By Clair Thunes
Q.I have an upper-level show jumper, and she’s had a few bouts of “tying-up” over the past few years. My veterinarian recently did a muscle biopsy and diagnosed my mare with recurrent exertional rhabdomyolysis (RER). As I understand it, RER isn’t the same as polysaccharide storage myopathy (PSSM, which also causes affected horses to tie up and requires nutritional management), but I’m wondering if diet changes might help?
A.I applaud you getting a definitive diagnosis by having the biopsy done so you know exactly which disease is afflicting your mare. As you state, RER and PSSM are not the same thing, even though both can result in a horse tying up. Researchers believe RER is due to an inherited abnormality in the regulation of muscle contraction and relaxation. There are different forms of PSSM: Horses with type 1 have genetic mutations of the glycogen synthase gene, whereas those with type 2 lack this mutation but show abnormal staining of glycogen when biopsy tissue undergoes microscopic examination.
NSCs and RER
Despite the different causes of tying-up between RER and PSSM, both conditions benefit from lower dietary nonstructural carbohydrate (NSC, the sum of the starch and water-soluble carbohydrates [WSC] in the diet) intake. Veterinarians recommend feeding PSSM horses diets low in NSC, because these carbohydrates have a direct impact on muscle glycogen; however, RER doesn’t affect muscle glycogen stores, so the reduced NSC is for a different reason. Horses with RER might tend toward excitability and tension, and high-NSC diets are more likely to cause these behaviors, which in turn may lead to muscle damage.
RER: Starch Calories vs. Fat Calories
RER is often seen in horses with Thoroughbred ancestry, so research in managing the condition has focused on this breed. Interestingly, mares tend to have higher incidences of RER than colts and geldings. Research using Thoroughbreds with RER showed that, when these horses were fed a moderate level of calories that included a high-starch concentrate at 2.5 kilograms per day, they showed few signs of tying-up when exercised. However, when researchers increased calorie and high-starch concentrate intake, the horses became more likely to tie up. This did not occur if fat provided the additional calories. For this reason, when horses with RER need extra calories, they should derive no more than 20% of their daily calories from NSC, and 20-25% of their calories should come from fat.
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