Merck Manual

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Pet Owner Version

Contagious Equine Metritis


Robert O. Gilbert

, BVSc, MMedVet, DACT, MRCVS, Reproductive Medicine, Department of Clinical Sciences, College of Veterinary Medicine, Cornell University

Reviewed/Revised Sep 2019 | Modified Oct 2022

Contagious equine metritis is a highly contagious, but uncommon, sexually transmitted disease of horses. The disease occurs primarily in Europe and is uncommon in the United States. The disease is caused by infection with Taylorella equigenitalis bacteria, also known as the contagious equine metritis organism. It is primarily transmitted at mating, but contaminated instruments and equipment also play a role. Stallions show no signs of infection and carry the bacteria in the prepuce and on the surface of the penis, especially in the opening of the urethra. The transmission rate is very high; virtually every mare mated by an infected stallion becomes infected.

The signs of infection include a large volume of vaginal discharge seen 10 to 14 days after mating. Mares may return to estrus after a shortened estrous cycle. Although the discharge subsides after a few days, mares may remain infected for several months. Chronically infected mares show no signs of the disease. Most mares do not become pregnant after an infected mating. If they do, they may infect the foal at or shortly after birth. Infected foals can become carriers of the disease when they reach sexual maturity. Diagnosis is based on identification of the causative bacteria. Although other bacteria may infect the mare’s genital tract, most do not produce the same large volume of vaginal discharge seen with contagious equine metritis, and no other sexually transmitted disease of horses is as contagious.

Stallions and mares can be treated by thoroughly cleaning the penis or vagina with antiseptics and applying antibiotic ointment. Strict import regulations are in place in many countries to prevent spread of the disease.

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