Contagious Equine Metritis: CEM Signs, Testing and Breeding Biosecurity

By Dr Duncan Houston

A mare with vaginal discharge or repeated difficulty conceiving needs a reproductive examination. Contagious equine metritis, or CEM, is one possible cause, but its importance extends beyond a visibly unwell mare because infected horses may carry the organism without obvious signs.

For breeders, rapid communication and accurate records are central to controlling a suspected exposure. Breeding history, semen collection and shared handling equipment can all help the veterinarian identify which horses need investigation.

Quick Answer

CEM is a bacterial reproductive infection caused by Taylorella equigenitalis. If infection or exposure is suspected, stop breeding and semen distribution, contact your veterinarian and follow local animal health authority instructions. Normal appearance, disappearance of discharge or one negative test does not by itself establish that a horse is cleared.

How CEM spreads

Transmission can occur during natural mating, through contaminated semen and through contaminated hands or equipment. Stallions are typically silent carriers, and mares can also retain the organism after visible signs settle. The World Organisation for Animal Health’s CEM overview describes a reproductive-tract infection that usually causes temporary infertility rather than generalised illness.

Hygiene matters outside mating itself. Shared semen-collection equipment and genital handling can move bacteria between animals. Healthy-looking horses therefore cannot be selected for breeding on appearance alone, and a yard’s exposure assessment may include animals not currently used for reproduction.

Signs and practical priorities

What to do after a suspected exposure

1. Call your veterinarian and explain the breeding dates, partner identities and any semen movements.
2. Stop mating, semen collection for distribution and use of potentially contaminated material until advised.
3. Follow instructions about horse movements, separation, equipment and staff routines. Do not move an exposed horse elsewhere to solve a space problem without clearance.
4. Gather laboratory reports, treatment records, collection logs and contact details for relevant facilities.
5. Identify shared instruments and genital-care supplies so the vet can assess possible indirect transmission.
6. Use the veterinarian or designated animal health contact to coordinate required notifications and testing.

CEM is regulated in many jurisdictions. Reporting duties, import requirements and quarantine release procedures depend on location and may change, so obtain the applicable instructions rather than relying on a generic internet schedule.

How testing works

Diagnosis uses properly collected swabs from specified genital sites. The WOAH diagnostic manual describes culture and real-time PCR methods. Sample transport, the sites sampled and recent treatment can affect results.

A routine vaginal swab taken for another purpose may not satisfy an official CEM investigation. The veterinarian and laboratory should agree on collection and testing requirements before sampling. Repeated testing or additional procedures may be required under the relevant programme; do not substitute a blood test or an improvised home swab.

Treatment and clearance

Treatment generally involves carefully performed cleaning and prescribed antimicrobial therapy, with follow-up testing. The USDA APHIS CEM guidance illustrates the importance of supervised treatment, tracing and retesting before quarantine release. Protocols should be chosen by the responsible veterinary team, not copied across countries or individual horses.

Owners should not begin repeated genital washing or apply concentrated disinfectants before assessment. This can injure delicate tissue, interfere with testing and create further contamination if equipment is shared. Improvement in fertility or absence of discharge is encouraging but does not prove that carriage has ended.

Other causes and emergency warning signs

Other bacteria, persistent inflammation after breeding and anatomical or hormonal problems can cause reproductive difficulties. Discharge following foaling can also accompany serious uterine disease. Fever, weakness, reduced appetite, colic or rapidly worsening condition warrants urgent veterinary care rather than waiting for routine breeding tests.

While arranging help, describe whether the mare has recently foaled, whether the placenta passed normally and any medicines given. Do not attempt uterine flushing or other internal treatment yourself.

Prevention and common mistakes

Agree on pre-breeding testing with your veterinarian, use traceable semen sources and maintain dated records for every mating and collection. Clean and disinfect equipment between animals using a validated facility protocol, and avoid sharing contaminated gloves or supplies. There is no effective vaccine to replace these measures.

• Assuming an outwardly healthy stallion cannot carry infection.
• Continuing breeding while waiting for clarification of an exposure.
• Treating disappearance of discharge as proof of clearance.
• Using the same genital-care equipment on several horses.
• Applying a fixed treatment or retesting schedule without checking the governing protocol.

Frequently asked questions

Can artificial insemination spread CEM?

Yes. Contaminated semen or equipment can transmit the organism.

Can a mare return to breeding after treatment?

Often, but only after the treating team and relevant authority confirm that the required clearance conditions are met.

Can humans catch CEM?

Taylorella equigenitalis is not known to infect humans. People can still carry contamination between horses on hands or equipment.

Prompt veterinary involvement protects reproductive health and limits disruption to other owners. Clear records and completion of the full testing plan are as important as treating the individual horse.

For help understanding your animal’s care and preparing questions for your veterinarian, visit ASK A VET™.