German Warmblood Breed Societies Support PSSM Genetic Research

Several German breed societies recently announced their support of research into polysaccharide storage myopathy (PSSM), a genetic muscle disease that affects certain horses, including many Warmbloods.^*

When PSSM in horses was first discovered, it was thought to be a one-dimensional disease. Initial research showed that Quarter Horses with clinical signs of tying-up accumulated abnormal amylase-resistant polysaccharide in muscle cells. With exercise, these horses had substantially more glycogen in their muscles and persistent elevations in certain serum enzymes indicative of muscle damage. Eventually, researchers learned that a low-starch, high-fat diet would keep horses from showing clinical signs of PSSM. Continued research into PSSM in Quarter Horses resulted in the discovery of a genetic mutation in the glycogen synthase 1 gene. The form of PSSM resulting from the genetic mutation was termed PSSM1 (type 1 PSSM).

However, not all horses with PSSM had the genetic mutation, revealing PSSM to be a multifaceted disease. Those horses without the mutation are now known as PSSM2 (type 2 PSSM). “Research has been focused on looking at PSSM2 in a breed-specific fashion and trying to understand this second form of PSSM,” said Stephane Valberg, D.V.M., Ph.D., D.A.C.V.I.M., Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University.

“When we chose to look more closely at Warmbloods, we used electron microscopy to evaluate the fine detail within a muscle cell. We found that the contractile proteins that are organized into units called myofibrils were disarranged in some areas of the cells,” she explained. “In an attempt to find an easier way to detect this disruption, we started staining for desmin, a protein that keeps myofibrils aligned. In areas where myofibrils were abnormal, desmin seemed to pool and clump. We used the stain as a way to classify some horses with PSSM2 as having myofibrillar myopathy, meaning they have a muscle disease in which some cells have disorganization of the contractile proteins.”

Warmbloods with myofibrillar myopathy (MFM) tend to progress normally in their training until they are about six to eight years old. Then, according to Valberg, they become more reluctant to exercise, including escalating resistance to certain aids that often progresses to sourness.

“Riders of these horses are often left feeling frustrated because they know the horses are physically capable of what is being asked of them, yet now they can’t or won’t perform,” she explained.

When riders encounter this roadblock, they often look at the larger picture, such as evaluating saddle fit, assessing management changes, and employing a veterinarian to perform a thorough lameness examination. When these bring no resolution, a muscle biopsy is often ordered and then a diagnosis is made.

While there is no panacea for MFM, dietary changes can help horses return to comfortable work.

Among the changes, the addition of n-acetyl cysteine to the diet in the form of MFM Pellet helps affected horses. “We’ve found these horses don’t seem to have an appropriate amount of cysteine-based antioxidants,” said Valberg. The n-acetyl cysteine in MFM Pellet forms cysteine, an amino acid that helps build antioxidants in deficient horses.

According to Valberg, about 80% of PSSM cases diagnosed by biopsy in Warmbloods are PSSM2, and affected breeds include Hanoverians and Westphalians. According to the report, the five most prominent breed societies in Germany—those that keep pedigrees for Hanoverians, Oldenburgs, Westphalians, Holsteiners, and Trakehners—are making DNA profiles available for research.^*

Watch a video that explains the discovery of myofibrillar myopathy and proper nutrition of affected horses.

Joe Pagan, Ph.D., founder of Kentucky Equine Research, and Valberg wrote a paper for the 2020 American Association of Equine Practitioners Conference, held virtually. Read the paper, Feeding Performance Horses with Myopathies.

^*German Breed Societies Support Gene Research into PSSM2. 2021. EuroDressage. (Accessed 28 March 2021).