Contagious equine metritis (CEM)

In the UK, isolation of the Contagious Equine Metritis Organism is notifiable by law. This is a statutory requirement under the Infectious Diseases of Horses Order 1987, and any positive samples must be reported by the testing laboratory to a Divisional Veterinary Manager (DVM) of the Department for Environment, Food and Rural Affairs (DEFRA) who will investigate all cases.

CEM is a highly contagious venereal disease seen in mares and stallions but the latter show no clinical signs. CEM is caused by the bacteria Taylorella equigenitalis, a gram-negative coccobacillus causing acute inflammation of the mare’s reproductive tract, vulval discharge and infertility in the mare. Carrier animals can exist in both the male and the female population. CEM is of economic importance to the breeding industry. Outbreaks have occurred in both Thoroughbred and non-Thoroughbred breeding programmes and carefully designed programmes are in place to minimise its incidence and effects.

The CEM organism can be carried asymptomatically in the sexual organs of the stallion, i.e. urethral fossa, urethral sinus, prepuce and urethra, or in the clitoral fossa and sinuses of mares, and is transmitted during reproduction. The organism can also be passed via fomites, e.g. veterinary instruments, bedding, tack, and by artificial insemination via the semen.

If your mare has CEM you will notice signs of the disease that include vulval discharge that usually appears 2 days after breeding and can last up to 2 weeks in untreated cases. The mare may come back into season sooner than expected. Conception rates will be lower than usual. Abortion has been documented, but is rare. There are no signs in carrier mares and stallions.

If you think your horse has CEM, you should contact your vet immediately. Your vet will take some swabs (samples) from the genitalia of your horse, which will be sent to an approved laboratory for culturing (testing). The laboratory will test for the presence of the CEM organism. If the results are negative, that means your horse is free from infection, but if the results are positive, this means your horse is infected and must be treated, re-tested and cleared. During this time your horse must not be used for breeding.
Your vet will follow a ‘Code of Practice’ issued by the Horserace Betting Levy Board (HBLB) which sets out voluntary recommendations to help owners, in conjunction with their vet, to prevent and control specific diseases in all breeds of horse and pony. The ‘Code of Practice’ is available on the HBLB website in a pdf format.

No vaccines are available for CEM, therefore prevention is essential.
It is important to establish freedom from infection before commencing breeding activities by taking bacterial swabs from certain sites of the reproductive tract. Horses should be checked regularly during breeding activities to ensure they remain free from infection.
It is also extremely important to exercise strict hygiene measures during breeding activities. Anyone coming into contact with breeding horses should be made aware of the risk of direct and indirect transmission of the disease. They should wear disposable gloves when handling the tail or genitalia of the horse and the gloves should be changed between each horse. Separate sterile and disposable equipment (where appropriate) and clean water should always be used for each horse.

Occurrence of the carrier state in horses mean that the prognosis is guarded and stringent adherence to the HBLB ‘Code of Practice’ will minimise the risk of an outbreak. Early treatment can resolve infection quickly with less likelihood of a carrier state occurring.