Contagious equine metritis (CEM) is a bacterial infection in horses caused by Taylorella equigenitalis. CEM is highly contagious and primarily spreads through venereal transmission and contact with contaminated breeding equipment.

CEM is characterized by severe inflammation of the reproductive tract in mares, leading to temporary infertility. While CEM only causes symptoms in mares, it can also infect stallions who act as asymptomatic carriers of the disease.

CEM is a significant reproductive disease in horses, known for its potential to disrupt breeding programs and cause economic losses. Treatment primarily involves antibiotics, including local treatment of the reproductive tract in mares and washing the external genitalia with antimicrobial solutions in stallions.

Prevention of contagious equine metritis involve rigorous screening of breeding horses, especially those travelling internationally. Strict hygiene, including proper disinfection of equipment and adherence to quarantine protocols, are key to controlling the spread of CEM.

Contagious Equine Metritis

Contagious Equine Metritis is venereal disease that can be transmitted through natural breeding, artificial insemination, or contaminated equipment.

The bacterium Taylorella equigenitalis is a gram-negative, rod-shaped bacterium belonging to the family Alcaligenaceae. It is known for its ability to colonize the genital tract of horses, leading to the development of clinical and subclinical infections.

CEM is characterized by mucopurulent vaginal discharge, endometritis (inflammation of the uterine lining), and, in some cases, infertility. CEM can result in prolonged estrous cycles and difficulties in becoming pregnant.

Infected mares can also transmit the bacterium to stallions during breeding, leading to the spread of the disease. Stallions can carry the bacterium in their reproductive tracts without showing clinical signs, making them notable carriers.

Although CEM is not life-threatening, it can cause significant economic losses from reduced breeding and costs associated with testing and treatment.

History of the Disease

The first recognized outbreak of CEM occurred in the United Kingdom in 1977. [1] Estimates suggest this outbreak cost the Thoroughbred industry between $20 – 30 million. [1] This led to a massive investigation into the disease, resulting in identification of the bacteria in 29 countries. [1]

In 1978, CEM was first identified in the United States, resulting in significant economic consequences with estimated costs of $13.5 million to resolve. [1] This outbreak arose from the import of two French stallions who were carriers of the disease. [2]

Small-scale outbreaks of CEM within the United States continued until 2008 when a widespread outbreak occurred. [2] This outbreak required extensive investigation to trace the infection source and horses that were potentially infected, with testing and treatment costs amounting to several million dollars. [1]

The first test-positive stallion lived on a large breeding farm that stood 22 stallions. [2] Through tracing, federal officials identified 28 carrier animals and 977 potentially exposed stallions and mares. [2] The horses resided in 48 different states and spanned 11 different breeds, highlighting the extensive reach of this outbreak. [2]

Many countries are currently CEM-free, including Canada, the United States, Australia, and the United Kingdom. However, testing protocols for stallions and mares imported into these countries are imperfect, which has led to outbreaks of CEM in the past.

Currently, researchers are investigating the role of imported embryos and semen as a source of potential CEM outbreaks. [3]

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Contagious Equine Metritis is not a life-threatening infection, but it is highly contagious and requires prompt detection and treatment to prevent spread to other horses.

Only mares develop symptoms of CEM, but not all mares will show signs of infection. [1] Studies suggest only 30 – 40% of mares show signs of the infection. [3]

Common signs of CEM include: [1][4]

  • Gray to white vaginal discharge
  • Failure to conceive
  • Abortion in rare cases

Usually, vaginal discharge develops around 2 – 3 days after the mare becomes infected with the bacteria, and worsens over the following 12 – 13 days before resolving completely. [1][4]

Infertility due to CEM is short-term, and no long lasting adverse effects have been reported. [1] However, failure to conceive can have significant economic impacts for horse breeders. Mares that conceive despite CEM infection are usually able to carry pregnancies to term and deliver healthy foals. [1]


CEM is a venereal disease, meaning that it spreads primarily through sexual contact between mating horses. [1]

During breeding, there is a risk of transmitting the bacteria from either an infected mare or a carrier stallion to an uninfected horse. The highest risk of transmission occurs during live cover breeding. [1][5]

Carrier Horses

The main source of CEM infections is carrier mares and stallions, who harbour the bacteria but do not show any symptoms. Carrier horses transported internationally have caused several outbreaks of CEM in the past. [1]

In stallions, the bacteria primarily reside in the urethral fossa (the area surrounding the exit of the urethra), the surface of the penis, and the prepuce. The bacteria can also shed in the semen. [1]

In mares, the bacteria can infect either the uterine lining (endometrium) or the clitoris. Most mares carry the bacteria in the clitoral sinuses, cavernous structures found on either side of the clitoris. [1]

Contaminated Equipment

Although CEM is traditionally considered a venereal disease, the bacteria can also spread through contaminated equipment used during breeding. [1]

Any equipment contacting vaginal discharge, smegma, or semen from infected horses has the potential to spread the disease to another horse. [1] Thorough sterilization of all equipment is critical for preventing spread.

