Myofibrillar Myopathy in Warmblood Horses

The understanding of muscle disease in horses has broadened considerably over the last few decades. Researchers continue to discover nuances in muscle dysfunction, and one disease continues to gain recognition within the Warmblood and endurance communities: myofibrillar myopathy (MFM).

Like those diagnosed with polysaccharide storage myopathy (PSSM), horses with MFM demonstrate exercise intolerance. Microscopic examination of muscle tissue reveals, however, that the underlying cause of MFM is much different than it is for PSSM.

“Initially, we were looking at PSSM thinking it might be one disease,” said Stephanie Valberg, D.V.M., Ph.D., Dipl. ACVIM, ACVSMR, Director of the Equine Neuromuscular Diagnostic Laboratory and Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University, Department of Large Animal Clinical Sciences. Her early research in Quarter Horses revealed a genetic mutation leading to a buildup of amylase-resistant polysaccharide (type 1 PSSM). However, upon further review, 82% of Warmblood muscle biopsies in storage at Dr. Valberg’s lab did not have this genetic mutation (nor did any of the Arabians), leading the researchers to identify MFM as a potential source of exercise intolerance in this group.

Endurance Arabians with MFM have episodes of tying-up whereas Warmblood horses with MFM will display a reluctance to collect. Using electron microscopy, Dr. Valberg found that some myofibrils, the contractile proteins within the cells, were degenerating and in disarray instead of being well aligned. This breakthrough enabled some horses that were previously classified as type 2 PSSM to be diagnosed with MFM.

Learn more about MFM in warmbloods and the development of feeding strategies to help affected horses in this interview with Dr. Valberg.