Feeding Horses with Certain Muscle Disorders

Karen E. Davison , Ph.D.

Director, Nutritionist, Equine Technical Solutions

Muscle disorders cause loss of performance and value in affected horses and may present with a variety of clinical signs ranging from muscle stiffness and pain to muscle atrophy, weakness, exercise intolerance, and muscle fasciculations. Although originally thought to be a single clinical syndrome, it is now clear that these clinical manifestations are common to several different muscle disorders with different etiologies.1 Recently metabolic, inflammatory, dystrophic and other inherited muscle diseases have been described in horses.  Proper diagnosis of the specific myopathy involved is critical for determining the appropriate targeted management regimen. This article will focus on muscle disorders with known nutritional management recommendations to help support affected horses. Specially discussed here are Polysaccharide Storage Myopathy, Recurrent Exertional Rhabdomyolysis and Hyperkalemic Periodic Paralysis.

Polysaccharide Storage Myopathy (PSSM)

Tying up was first described in heavy draft horses and officers’ riding horses as early as 1840 by a French army veterinarian. The term “Monday Morning Disease” was coined due to the frequency of symptoms appearing in working draft and carriage horses on the Monday following a weekend of rest, while consuming a full ration of oats or sweet feed.
Muscle biopsy studies reported by Dr. Stephanie Valberg in 1992 revealed that horses affected with PSSM store 1.5 to 4 times the muscle glycogen as normal horses. In addition, an abnormal polysaccharide that doesn’t break down properly during exercise was found in the muscle fibers of some PSSM horses. This led to the conclusion that there were possibly multiple forms of PSSM. Currently, horses with both elevated glycogen and the abnormal polysaccharide are identified as type 1 PSSM, whereas horses with elevated glycogen stores but no abnormal polysaccharide are identified as type 2 PSSM.

Type 1 PSSM

Horses with PSSM1 are more sensitive to dietary starch and sugar levels, because they accumulate more muscle glycogen than normal horses or horses with PSSM2. PSSM1 horses also benefit from more calories supplied by dietary fat because they possess the GYS1 mutation and store an amylase-resistant polysaccharide which cannot be used as substrate for working muscle fibers. Training adaptation and a diet consisting of higher dietary fat can shift certain muscle fibers to rely more on fatty acids, and less on glycogen, to fuel exercise.
Diets for horses with PSSM1 should be low in starch and sugar (nonstructural carbohydrates – NSC) and supply more calories from vegetable oils. “Easy keepers” or less active PSSM1 horses may not be able to consume higher fat levels without becoming overweight. In these cases, use forages low in starch and sugar (<12% NSC) and a ration balancer, like Purina® Enrich Plus® horse feed, to meet nutrient requirements. Rather than provide dietary fat to an overweight horse, fasting for 6 h prior to exercise can be used to elevate plasma free fatty acids prior to exercise and alleviate any restrictions in energy metabolism in muscle. Specific dietary NSC and fat levels that best mange each case will vary by horse and total calorie demand.

Type 2 PSSM

Horses with PSSM2 have abnormal muscle histology with glycogen aggregation, but do not possess the GYS1 mutation, accumulate excess glycogen or store an abnormal polysaccharide. Dietary recommendations for PSSM2 are not as well defined, but PSSM2 horses don’t seem to be as sensitive to dietary starch and sugar levels as PSSM1 horses. Current recommendations for PSSM2 cases call for the use of low- to moderate-NSC feeds and fat supplementation based on the horse’s energy needs. Research suggests that amino acid supplementation, using whey protein-based supplements like Purina® SuperSport® Amino Acid Supplement, can be helpful in optimizing muscle recovery following exercise.1 These supplements may also be helpful when managing symptoms in horses with PSSM2.2

There is no one-size-fits-all diet for horses with PSSM1 or PSSM2. Finding the levels of soluble carbohydrates, fat and other supportive nutrients such as amino acids that best manage PSSM symptoms for an individual horse may require some trial and error.

Recurrent Exertional Rhabdomyolysis (RER)

RER is a complex disease with no identified genetic cause. It is an intermittent form of tying-up that appears to involve an abnormality in intracellular calcium regulation currently diagnosed by a history of repeated episodes. Managing horses with RER revolves around managing stress, avoiding stall rest, and administering medication if needed. Diet management is less restrictive than for PSSM but RER horses may also be better managed by substituting fat calories for starch and sugar calories. This shift in calorie sources may reduce anxiety and excitability, which may trigger episodes in susceptible horses.

