New muscle disorder challenges British vets

While tying up is well known on British shores, a totally different, but also possibly inherited, muscle disorder has come to light in recent years.

The condition is known as equine polysaccharide storage myopathy (often talked of as EPSM, PSSM or even EPSSM) and is linked with abnormal energy storage in muscle tissue. It has been seen particularly in Quarter Horses in America, as well as warmbloods and draught breeds.

Affected horses are thought to have an abnormally rapid uptake of glucose from the bloodstream and to store glycogen and an abnormal polysaccharide within their skeletal muscle fibres. Consequently, such horses can experience painful muscles, but how it all goes wrong is not fully understood.

Alarmingly, EPSM cases are not detectable using the blood tests routinely undertaken by vets for muscle disease. Instead, muscle biopsies are required to make a definite diagnosis. A sample is required for analysis from specific muscles in the hindquarters.

Such biopsies can be rapidly performed using local anaesthetics in conscious horses. The horse may be a bit sore for a few days, but at least a biopsy should provide a definitive answer. When viewed under the microscope by an experienced pathologist, abnormal polysaccharide inclusions can be seen in the muscle samples from genuine EPSM cases.

This is a disease that has been well recognised in America, while the first official report of four cases in the UK was published in veterinary literature only in 2003. There have been more recent reports of this muscle disease occurring in a family of Appaloosa horses in the UK.

Correspondence in the Veterinary Record suggests that the overall prevalence of EPSM may be even higher than suspected because few vets take muscle biopsies from horses with a history of tying up.

It seems that there is a low prevalence of EPSM in pure Thoroughbreds, but many pleasure and competition horses in the UK have draught or warmblood breeding and both types are prone to EPSM. It is very much a case of “if you don’t look for it, you won’t find it”.

As there are concerns that this condition could be inherited, it is sensible to consider biopsies in horses with repeated and unexplained episodes of muscle disease, particularly if there is any chance of breeding from the horse concerned.

The good news is that many EPSM and recurrent tying up cases will respond to a change in diet. This means increased fat and reduced carbohydrate (so cutting down or avoiding grain and minimising sweet hays and lush pastures), plus careful management, including plenty of turnout and regular exercise.

To be effective, a very high-fat diet is required, so it is sensible to establish whether it is really necessary before you start pouring in the oil. Horses usually respond to the dietary changes within four months, but before radically altering a horse’s diet, it is worth consulting your vet.

This veterinary feature was published in full in Horse & Hound (2 June, ’05)