Contagious equine metritis (CEM) is a highly contagious venereal disease affecting horses, with potentially severe implications for breeding operations. Caused by the bacterium Taylorella equigenitalis, CEM primarily affects mares, causing vaginal discharge, temporary infertility, and, in some cases, long-term carrier states. Stallions, while asymptomatic, can be silent carriers capable of spreading the disease through natural breeding or artificial insemination.

CEM Transmission and Risks in Horses

Contagious equine metritis spreads in three primary ways:

• Between a mare and stallion during live-cover breeding.
• From stallion to mare via contaminated semen used in artificial insemination.
• From stallion to stallion through shared breeding equipment.

Mares can sometimes clear the infection on their own, but many become chronic carriers, maintaining the bacteria in their reproductive tract and serving as a long-term source of transmission. Stallions, unless diagnosed and treated, can transmit the bacteria indefinitely.

CEM Outbreaks in the U.S.

Although the United States has been considered CEM-free since the mid-1980s, multiple outbreaks have occurred. Six reported incidents have emerged since 2006, with three classified as full-scale outbreaks. The most significant outbreak, spanning 2008 to 2010, involved 23 stallions and five mares. More than 1,000 exposed horses across 48 states were tested, with the infection traced to a stallion imported from a CEM-affected country in 2000.

Between May 2024 and March 2025, the USDA confirmed 52 cases of CEM in an outbreak associated with transmission at a single farm in Florida. The outbreak was first identified when a pony stallion on the farm infected a pony mare that subsequently showed clinical signs of CEM and tested positive for T. equigenitalis. These two animals and one other pony stallion were the only breeding animals involved in the case—the other 49 were geldings who acquired the infection through sheath-cleaning practices on the index farm. None of the geldings arrived at the farm as stallions; current epidemiology (the study of disease, health, and their determinants in populations) indicates one must have arrived infected. In addition to the 52 confirmed cases, officials said at least another 140 were potentially exposed. This is the first finding of extensive gelding-to-gelding spread of T. equigenitalis through inadequate biosecurity during sheath-cleaning practices in the U.S.

Prevention and Biosecurity Measures

To mitigate CEM risks veterinarians recommend the following precautions for breeding operations:

• Adhering to the American Association of Equine Practitioners’ biosecurity guidelines for venereal disease prevention.
• Regularly testing active breeding stallions before the breeding season using direct swab culture for T. equigenitalis. Testing requires collecting swab samples from multiple sites on a stallion’s genitalia and submitting them to a CEM-approved laboratory.

Even a single negative test provides valuable insight into a stallion’s CEM status. However, veterinarians encourage multiple tests to ensure accuracy and prevent undetected carriers from spreading the disease.

To reduce the risk of CEM spread to geldings, experts recommend:

• Wear disposable gloves and change gloves between geldings.
• Use a disposable bucket liner in the wash bucket and paper towels or disposable rags that will not be shared between geldings.
• Avoid using a hose that can become contaminated during handling.

The Stakes for Stallion Owners

The potential financial and horse-health consequences of a CEM outbreak are significant. Beyond the health risks to breeding stock, outbreaks can lead to quarantine measures, disruptions in breeding schedules, increased costs to treat outbreaks, and disruption in the international movement of U.S. horses and semen/embryos.

Ensuring compliance with testing and biosecurity measures is not just about protecting individual breeding programs—it is a collective effort to safeguard the equine industry. Stallion owners, veterinarians, and breeding facility managers play a vital role in preventing future outbreaks and maintaining the country’s CEM-free status.

By taking proactive steps, the equine industry can continue thriving while minimizing the risks associated with this serious but preventable disease.

Editor’s Note: This article has been expert reviewed by Angela Pelzel-McCluskey, DVM, MS, equine epidemiologist at the USDA for accuracy.