Teaser Stallions

Teaser stallions are another known source of infection. These stallions typically acquire the bacteria through direct genital contact or through contaminated equipment. [1]

Left unnoticed, teaser stallions have the potential to spread CEM to multiple mares during a breeding season. [1]

Newborn Foals

CEM can spread to newborn foals if their mother carries the bacteria. [1][4] Although CEM does not cause illness in these foals, affected foals can become carriers of the bacteria and go on to spread the disease when they reach sexual maturity. [1][4]

Foals born to mares known to carry CEM must be tested for the bacteria before breeding. [1]

Artificial Insemination and Embryo Transfer

Since CEM can shed in the semen, there is a risk of infection when using contaminated semen for artificial insemination. However, the risk of infection appears to be relatively low based on research into the 2008 United States outbreak.

During this outbreak, only five mares bred via fresh-cooled semen from carrier stallions became infected. [5] There is some evidence to suggest that frozen semen poses an even lower risk than fresh-cooled semen, but this has not been thoroughly investigated. [1]

Currently, the risk of CEM transmission during embryo transfer is relatively unknown. [1] However, since mares can carry the bacteria in their uterine lining, it seems likely that embryos flushed from a CEM carrier mare may be contaminated. [1]


Contagious equine metritis can cause similar symptoms to other bacterial infections, including those that cause endometritis. [1] The main method of diagnosing CEM is bacterial culture, which allows the veterinarian to confirm the bacterial species causing the infection. [1][6]

To diagnose the infection, the veterinarian uses swabs to collect samples, which are then cultured in a laboratory to identify the bacteria.

  • Mares: Swabs are taken from the clitoral fossa, clitoral sinuses, and endometrium. [1][6]
  • Stallions: Swabs are taken from the prepuce, urethral fossa, and penile sheath (external surface of the penis). [1][6]

More sensitive and rapid techniques such as Polymerase Chain Reaction (PCR) tests are also available. [1] This method is able to detect the presence of T. equigenitalis DNA in samples from the reproductive tract.

Since CEM can shed intermittently, many veterinarians perform three tests at 3 – 7 day intervals, to ensure that a negative result is truly negative. [1] All horses that test positive for CEM should be prevented from breeding until the infection is treated and cleared. [1]


Since stallions do not develop disease from CEM, treatment to resolve symptoms only applies to mares. The goal of treating mares is to eliminate the infection and to restore the mare’s fertility. [1]

The main treatment for symptomatic CEM is intrauterine infusion of antibiotics. [1][6] Commonly used antibiotics include:

  • Potentiated sulfonamides
  • Gentamicin
  • Penicillin
  • Ceftiofur
  • Enrofloxacin
  • Topical nitrofurazone, particularly for disinfecting the external genitalia

The choice of antibiotic should be guided by the results of antibiotic sensitivity testing, as Taylorella equigenitalis can exhibit resistance to certain antibiotics. Enrofloxacin is sometimes used in resistance cases.

Treatment typically requires multiple infusions over several days, complemented by topical disinfectants for the external genitalia. Horses should be retested for CEM within 21 days of their treatment to confirm they are free of the disease before they return to breeding. [1]

Some mares and stallions require multiple treatment sessions to fully resolve the infection. [1] Some horses may also require systemic antibiotics. In rare cases, surgical removal of clitoral sinuses may be necessary for mares who do not respond to treatment. [1]

Carrier Horses

Carrier mares typically harbour the bacteria in and around the clitoris, requiring specific treatment to resolve the infection. Treatment takes place over 4 – 5 days and involves: [1][6]

  • Thorough cleaning in and around the clitoris to remove debris
  • Washing the area with antibacterial soaps, such as chlorhexidine
  • Packing the clitoral area with antibiotics

Carrier stallions undergo a similar treatment process, focusing on the urethral fossa, prepuce, and surface of the penis. [1][6]

Semen Processing

Stallions carrying CEM can shed the bacteria in their semen, which poses an infection risk for mares bred by artificial insemination.

