do not show clinical signs but become carriers that infect the mares they breed. Diagnosis can be difficult. Blood serology tests in mares indicate infection but have no value in the stallion because the bacteria are only a surface contamination on the stallion’s genitalia and do not invade the body. A definitive diagnosis in mares and stallions can only be made by isolation of the bacterium Taylorella equigenitalis in culture. The sites cultured in the mare include the uterus, cervix and clitoris. In the stallion, swabs should be taken from the urethra, the external surface of the penis and the prepuce. The stallion’s semen should also be cultured, as it can carry the bacteria to the mare.

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Cultures should be repeated three times to ensure the bacteria is not missed. The most fool-proof method used to ensure stallions are CEM-free is to test-breed the stallion to two negative mares and then culture the mares on days 3, 6 and 9 post breeding. The mares are also blood tested 15 to 40 days after breeding. If a stallion or mare is found to be infected with CEM, treatment involves thoroughly washing the affected areas with a chlohexidine solution and applying a 0.2 percent nitrofurazone ointment daily for five days. The animal is then retested to ensure the condition is cured. Although not a life-threatening disease, CEM is detrimental to the individual animal’s ability to produce offspring and can have a devastating effect on the horse industry. We should be extra vigilant when we have a mare who produces large volumes of cloudy vaginal fluid or fails to settle after repeated breedings. Additional information on CEM can be obtained by contacting the United States Department of Agriculture, or your state or local veterinarian.