Contagious equine metritis (CEM) affects horses, donkeys, mules, zebras and other members of the equid family. It spreads from horse to horse during mating. It can also spread when horses are artificially inseminated.
Stallions do not show clinical signs of the disease, but they can carry it. In mares you should look out for discharge from swollen genitals, especially 1-6 days after mating.
Infected mares may also be temporarily infertile, so if your animal is failing to get into foal, get it examined by a vet.