Contagious equine metritis (CEM) is a venereal disease of horses caused by the bacteria Taylorella equigenitalis. It is spread during breeding or through contact with contaminated objects. This disease occurs very rarely in the United States and does not affect other livestock or people. However, it is highly contagious among horses and can be difficult to detect and control. Signs of illness in infected mares may not be obvious, and stallions carry the bacteria without showing any signs at all. CEM can have a negative impact on fertility in both mares and stallions. If the disease became widespread in the United States, the horse industry could suffer considerable economic losses.
Stallions show no signs of the disease, but they can carry the bacteria for years, and it can survive in chilled and frozen semen. CEM-positive horses that show no signs of illness (i.e., carriers) can cause outbreaks at breeding facilities. During the breeding season, a carrier stallion may spread the disease to several animals, including other stallions using the same facility, before it is suspected or diagnosed.
Test breeding is used to detect CEM in stallions under certain circumstances. A stallion is bred to 2 test mares known to be CEM-free. Those mares are then tested for CEM. It takes 4- 5 weeks after the test breedings to declare the stallion CEM-free. Test breeding will sometimes reveal bacteria that direct cultures of the stallion did not find. The USDA requires stallions to be tested by both bacterial culture and test breedings when they are being imported into the United States from countries known to have had problems associated with this venereal disease organism. Test breeding is not normally required to export stallions or semen to another country.