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In 1977 — long enough ago to be nearly forgotten, but important enough to remember — a serious outbreak of an infectious form of horse venereal disease (VD) occurred in Newmarket and rapidly spread among nearby stud farms. The cause was a previously unidentified bacterial infection known as contagious equine metritis (CEM).

The key features of this condition are in the name — contagious means it is infectious, the equine bit is obvious and metritis means inflammation of the lining of the mare’s uterus or womb, which is the area attacked by the bug.

Contagious equine metritis is horse VD, so it is transmitted from broodmare to stallion or vice versa at mating. A teaser may also spread it, if they are allowed close contact with a mare. There is a risk of CEM being transmitted to mares through artificial insemination (AI), if the semen used comes from infected stallions. Infection can also be spread by people handling infected horses, so hygiene is important. Foals born to infected mares may also pick up the infection at birth and the disease is highly contagious.

The difficulty in controlling CEM is that infected stallions do not appear to have anything wrong with them. In mares, the severity of the infection varies: some appear normal; others will have an obvious dirty vaginal discharge, which usually shows two to seven days after mating. In the Newmarket outbreak, many mares showed obvious signs of infection, but this is not always so. Any mare coming into season early or breaking early in her cycle should be viewed with suspicion.

Affected mares have a reduced chance of conceiving during the acute phase of the disease. If an infected mare that is not in-foal is covered again, she may spread the infection to the stallion. He will infect other mares he covers and so it goes on — a depressing downward spiral of reduced fertility and serious disruption to breeding operations. The disease can have serious economic consequences.

Preventing CEM

To control the spread of CEM, the Horserace Betting Levy Board (HBLB) set up an advisory committee, which developed codes of practice for prevention of this and other infections. This code is revised and republished annually and has been effective in controlling outbreaks.

One outbreak of CEM was detected by swabbing a warmblood stallion in Somerset in spring last year. Prompt action and careful checking of other horses in contact with the stallion controlled it, so restrictions were lifted in May rather than the infection disrupting the whole stud season nationally, which could have happened.

More recently a mare in Oxfordshire tested positive to the notifiable disease contagious equine metritis (CEM). The mare had been imported from Germany. A movement order has been placed on her stables, and an investigation is under way to establish whether she has been used for breeding in the UK.

For CEM and other bacterial venereal infections, it is impossible to tell by looking at a horse whether they carry infection, so a scientific system of taking swabs has been devised. Your vet will be able to advise if your mare is a high or low risk. You should ask your vet to take the appropriate swabs from any horses used for breeding every year after 1 January but before any teasing or mating starts.

These swabs are sent to laboratories specially designated and checked by the HBLB. The swabs need to be tested within 48hr, hence you should avoid having it done on Fridays or weekends, when delays are more likely. It takes almost a week to produce the results, so do not leave it until the last moment. The results will be returned on an official laboratory certificate, which is this year coloured blue with a 2006 background.

Owners should not send any mare to a stallion unless he has an up-to-date negative CEM certificate. Stallion owners should not accept mares unless they have up-to-date negative certificates. If a case of CEM is confirmed, the stud must stop breeding operations, seek immediate veterinary advice and inform DEFRA because CEM is a notifiable disease.

Careful control, proper testing and good hygiene measures reduce the risk of an outbreak. If these guidelines are followed rigorously and responsibly by all involved with British horse breeding, then such problems can be avoided.

This information formed part of a breeding feature in Horse & Hound (26 January, ’06)

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