Warmbloods with PSSM2 often exhibit chronic signs related to poor performance, including undiagnosed gait abnormalities, sore muscles, decreased energy levels, firm and painful back and hindquarter muscles, reluctance to collect and/or engage the hind end, poor rounding over fences, and slow-onset atrophy. | Photo: iStock

It wasn’t long ago that horses with polysaccharide storage myopathy—a condition that results in abnormally high levels of glycogen (stored sugar) in their muscles, lack of energy during exercise, and persistently elevated serum creatine kinase levels (indicative of muscle damage) during and after exercise—had just that: PSSM.

Researchers have learned, however, that some affected horses have a genetic mutation in glycogen synthesis behind their clinical signs while others do not, suggesting there are at least two subtypes of the disease: type 1 PSSM (PSSM1) and type 2 PSSM (PSSM2), respectively.

Study results have shown that horses with PSSM1 benefit from a carefully crafted diet and a regular exercise program. While veterinarians often recommend the same protocol for PSSM2 horses, how these horses respond to that treatment hasn’t been documented. So Zoë Williams, a DVM and PhD student at Michigan State University (MSU), in East Lansing; Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, neuromuscular disease expert and Mary Anne McPhail Dressage Chair in Equine Sports Medicine Professor at MSU; and MSU colleagues conducted a retrospective study to fill in the gap. Williams shared the team’s findings at the 2018 American College of Veterinary Internal Medicine Forum, held June 14-16 in Seattle, Washington