Contagious Equine Metritis in Horses: Breeding Disease and Carrier Risk

Contagious equine metritis (CEM) is a sexually transmitted bacterial disease of horses caused by Taylorella equigenitalis. It mainly affects breeding animals because the bacteria spread during natural cover, artificial insemination with contaminated semen, or contact with contaminated hands and breeding equipment. In the United States, CEM is considered a reportable foreign animal disease, so your vet may need to involve state or federal animal health officials if it is suspected.

One reason CEM is so important is that infected stallions usually look completely normal. They can carry the bacteria on the penis, prepuce, urethral fossa, and nearby tissues without any outward illness. Mares are more likely to show reproductive signs, but some infected mares also become silent carriers after the obvious discharge goes away.

For breeding programs, the biggest concern is fertility disruption and hidden spread. A mare bred to a carrier stallion has a high chance of becoming infected, and infected mares may fail to conceive or return to heat early. Foals born to infected or carrier mares may also become carriers later in life, which adds another layer of risk for future breeding management.

CEM is caused by the bacterium Taylorella equigenitalis. The organism spreads mainly through breeding contact. That includes natural mating, artificial insemination with contaminated semen, and contaminated breeding equipment or hands used during semen collection, reproductive exams, or insemination. Because the bacteria can survive in protected genital sites, a horse may spread infection even when it looks normal.

Carrier risk is central to this disease. Stallions commonly carry the bacteria without signs, especially on the external genital tissues. Mares may show discharge for a few days and then appear normal, yet still remain infected for months. Foals born to infected or carrier mares may also become long-term carriers and later introduce the organism into breeding populations.

In the United States, imported breeding horses from CEM-affected countries receive special quarantine and testing because international movement is a recognized pathway for introduction. Day-to-day farm spread can also happen when hygiene slips during breeding work, such as reusing gloves, buckets, sleeves, speculums, or collection equipment between horses without proper disinfection.

Diagnosis depends on identifying the organism through official testing, not on signs alone. Your vet will usually coordinate with state or federal animal health officials because CEM is reportable in the United States. USDA describes three testing approaches: bacterial culture, blood testing, and test breeding. Blood testing is a complement fixation test used in mares to look for recent exposure, while stallions do not develop a useful antibody response for that test.

Sampling is very specific. In mares, official culture commonly targets the clitoral fossa, clitoral sinuses, and either the cervix or endometrium depending on reproductive status. In stallions, culture sites may include the distal urethra, fossa glandis, urethral sinus, and shaft of the penis and prepuce. Special transport media, careful timing, and approved laboratories are needed because T. equigenitalis requires special culture conditions and samples must be handled quickly.

Testing often takes more than one visit. USDA notes that three sets of swabs are typically collected over a 7- to 12-day period for culture, and some exposed or imported stallions may also need test breeding to confirm they are negative. Horses cannot be officially cleared for breeding until required testing, treatment if needed, and follow-up negative results are complete.

Prevention starts with breeding biosecurity. Test breeding stallions as recommended by your vet before the breeding season, especially if there is any import history, unknown status, or prior fertility concern. Keep accurate breeding records for every mare, stallion, semen collection, and insemination date. Good records make trace-outs much faster if a problem appears.

Strict hygiene matters every time reproductive work is done. Use new disposable gloves between horses. Change sleeves, tail wraps, speculums, and other disposable supplies between animals. Clean and disinfect non-disposable equipment such as semen collection tools, ultrasound covers, hoses, and reusable breeding equipment between horses. Avoid sharing wash buckets, cloths, or supplies without proper cleaning.

Imported horses need special attention. USDA requires stallions and mares imported from CEM-affected countries to complete quarantine and test negative before entry into the United States. There is no vaccine for CEM, so prevention depends on screening, hygiene, and breeding management. If a mare develops discharge or fertility problems after breeding, or if a stallion is linked to multiple affected mares, pause breeding activity and contact your vet promptly.