Contagious Equine 

Metritis

Contagious equine metritis (CEM) is a transmissible, 

exotic, venereal disease of horses caused by the 

bacterium Taylorella equigenitalis. Thoroughbred 

horses appear to be more severely affected by the 

disease than other breeds. Because animals may 

be asymptomatic, the disease is difficult to detect 

and control. There is no evidence that CEM affects 

people.

CEM is a serious disease because it is highly 

contagious. When coupled with the fact that mares 

can be bred only during certain seasons, CEM can 

have a devastating effect on equine reproductive 

efficiency. Should CEM become established in the 

United States, the horse industry would suffer great 

economic losses.

History

The first case was diagnosed in England in 1977. 

Because of the insidious nature of the disease, 

it is difficult to determine its origin or how widely 

it is distributed throughout the world. Countries 

known to be affected with CEM include Austria, 

Belgium, Bosnia-Herzegovina, Croatia, the Czech 

Republic, Denmark, Finland, France, Germany, Great 

Britain, Greece, Guinea-Bissau, Ireland, Italy, Japan, 

Luxembourg, the former Yugoslavian republic of 

Macedonia, Montenegro, the Netherlands, Norway, 

Serbia, the Slovak Republic, Slovenia, Sweden, and 

Switzerland. 

The first cases of CEM in the United States were 

diagnosed on March 9, 1978, on thoroughbred farms 

in central Kentucky. In April of the following year, 

an outbreak occurred in Missouri. The disease was 

rapidly eradicated from both States and has not subsequently been found in the U.S. horse population.

Transmission

CEM is commonly transmitted directly during 

sexual intercourse between undetected CEMpositive breeding mares and stallions. Transmission 

may also occur indirectly by artificial insemination or 

contact with fomites, such as contaminated hands 

or instruments. Outbreaks usually occur at breeding 

facilities following international horse shipments.

Undetected carrier mares and stallions are the 

source of infection for acute outbreaks of the disease. During the breeding season, a carrier stallion 

may infect several mares before the disease is suspected or diagnosed.

Clinical Signs

Initial exposure to the disease usually results 

in infertility. An infected mare may fail to conceive 

(revealed by an early return to estrus after breeding) 

or she may spontaneously abort. Abortions related 

to CEM are rare, however. Stallions exhibit no clinical signs but can carry the CEM bacteria on their 

external genitalia for years.

There are three general degrees of infection in 

mares.

•	 Acute: Active inflammation of the uterus causes

an obvious thick, milky, mucoid vulvar discharge 

10 to 14 days after breeding.

•	 Chronic: Milder uterine inflammation causes less

obvious vulvar discharge, and infection may be 

more difficult to eliminate.

•	 Carrier: The bacteria are established in

the reproductive tract. The mare, though 

asymptomatic, is still infectious and can remain a 

carrier for several months or longer.

 

Diagnosis

Two of the most common genital infections in 

mares are caused by Klebsiella and Pseudomonas 

spp. bacteria. Differentiating these infections from 

CEM cannot be reliably done except by laboratory 

isolation of T. equigenitalis. 

Swabs for bacteriologic cultures from mares are 

taken from the cervix or endometrium of the uterus 

during estrus, clitoral fossa, and clitoral sinuses. 

Swabs from stallions should be taken from the penile 

sheath, fossa glandis, and urethral sinus. Bacterial 

samples must be delivered in Aimes transport medium (with charcoal) under refrigeration (4 to 6  ̊C) to 

an approved laboratory within 48 hours.

In mares, various blood tests may be used to 

detect antibodies to the CEM bacteria. In stallions, 

detectable antibodies do not develop.

 

Treatment

The mare cannot be successfully treated until 

the CEM bacteria clear from the uterus, a process 

that may take several months. The external genitalia 

of the mare and stallion can be treated with disinfectants and antibiotics. Once daily for 5 consecutive days, the external genitalia should be gently 

scrubbed with 2-percent chlorhexidine in a mild 
detergent solution and rinsed with a warm saline 

solution. The external genitalia should then be coated with an antibiotic ointment, such as nitrofurazone. 

Due to the effectiveness of this treatment, surgical 

removal of the clitoral sinuses is rarely required.

Prevention and Control

•	 Quarantine and test all imported fillies, mares, and

stallions of foreign origin, and mares and stallions 

not previously bred in the United States that are 

older than 731 days (2 years). 

•	 Quarantine and test the first three mares bred to a

stallion of foreign origin.

•	 Quarantine all suspects until all test results are

negative.

•	 Avoid breeding any CEM-positive horses until they

have been successfully treated and certified CEM 

negative.

•	 Maintain strict hygiene when handling mares and

stallions (e.g., use disposable gloves, change 

gloves between horses, and thoroughly clean and 

disinfect instruments).

Report Suspicious Cases

Veterinarians and equine owners who suspect 

that an animal may have CEM or any other foreign 

animal disease should immediately contact State or 

Federal animal health authorities.

Additional Information

For more information, contact 

U.S. Department of Agriculture

Animal and Plant Health Inspection Service

Veterinary Services, Emergency Management

4700 River Road, Unit 41

Riverdale, MD 20737

Telephone (301) 734–8073

Fax (301) 734–7817

Current information on animal diseases and suspected outbreaks is also available on the Internet; 

visit the APHIS Web site at 

http://www.aphis.usda.gov/animal_health.

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