Muscle disorders are an area of equine research that have made tremendous progress over the last two decades. Ongoing research has revealed that there are a number of underlying disease processes that cause a similar set of symptoms. PSSM Type-1  was the first of these muscle disorders that was identified and the body of knowledge surrounding this disorder is the most comprehensive, but information on newer muscle disorders, like PSSM Type-2 and Myofibrillar Myopathy (MFM), is published frequently.

Symptoms of Muscle Disorders

Symptoms of muscle disorders vary from reduced performance and behavioral issues, to muscle soreness, elevated muscle enzymes or episodes of tying up. The muscle enzymes CK and AST increase after exercise in healthy horses, but increases in these enzymes outside of the standard range is indicative of muscle damage and is often used as a diagnostic indicator of a muscle disorder; however, increases in CK and AST are not present in all muscle disorders. Tying up, or exertional rhabdomyolysis, is characterized by a horse who becomes reluctant to move, stiff, and sweats excessively during exercise.

What Breeds of Horses Are Impacted

Horses of many breeds are impacted by the various muscle disorders. PSSM Type-1 is found in a number of draft breeds, as well as stock horse breeds. PSSM Type-2 is also found in stock breeds and tends to be more prevalent in quarter horses and related breeds that are bred for performance disciplines, as compared to the tendency for PSSM Type-1 to be more prevalent in horses bred for halter classes. PSSM Type-2 has also been diagnosed in a number of other light horse breeds, including Thoroughbreds, Arabians, Standardbreds, and Warmbloods.

PSSM Type-1 vs PSSM-Type 2 and MFM

PSSM Type-1 is a glycogen storage disease caused by a mutation in the GYS1 gene. This mutation causes an overabundance of glycogen to be stored in the muscle. A genetic test has been developed for PSSM Type-1, allowing this disease to be definitively diagnosed in horses.

Horses with similar symptoms to PSSM Type-1, including the appearance of the muscle under a microscope, that are negative for the GYS1 genetic mutation are characterized as PSSM Type-2. The underlying disease process for PSSM Type-2 is unknown at this time; however, further research has separated out some horses that would have previously been diagnosed with PSSM Type-2 with the diagnosis of Myofibrillar Myopathy (MFM) based on additional testing procedures.

Horses with MFM are characterized by a disruption of the orderly alignment of the contractile proteins in muscle that are called myofibrils. Myofibrillar Myopathy has been identified in Warmbloods and Arabians and is thought to be the result of improper signaling to the muscles from the stresses of exercises, which causes lack of muscle repair, damage to muscle cells and oxidative stress.

Proper Diet for Horses with PSSM Type-2

Horses with PSSM Type-2 were originally fed with the same dietary interventions as horses with PSSM Type-1: namely low non-structural carbohydrate (NSC; sugar and starch) diets with an increased level of fat. As PSSM Type-2 is ultimately not a glycogen storage disease, this type of diet often had little impact. While the exact mechanism of muscle damage is unknown in PSSM Type-2, more recent research suggests that a focus on high quality amino acid intake is beneficial for these horses. Diets that are low to moderate in NSC with added fat for calories, as needed, is appropriate. Often, adding an extra pound of a ration balancer, such as Essential K® or Wholesome Blends® Balancer, to the diet is beneficial to increase essential amino acid intake. For example, a healthy 1,000-pound, lightly active horse that is fed 5 pounds daily of Kalm ‘N EZ® would receive all of the necessary essential nutrients and amino acids to support their dietary needs. However, the same size horse with PSSM Type-2 would benefit from 5 pounds of Kalm ‘N EZ® plus 1 pound of Essential K® for the added boost of essential amino acids.

Diet for Myofibrillar Myopathy

A high fat, low NSC diet would not be suggested for horses with MFM based on the current understanding that oxidative stress is a contributor to muscle damage. High fat diets have many positive attributes, but in horses already experiencing abnormal response to oxidative stress, the addition of fat may exacerbate symptoms.

Instead, a low to moderate NSC and low fat diet with additional essential amino acids is recommended. Similar to PSSM Type-2, the addition of an extra pound of a ration balancer to the diet is beneficial to increase the horse’s intake of essential amino acids.

Conclusion

There is much that is still unknown about the more recently discovered muscle disorders. Dietary interventions will continue to be refined as the underlying causes of these diseases are better understood. Do you have a horse with a muscle disorder? Let us help you with a personalized feeding plan!
 

Nicole Rambo, Ph.D.