APHIS Factsheet

Veterinary Services May 2014

Contagious Equine 

Metritis

Contagious equine metritis (CEM) is a venereal 

disease of horses caused by the bacteria Taylorella 

equigenitalis. It is spread during breeding or through 

contact with contaminated objects. This disease occurs 

very rarely in the United States and does not affect 

other livestock or people. However, it is highly contagious among horses and can be difficult to detect and 

control. Signs of illness in infected mares may not be 

obvious, and stallions carry the bacteria without showing any signs at all. CEM can have a negative impact 

on fertility in both mares and stallions. If the disease 

became widespread in the United States, the horse 

industry could suffer considerable economic losses.

The U.S. Department of Agriculture (USDA) works 

actively with its State partners to protect our country’s 

horses against CEM and keep it from becoming 

established here. 

Background

The first cases of CEM were diagnosed in the 1970s, 

beginning in England and then in other countries in 

Europe and in the United States. The disease has 

been detected sporadically in the United States since 

that time, most recently in 2013. Animal health officials 

investigate each detection to identify, test, and treat all 

infected or exposed horses. Affected horses are treated 

with antimicrobials to eliminate T. equigenitalis; they are 

then retested and not cleared for release from quarantine until negative results are obtained. 

Due to the nature of the disease, it is difficult to determine how widely CEM is distributed throughout the 

world. USDA currently considers all countries of the 

European Union, and a small number of others, to 

be affected with CEM. To protect the U.S. equine 

industry, horses imported from CEM-affected countries 

must test negative for the disease before entering the 

United States. 

For the latest information on CEM reporting around the 

world, visit the World Animal Health Information Database 

Interface at www.oie.int/wahis/public.php?page=disease_ 

status_detail.

How It Spreads

CEM can be transmitted by an infected horse during 

breeding; through artificial insemination with contaminated semen; or by contact with contaminated objects, 

such as hands or instruments. It can also be spread 

during the semen collection process.

CEM-positive horses that show no signs of illness 

(called “carriers”) can cause outbreaks at breeding 

facilities. During the breeding season, a carrier stallion 

may spread the disease to several animals, including 

other stallions using the same facility, before it is suspected or diagnosed. Infected mares can transmit the 

disease to their foals, but this happens rarely.

Signs of Infection

In mares, the infection can appear in three general forms. 

1) Acute: Active inflammation of the uterus causes 

an obvious thick, milky, mucus discharge 10 to 

14 days after breeding.

2) Chronic: Milder uterine inflammation causes 

less obvious discharge, and infection may be 

more difficult to eliminate.

3) Carrier: The bacteria are established in the 

reproductive tract. The mare, though not showing signs of illness, is still infectious and can 

remain a carrier for several months or longer. 

Infected mares are likely to return to heat (estrus) 

more quickly than normal.

CEM rarely induces abortion or permanent infertility in 

mares. Stallions show no signs of the disease, but they 

can carry the bacteria for years, and it can survive in 

chilled and frozen semen.

Testing for CEM

Private veterinarians work with State or Federal animal 

health officials to test any horse suspected of having 

CEM, or any horse exposed to a CEM-positive animal. 

You may also need to test for CEM to meet requirements for exporting horses, semen, or embryos to 

another country. There are three different ways to 

test for CEM: bacterial culture, blood testing, and 

test breeding.

Bacterial culture tests look for the bacteria that cause 

CEM. Veterinarians collect swabs and send them to 

approved laboratories. They require special handling 
and must be processed within 48 hours of collection. It 

takes 7 days to get a result. Typically, the veterinarians 

will collect three sets of swabs for culturing during a 

7- to 12-day period. 

Blood testing looks for an immune response to CEM, 

which helps determine if a horse has recently been 

exposed to the disease. This complement fixation test 

can only be used in mares because stallions do not 

develop an immune response to CEM.

Test breeding is used to detect CEM in stallions under 

certain circumstances. A stallion is bred to two mares 

known to be CEM-free. Those mares are then tested 

for CEM. It takes 5 weeks after the test breeding to 

declare the stallion negative. Test breeding will sometimes detect bacteria that direct testing of the stallion 

did not detect. To maximize the chances of detecting 

the bacteria, USDA requires stallions to be tested by 

both bacterial culture and test breeding if they are 

known to have been exposed to the CEM bacteria or 

are being imported to the United States. Test breeding 

is not normally required to export stallions or semen 

to another country. Mares are tested with bacterial 

culturing and blood tests.

Treatment and Recovery

Although the occurrence of CEM in the United States 

is a serious concern, there is some good news. Both 

mares and stallions can be successfully treated using 

disinfectant scrubs and antimicrobials. Most importantly, 

after treatment, fertility appears to return to normal in 

recovered horses.

Preventing CEM

Even if you don’t suspect that your horses have CEM, 

you can help prevent its introduction and spread by using 

proper biosecurity measures in your daily procedures. 

For example, always clean and disinfect semen collection equipment between stallions. Also, be sure to keep 

records on horse movements and semen shipments so 

they can be traced if a case of CEM is detected. For 

more information on equine biosecurity practices, see 

USDA’s brochure “Biosecurity—The Key To Keeping Your 

Horses Healthy,” available at www.aphis.usda.gov/

publications/animal_health/2011/bro_keep_horses_

healthy.pdf. 

In addition to biosecurity practices, you can also help 

prevent the spread of CEM by conducting bacterial 

culture tests on breeding stallions prior to each breeding 

season. Ideally, you should ask your veterinarian to 

collect a series of swab sets for CEM testing as part of 

your stallion’s annual breeding soundness exam. 

Report Suspicious Cases

If you suspect that an animal may have CEM or any 

other foreign animal disease, immediately contact State 

or Federal animal health authorities.

Learn More

For more information on CEM and other animal diseases, 

visit USDA’s Animal and Plant Health Inspection Service 

(APHIS) Web site at www.aphis.usda.gov. Click on 

“Animal Health” and then “Animal Disease Information.”

Or, contact us at:

Veterinary Services

USDA/APHIS

4700 River Road, Unit 41

Riverdale, MD 20737

Phone: (301) 851-3595

Fax: (301) 734-7817

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture

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