Myosin heavy-chain myopathy in 2 American quarter horses.
Authors: Faccin Mayane, Landsgaard Kirsten A, Milliron Sarai M, Jennings Alexis H, Keith Chaffin M, Giaretta Paula R, Rech Raquel R
Journal: Veterinary pathology
Summary
# Myosin Heavy-Chain Myopathy in American Quarter Horses Two young American quarter horses—a 1.5-year-old gelding and an 11-month-old filly—presented with acute pelvic lameness, lethargy, and elevated serum creatine kinase, prompting investigation into an underlying myopathy. Muscle biopsies from both cases revealed strikingly similar patterns of polyphasic myositis characterised by histiocytic and lymphoplasmacytic infiltration, fibre necrosis, mineralisation, and regenerative changes; genetic testing identified heterozygous and homozygous E321G MYH1 variants respectively, confirming myosin heavy-chain myopathy. The gelding additionally developed suppurative retropharyngeal lymphadenitis secondary to *Streptococcus equi* subsp. *equi* with concurrent renal involvement (purpura haemorrhagica and myoglobinuric nephropathy), whilst the filly presented with rapid muscle wasting and a focal pulmonary abscess caused by *Actinobacillus equuli*. These cases suggest that concomitant bacterial infections may act as clinical triggers for expression of underlying myosin heavy-chain defects, with homozygous carriers appearing to manifest more severe phenotypes. Clinicians should consider MYH1 myopathy in young quarter horses presenting with acute myositis and lameness—particularly when secondary bacterial disease is documented—as genetic counselling and breeding recommendations may be warranted.
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Practical Takeaways
- •Acute pelvic lameness with elevated muscle enzymes and rapid muscle atrophy in young Quarter Horses warrants genetic testing for MYH1 variants, as this hereditary myopathy may be triggered or worsened by concurrent bacterial infections
- •Homozygous carriers of the E321G MYH1 variant appear more severely affected than heterozygotes; screening breeding stock is prudent to reduce disease incidence in Quarter Horse populations
- •When myosin heavy-chain myopathy is suspected, investigation for concurrent infections (strangles, respiratory infections) is important as bacterial disease may precipitate clinical signs in genetically predisposed animals
Key Findings
- •Two American Quarter Horses (1.5 years and 11 months old) presented with acute pelvic lameness, elevated creatine kinase, and histologic myositis with necrosis and regeneration
- •Genetic testing identified E321G MYH1 variant: case 1 heterozygous, case 2 homozygous, confirming myosin heavy-chain myopathy diagnosis
- •Concurrent bacterial infections (Streptococcus equi and Actinobacillus equuli) were identified as potential disease triggers in both cases
- •Homozygous mutation (case 2) presented with more severe clinical signs including rapid muscle atrophy compared to heterozygous presentation (case 1)