Equine Sexually Transmitted Diseases

Contagious equine metritis and equine viral arteritis are sexually transmitted diseases (STDs) that affect horses, causing problems such as low conception rates, abortion, and sick or weak newborn foals. These diseases have an economic impact because they disrupt breeding schedules, can be expensive to treat, and are sometimes difficult to eliminate completely from infected animals.

Contagious equine metritis (CEM) was first reported in England 1977 and in the U.S. within the next few years. This bacterial infection has been found in several countries including Germany, France, and Japan.

Stallions show no signs of infection, but they can spread the disease to mares during breeding. Infected mares may also show no signs, though they often develop a creamy vaginal discharge within two weeks after being bred to an infected stallion. Most mares don’t seem to be very sick, and recover well without or without antibiotic treatment, but they will not have become pregnant from the breeding. Fertility returns after the acute infection subsides, though these mares will have lost a month or more of the breeding season and so may not produce a foal for that year. Mares that are not treated may become asymptomatic carriers, able to spread the disease to a stallion at the next breeding.

Mares artificially inseminated with semen from an infected stallion can develop CEM unless the semen has been treated with antibiotics. In this scenario, many horses can theoretically be infected before the disease is discovered. Testing of breeding stallions can be conducted to be sure they are free of disease before the season begins.

Another problem for horse breeders is equine viral arteritis, or EVA. This is not strictly an STD as it is most commonly spread from horse to horse by contact with nasal secretions. The virus can also be present in semen from infected stallions even if they show no disease signs. Mares impregnated with semen from these stallions may develop mild upper respiratory disease with a cough and nasal drainage, but these signs may be so slight that they are overlooked. The mare becomes a carrier of EVA but suffers no ill effects and usually goes on to deliver a healthy, full-term foal. If she is turned out with other late-pregnant mares, however, those mares may pick up the virus by contacting the carrier mare’s nasal secretions or coughing. Some of those mares will then suffer late-term abortions, even if all mares in the field appear healthy. Vaccinating mares against EVA before they are bred can help to stop the spread of this disease.

Interestingly, Standardbreds and Warmblood horses seem to be more tolerant of EVA infection than some other breeds, with few late-term abortion even among mares that are infected. On the other hand, Thoroughbreds and Quarter Horses have fewer carriers but are far more likely to have pregnancy losses because of the virus.