Contagious Equine Metritis: Symptoms and Treatment

Contagious equine metritis (CEM) is a sexually transmitted disease that can be spread by direct contact during breeding or contact with contaminated tools and infected semen.  

It’s important for horse owners, especially horse breeders, to know the signs of CEM and quarantine horses if necessary. 

What Is Contagious Equine Metritis?

Contagious equine metritis is a venereal disease of horses caused by the bacterium Taylorella equigenitalis. Because it’s a sexually transmitted infection, breeding mares and stallions are most at risk.  

Historically, CEM caused large economic losses for the horse industry in the United States. As a result, the US Department of Agriculture (USDA) implemented strict testing and quarantine procedures to help prevent outbreaks. CEM is also labeled as a Foreign Animal Disease, meaning it’s very rare in the United States and any case must be reported to veterinary health officials. 

Symptoms of Contagious Equine Metritis

CEM is a disease limited to the reproductive tract of mares and stallions. In stallions, there are rarely any symptoms. In mares, symptoms are typically mild and self-limiting, so they resolve on their own.  

The biggest concern with CEM is that mares, stallions, and even foals born to infected mares can be asymptomatic carriers and continue to spread the disease.  

Infected mares may show the following symptoms:  

• Vaginal discharge (most common) 

• Short-term infertility 

• Abortion/miscarriage (loss of an unborn foal) 

• Short-cycling (going back into estrus more quickly than typical) 

What Causes Contagious Equine Metritis?

The bacterium Taylorella equigenitalis can be transmitted in the following ways: 

• Direct contact between mares and stallions during breeding 

• Infected semen during artificial insemination 

• Contaminated breeding or collection equipment 

How Do You Test for Contagious Equine Metritis?

Two tests are used to diagnose CEM:  

• Bacterial culture: Swab samples are collected from the urogenital tract of stallions or mares. Multiple samples are collected at weekly intervals.  

• Complement fixation (CF) test: For mares only, this test detects antibodies against Taylorella equigenitalis. 

Treatment of Contagious Equine Metritis

If a horse tests positive for CEM, treatment is performed under the direction and supervision of a state veterinarian. Horses must be quarantined to prevent spreading the disease until they clear it. 

​​​​​Antibiotics are typically used topically for five days. At the end of the treatment period, horses are retested to confirm they are negative before being released from quarantine. 

Recovery and Management of Contagious Equine Metritis

Once the infection has been treated with antibiotics and the horse has tested negative, there are rarely any long-term complications and fertility appears to return to normal after treatment. 

Prevention of Contagious Equine Metritis

Following USDA guidelines for testing and quarantining is the first line of defense against CEM. This means testing all imported mares, stallions, and semen samples.  

Horse breeders should test stallions annually prior to breeding and maintain thorough breeding records in case of an outbreak.  

Veterinarians and breeding facilities must practice good sanitization and sterilization between breedings, collections, and artificial inseminations.  

Finally, horse owners and breeders should immediately report any new vaginal discharge or fertility issues to their veterinarians.