Contagious equine metritus (CEM) is a venereally transmitted bacterial disease of horses. CEM was first reported in the UK in 1977.

The disease is notifiable: if you suspect the disease, you must immediately notify the duty vet in your local Animal Health Veterinary Laboratories Agency (AHVLA) office.

Latest situation

• The last outbreak was in 2010

About the disease

Three species of bacteria are recognised as liable to cause outbreaks of infectious reproductive disease in the horse:

• Taylorella equigenitalis (the CEM Organism, or the CEMO)
• Klebsiella pneumoniae
• Pseudomonas aeruginosa

Infection with these bacteria can be highly contagious.

Spread/transmission

Infection spreads through direct transmission of bacteria from mare to stallion or teaser or from stallion or teaser to mare at the time of mating or teasing. It is also transmitted to mares if semen used in artifical insemination (AI) comes from infected stallions.

Indirect infection also occurs, for example:

• through contaminated water, utensils and instruments
• on the hands of staff and veterinary surgeons who handle the tail and genital area of the mare, or the penis of the stallion or teaser
• genital to genital or nose to genital contact between stallions/teasers and mares

Indirect infection is a significant risk for the transmission of the CEMO, and of Klebsiella pneumoniae and Pseudomonas aeruginosa between horses.

In the mare, the severity of disease caused by the CEMO varies. The main outward clinical sign is a discharge from the vulva, resulting from inflammation of the uterus, usually 1 to 6 days after infection at mating. There are 3 states of infection:

• In the acute state, there is active inflammation and obvious discharge, seen 1-6 days after infection at mating.
• In the chronic state, the signs may be less obvious but the infection is often deep seated and may be difficult to clear. Discharge may not be seen for up to 80 days after infection.
• There is also the carrier state. The bacteria have become established as part of the bacterial flora in the genital areas and there are no signs of infection. However, the mare is still infectious.

Infected stallions and teasers are usually passive carriers, meaning that they do not show clinical signs of infection but have the bacteria colonised as part of the flora on their external genital organs. Stallions pass the bacteria on to mares during mating. Bacteria may also pass to mares, directly or indirectly, from infected teasers.

Although internal spread in the male is rare, the bacteria may occasionally invade the urethra and sex glands, causing pus and bacteria to contaminate the semen.

Disease prevention

You are advised to consult the Industry Common Code of Practice for the Prevention and Control of CEM.

The main ways of preventing infection are:

• check stallions, teasers and mares for infection before they are mated: this is done through swabbing
• if a horse proves to be infected, do not use it for mating until the infection has been successfully treated
• always exercise strict hygiene measures when handling mares, stallions and teasers. Further information is available in Equine Veterinary Education 1996 Volume 8 (3) 166-170.

The main ways of stopping the spread of infection if it does occur are to:

• stop mating by the infected horse(s)
• treat the infection and re-swab to check that the infection has cleared up before resuming mating
• exercise strict hygiene measures when handling the horses involved

NB: For non-Thoroughbreds, Artificial Insemination (AI) is a useful disease control measure. However, mare owners should only use semen collected from stallions proven free of infection at the time of semen collection.

Klebsiella pneumoniae and Pseudomonas aeruginosa

The means of spread of infection and the signs and stages of infection with Klebsiella pneumoniae and Pseudomonas aeruginosa are similar to those described for CEM above, but infection may also become established in the bladder and urinary system as well as the reproductive system.

Infection by these bacteria can be prevented by implementing the same measures as for CEM. All swabs should therefore be screened for these bacteria as well as for the CEMO.

Legislation

Notification is a statutory requirement under the Infectious Diseases of Horses Order 1987.