Research carried out by Dr. Stephanie Valberg of the University of Minnesota has thrown new light on to exertional myopathies (“Tying up”) in horses. Valberg’s work focused on thoroughbreds and quarterhorses. However, there are practical take home messages for all horses and disciplines from this work.
Valberg has identified an apparent inherited condition known as Recurrent Exertional Rhabdomyolysis (RER) in thoroughbreds. Affected horses can be identified because of chemical differences in the contractility of their muscles. It appears that approximately 5% of thoroughbreds carry the gene for RER, however, not all horses with the gene suffer tying up. Certain stressors appear necessary for horses with this gene to suffer tying up and minimizing these stressors offers a practical approach to avoiding or managing the condition.
Nervous and excitable horses are prone to develop tying up. This includes young horses, especially fillies. Horses on a high grain diet receiving excessive calories are also at risk. Holding horses back in their work, fighting to keep them at a slow pace is another risk factor.
Managing RER in thoroughbreds involves decreasing excitability, ensuring that there is not an overfeeding of calories and decreasing soluble carbohydrate intake by replacing grain with fat. Regular daily exercise is essential and horses should not be wound up. Owners should also remember that the condition is inherited and affected parents are likely to produce affected offspring.
Valberg identified a different inherited condition in quarterhorses. This is known as a polysaccharide storage disorder. Affected horses have up to one-and-a-half times more glycogen in their muscle cells than normal horses and this can be identified with special staining techniques on muscle biopsies. Affected horses have an enhanced glucose clearance test and increased insulin sensitivity. This explains why they are very good-doers or easy keepers.
In terms of managing these horses it is essential that they are introduced to training slowly with a lot of daily exercise including turn out whenever possible. Grain should be removed from the diet and replaced with 1 to 4 lbs. of rice bran which is a fat supplement. The rest of the diet can usually be made up with grass-hay. Valberg acknowledged the generous support of the American Quarter Horse Association, The Grayson Foundation and the Morris Animal Health Foundation.
Reprinted from Equine Canada, Vol. 2 No. 2
Journal of the Equine Research Centre
Guelph, Ontario, Canada