There has been considerable debate over the past two years about the occurrence of the inherited disease Polysaccharide Storage Myopathy (PSSM) in horses in South Africa. This genetic condition occurs in several draught breeds, Quarter Horses, warmbloods, Paints and Morgans, as well as Appaloosas.
The breed most affected is the Belgian Draught Horse, but PSSM is also common in Percherons. On the other hand, the Clydesdale, a British Draught breed, appears to be ‘immune’. In addition, PSSM is found in only a very low proportion of English Thoroughbreds and Arabian horses.
What it is
‘Tying up syndrome’ or ‘Monday morning disease’ has been well known in working horses for more than a century. After resting all Sunday, draught and carriage horses could only pull a cart or plough for a short distance before becoming so stiff and sore they had to stop. It was always thought this was from lactic acid build-up in the muscles.
Recently, however, researchers at the College of Veterinary Medicine in Minnesota in the US, found that there are several different causes of ‘tying up syndrome’. One of them is PSSM, where there’s an abnormal accumulation of glycogen, as well as polysaccharide, in skeletal muscle. The muscle cells in PSSM horses take up blood sugar at a faster rate than in normal horses, mainly because they’re more sensitive to the effects of insulin.
Inside the PSSM muscle cell, an over-active enzyme called ‘glycogen synthase’ converts the blood sugar to glycogen and polysaccharides. These polysaccharides can clearly be seen in muscle biopsies of PSSM horses. This is due to a dominant gene and 50% of foals from a carrier will show symptoms. The college suggests that no carrier horses should be used for breeding. But in SA so far, only the Appaloosa Horse Breeders’ Society has included this criteria in its constitution recommending that all horses used for breeding should test negative for PSSM.
The first signs appear when horses are broken in at about two years old. PSSM horses are unwilling to work, become very stiff and often tend to buck. Saddle fitting is difficult as they often have sore backs. In severe cases they can stop dead, refuse to move and stand sweating and trembling. When ridden, they twist their tails and kick out due to sore muscles.
In older horses, the muscles of the back and hindquarter atrophy and the underlying bones become more visible. Horses on a diet high in oats, maize or other starches are particularly badly affected. A vet can check for muscle breakdown products in the bloodstream or take a muscle biopsy, but to definitely diagnose PSSM, hair or blood must be sent to Minnesota University for analysis.
The effects can be minimised by substituting vegetable oil for the usual concentrated feed and feeding mainly hay. Regular exercise is also important as it prevents the build-up of glycogen in the muscles. There is, however, no cure, and the horse eventually becomes unsteady on its hind legs as its muscles are no longer able to support its weight.
Contact Dr Mac at [email protected]. Please state ‘Horse talk’ in the subject line of your email.