Contagious Equine Metritis (CEM) in Horses

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.


Anything that contains pus


A horse that is four years of age or older; a stallion is intact


The hollow bodily organ that holds the embryo and fetus and provides nourishment; only found in female animals.


A condition in which the uterus becomes inflamed


An in-depth examination of the properties of urine; used to determine the presence or absence of illness


The term for a female horse over the age of four that has not been sterilized


The excretion of semen from the reproductive system of the male


The name for the species of horses, donkeys, mules


The name for the reproductive organs


Used to refer to any drug or medical substance that has the ability to slow down or stop the growth of bacteria and other such organisms.

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