Summary and Diet Examples

Managing symptoms in horses with RER or PSSM through diet and regular exercise has shown to be quite effective. Best results are reported when regular exercise is combined with diet management, suggesting that the exercise component may be equally important to dietary alterations. The general guidelines of reducing dietary starch and sugar content and increasing the fat content of the diet consistently proves beneficial but the specific best diet seems to vary from case to case. This variation may be partially because horses with these conditions represent quite a range in age and activity level, but also may be due to variation in individual starch/sugar sensitivity and need for dietary fat to control symptoms.
  • Example diets:
    • PSSM – 1100 lb QH, moderate work, 23.31 Mcal/day
      1. 7.5 lbs Ultium®, 0.5 lbs SuperSport® 12 lbs good quality grass hay
      2. 6 lbs WellSolve® L/S, 2 cups oil, 0.5 lbs SuperSport® 16 lbs good quality grass hay
      3. 1 lb Enrich Plus®, 4 lbs Amplify®, 18.5 lbs good quality grass hay
    • RER – 1100 lb TB – intense work, 34.48 Mcal/day
      1. 8.5 lbs Race Ready®, 4 lb Amplify®, 16.5 lbs good quality grass hay
      2. 12.5  lbs Ultium®, 14 lbs good quality grass hay
      3. 10 lbs Omolene® 400, 4 lbs Amplify®, 16.5 lbs good quality grass hay
The diet recommendations above are examples of diets that have successfully supported horses while managing these health issues. There are many options available and specific diets can be designed based on evaluation of the horse, available hay or pasture and considering the age/activity level of the horse, as well as preferences expressed by the horse owner. Often, arriving at the best diet recommendation for an individual horse involves starting within the established general guidelines and then fine tuning. Also, over time, the horse may change in their response and further diet adjustments may be needed for continued benefit.          

Hyperkalemic Periodic Paralysis (HYPP)

Horses with HYPP have a genetic mutation that causes an interruption of the flow of sodium and potassium in and out of muscle cells. Many HYPP-positive horses only show symptoms when stressed, anesthetized or when consuming higher potassium diets. As with other muscle disorders, controlling symptoms involves managing stress and providing regular exercise along with specific dietary modifications. The diet modification most noted for managing HYPP episodes is controlling dietary potassium (K) to ≤1.1% in the total diet. 
Hay, pasture and many higher fiber ingredients typically contain 1.1 – 3.36% potassium.  Alfalfa tends to be higher than grass hay varieties, but fertilization and other factors can significantly affect potassium content in any given hay. Analyzing hay for potassium content may be helpful in more consistently managing HYPP horses. In addition, soaking hay for 1 hour in tap water and draining the water can leach out potassium and help lower potassium content of the hay. 
Grains, such as oats, are low in potassium but do not provide balanced nutrition to support long-term health. Grain mixes with fortification, such as Purina® Omolene® typically contain ≤1.1% potassium in a balanced formulation. Molasses is a high-potassium ingredient and generally recommended against feeding to horses with HYPP. However, Purina® uses an oil-blended molasses rather than straight cane or sugar beet molasses. This molasses blend combined with lower potassium ingredients results in Purina® sweet feed formulas that fall within the recommended potassium guidelines. In addition to these textured feeds being lower in potassium than most pelleted feeds, the higher starch content may benefit HYPP horses by causing a slightly higher insulin response to a meal, which helps drive potassium back into the cells. This is the principle behind administering 60 – 120 mL lite Karo syrup orally during an HYPP episode.
Restricting potassium to ≤ 33 grams per meal may help keep HYPP horses asymptomatic. This suggests that the amount of potassium per meal is more critical than the percent potassium in the diet components. Considering the amount of potassium per meal is important in designing a diet for HYPP horses. For example, an HYPP horse that is an easy-keeper and can’t eat the minimum recommended feeding rate of Omolene 200 without becoming overweight, may be well supported by 1 – 2 lbs/day of Purina® Enrich Plus® ration balancer and a controlled amount of hay, even though Enrich Plus® contains more than 1.1% potassium. It is important to note that hay or pasture contributes the greatest amount of potassium in the horse’s diet. 
Example diets:
Horse Concentrate Concentrate Potassium g/day Hay Hay  Potassium g/day Total Potassium
Recommended number of meals/day
1100 lb horse – moderate work 5 lbs Omolene 200 (1.0% K) 22.7 20 lbs Timothy Hay
(1.5% K)
136.2 158.9 5
1100 lb horse – maintenance 2 lb
Enrich Plus (1.5% K)
13.62 18 lbs Bermudagrass Hay
(1.5% K)
81.72 108.96 4
1100 lb horse – moderate work 4.4 lbs
Ultium Competition (1.25% K)
24.97 20 lbs Bermudagrass Hay
(1.1% K)
99.88 124.85 4
HYPP horses are extremely sensitive to dietary changes but appear to adapt to higher potassium diets over a 2-week period. Using slow feeders for hay and dividing concentrate supplementation into smaller, more frequent meals are management options that support a more consistent plasma potassium concentration. 
Additional diet recommendations for HYPP horses include providing access to clean, fresh water and free-choice salt. If individual horses do not consume adequate salt from a salt block, adding 1-2 TBS/day of loose salt to the concentrate ration would be recommended. Water and salt intake is important for all horses but especially for HYPP horses being maintained on diuretic medication. Electrolyte supplements are not recommended due to the potassium content.
Successful management of HYPP horses requires controlling dietary potassium levels and relies on maintaining a low stress environment, a consistent routine and making very slow changes in the diet. This slow change in diet includes the source of hay. Even when feeding the same variety of hay, a new supply of hay can be significantly different in digestibility and potassium content which can increase the risk of triggering HYPP symptoms. 
1 Vineyard, K.R., M.E. Gordon, P. Graham-Thiers, and M. Jerina. 2013. Effects of daily administration of an amino acid-based supplement on muscle and exercise metabolism in working horses. Journal of Equine Veterinary Science, Vol 33(5).
2 Williams ZJ, Bertels M, Valberg SJ. 2018. Muscle glycogen concentrations and response to diet and exercise regimes in Warmblood horses with type 2 Polysaccharide Storage Myopathy. PLoS ONE 13(9): e0203467.

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