Estimated prevalence of polysaccharide storage myopathy among overtly healthy Quarter Horses in the United States.

Abstract

OBJECTIVE: To estimate the prevalence of polysaccharide storage myopathy (PSSM) among Quarter Horses in the United States and evaluate possible relationships between muscle glycogen concentration, turnout time, and exercise level. DESIGN: Cross-sectional study. ANIMALS: 164 overtly healthy Quarter Horses > 2 years old from 5 states. PROCEDURES: Horses with a history of exertional rhabdomyolysis or any other muscular disease were excluded. Muscle biopsy specimens were examined histologically for evidence of PSSM and were submitted for determination of muscle glycogen concentration. A diagnosis of PSSM was made if amylase-resistant inclusions that stained with periodic acid-Schiff stain were detected. RESULTS: Prevalences of PSSM on the 2 farms with a history of PSSM were 20% (1/5) and 40.7% (11/27); mean prevalence for the other 4 farms was 6.1% (8/132). Sex was not significantly associated with a diagnosis of PSSM, and age was not significantly different between horses with and without PSSM. Total histologic score, serum creatine kinase activity, and muscle glycogen concentration were significantly higher in horses with PSSM than in horses without. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that the prevalence of PSSM among overtly healthy Quarter Horses in the United States is likely to be between 6% and 12%.