Antibiotic treatment of fresh-cooled semen greatly reduces the number of bacteria, but does not completely eliminate the risk of infection. [1][5] Freezing semen may further reduce the bacterial load. [1][5]

Most currently used semen extenders contain appropriate antibiotics for CEM control. [1]


Since there is no vaccine for CEM, prevention strategies are critical to reducing the spread of the disease. [1] Many countries, particularly those considered “CEM-free,” have specific testing and quarantine requirements for imported horses. [1]

The main prevention measures are routine testing of breeding animals, quarantine and testing of imported animals.

Routine Testing of Breeding Animals

Most countries do not have specific requirements for CEM testing within their breeding population, including for artificial insemination or embryo transfer. [7]

For example, CEM testing is voluntary within the United States, despite previous outbreaks. [8] Similarly, the United Kingdom has a voluntary code that provides guidelines on CEM control, but does not enforce testing before breeding. [9]

Without stringent testing requirements, it is possible that carrier horses not identified during quarantine could spread the disease unnoticed. [7] Many researchers and veterinarians in CEM-free countries are encouraging governments to develop stricter protocols for testing prior to domestic breeding. [2][3][7][10]

Import Quarantine and Testing

Many countries considered “CEM-free” require all imported breeding animals undergo a quarantine period and encourage testing to rule out CEM carrier status. [1]

For mares, bacterial culture protocols based on repeated clitoral and uterine swabs are usually sufficient to release the mare from quarantine. [1] Even when mares test negative for the bacteria, they are usually treated with antibiotics for five days before being released as an additional safeguard. [3]

Stallions require more intensive testing, as a single stallion can breed with and potentially infect numerous mares each season. An imported carrier stallion that went undetected during quarantine is the likely cause of the 2008 United States CEM outbreak. [1]

Stallions usually undergo bacterial culture testing as well as “test breedings”, where the stallion covers mares known to be free of CEM. Testing these mares for CEM provides another safeguard against introducing a CEM-positive stallion into the general population. [1][3]

Estimates suggest that up to 70% of CEM-positive stallions have negative bacterial cultures during their quarantine period, and are only identified as carriers during test breeding. [1][11]

Stallions and mares that test positive during the quarantine period are treated, then retested 21 days after their final treatment. [3]

Interestingly, foals, weanlings, and yearlings are not typically tested for CEM at import. Since foals born from a positive mare can be carriers, these imported young horses represent a possible source of CEM under current import policies. [3]

Semen and Embryo Import

Increased use of assisted reproductive technologies, such as artificial insemination and embryo transfer, poses a risk of CEM transfer, as historically semen and embryos were not tested for the bacteria. [10]

During recent CEM outbreaks in the United States and South Africa, contaminated semen was identified as a common source of infection for affected mares. [10]

Based on these outbreaks, some countries, including Canada, have increased their import requirements for semen and embryos. [3] Policies may require that semen is stored in an extender containing antibiotics. [3] Some countries require that the flushing medium used to collect embryos contains antibiotics. [3]

New PCR tests are under development for detecting CEM in semen based on identifying DNA of the T. equigenitalis bacteria. [12] These tests are not currently reliable enough for widespread use, but may be available in the future. [12]


  • Contagious Equine Metritis (CEM) is a highly contagious reproductive disease in horses caused by the bacterium Taylorella equigenitalis.
  • Affected mares may experience temporary infertility. While CEM only causes symptoms in mares, both mares and stallions can spread the disease.
  • Treatment involves intrauterine infusions of antibiotics to eliminate the bacteria.
  • CEM most commonly spreads through live cover breeding, artificial insemination or contact with contaminated breeding equipment.
  • Imported carrier animals are the most common cause of outbreaks in CEM-free countries. Semen and embryo import also pose a risk for transmission.
  • Good biosecurity policies including routine screening and quarantine of imported animals are key to preventing the spread of CEM.

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  2. Timoney, P. J. HORSE SPECIES SYMPOSIUM: Contagious Equine Metritis: An Insidious Threat to the Horse Breeding Industry in the United States1. Journal of Animal Science. 2011.
  3. Luddy, S. and Kutzler, M. A. Contagious Equine Metritis Within the United States: A Review of the 2008 Outbreak. Journal of Equine Veterinary Science. 2010.
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  12. Mawhinney, I. et al. Screening for Taylorella Equigenitalis in Equine Semen: An Exploratory Study. Journal of Equine Veterinary Science. 2022.