Contagious Equine Metritis (CEM) in Horses

By PetMD Editorial on Nov. 3, 2011

Taylorella equigenitalis in Horses

Contagious equine metritis (CEM) is an extremely contagious venereal disease that is acquired primarily via breeding. While this disease can be carried by either mares or stallions, it is the mare that suffers the ill effects of the infection. Stallions do not show any symptoms of CEM, but mares often will have a thick discharge from the vagina, and will be unable to conceive during the point at which the infection is active.

This is generally a non-lethal disease, and even if left untreated, the mare's system will typically clear the infection on its own over a few weeks time. Blood tests can identify the infection, but it may only indicate that the mare has had the infection, and not whether the infection is still active.

CEM is a bacterial disease caused by Taylorella equigenitalis, and can be treated with antibacterial washes, which is recommended.

Symptoms and Types

Symptoms in the mare will typically become apparent between 10 – 14 days after mating with infected stallions. (Note: stallions do not exhibit symptoms.) Note that only about 40 percent of infected mares will exhibit clinical signs. Those that do will exhibit milky, purulent vaginal discharge. The discharge may be grey in color and is often of a thick consistency. Other symptoms include:

  • Inflammation of the lining of the uterus (endometritis)
  • Inflammation of the cervix
  • Failure to conceive


CEM is caused by the bacteria T. equigenitalis. It is contracted during sexual contact with an infected horse, typically from stallion to mare. It can also be transmitted via contaminated instruments. CEM is primarily a disease seen overseas. Currently, it has only been present in the U.S. very sporadically. It is considered a reportable disease, meaning if it diagnosed, the attending veterinarian must report it to the USDA for further monitoring.


The only way to verifiably diagnose contagious equine metritis is to conduct laboratory tests in a clinical setting. As it is highly contagious, the horse must remain in complete isolation until your veterinarian has had an opportunity to examine your horse and obtain a diagnosis.

Your veterinarian will perform a thorough physical exam on your horse, with a complete blood profile and a urinalysis. Your doctor will also need to take a sample of the vulvar discharge and tissue cells of the genital tract in the mare, and a sample of ejaculate or pre-ejaculate from the stallion for laboratory testing.


While the infection is more of an inconvenience than a cause for serious worry, it is still beneficial to the animal to have the infection treated. Fortunately, contagious equine metritis is fairly easy to treat. The organism that causes it seems to respond rather well to most antibiotic treatments, as well as to disinfecting washes of the genitalia. The organism can easily hide in the folds of the genitals, making it difficult for the disease to be completely eliminated on the first go-round.

Both stallions and mares can be treated with clorhexidine solution and nitrofurazone ointment, which will be used to cleanse and treat the genitals until the infection has passed.

Living and Management

Because of its highly infectious nature, CEM is a serious issue among horse breeders. Allowing the horse enough time to rest and thoroughly heal from the effects of this affliction is a must, and isolation from other horses, particularly of the opposite gender, is essential.

It often takes more than one try to get the organism expelled completely from the system, so giving enough time to the treatment of this infection is important for controlling the spread of this disease among your equine population.


Prevention is the main way to control this sexually transmitted disease. There are tests available to identify CEM, so making sure that all of your horses are checked, and any horses brought into your group for mating purposes are tested will help to drastically reduce the effects of this disease on the horse population as a whole.

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