Sarcocystis fayeri-Induced Granulomatous and Eosinophilic Myositis in 2 Related Horses.
Authors: Herd H R, Sula M M, Starkey L A, Panciera R J, Johnson E M, Snider T A, Holbrook T C
Journal: Veterinary pathology
Summary
# Sarcocystis fayeri-Induced Granulomatous and Eosinophilic Myositis Two genetically related paint mares presented with chronic weight loss and gait abnormalities, both displaying marked persistent eosinophilia and intramuscular lateral thoracic masses; histological and molecular analysis (18S rRNA nested PCR with >99% homology) confirmed these lesions as granulomatous eosinophilic myositis caused by *Sarcocystis fayeri*, an apicomplexan protozoan parasite typically considered clinically benign in horses. The pathological presentation differed markedly between cases, with one mare developing severe refractory myalgia and recurrent oesophageal dysphagia progressing to disseminated, grossly visible granulomas throughout all striated muscles at necropsy, suggesting variable pathogenic potential depending on parasitic burden or individual immune response. Although *Sarcocystis* species routinely infect the skeletal musculature of omnivorous hosts without producing clinical signs, this case series documents a rare but clinically significant manifestation—one that clinicians should consider when presented with chronic, progressive myositis accompanied by unexplained eosinophilia and oesophageal dysfunction. The genetic relationship between affected animals raises the possibility of inherited susceptibility to severe disease, though further epidemiological investigation would be warranted to distinguish heritable factors from shared environmental exposure or parasitic strain virulence.
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Practical Takeaways
- •Consider Sarcocystis fayeri as a differential diagnosis in horses presenting with chronic weight loss, abnormal gait, persistent eosinophilia, and intramuscular masses, particularly if granulomatous myositis is confirmed on biopsy
- •Sarcocystosis can cause severe, refractory clinical signs including muscle pain and dysphagia; genetic predisposition may play a role as both affected horses were related
- •Early tissue diagnosis via biopsy and PCR is important for confirming this rare but potentially disseminated parasitic infection
Key Findings
- •Two genetically related paint mares presented with chronic weight loss and gait abnormalities associated with marked persistent eosinophilia and multiple intramuscular lateral thoracic masses
- •Histologic examination revealed eosinophilic, centrally necrotic granulomas with intralesional Sarcocystis fayeri material and intact protozoal cysts in adjacent muscle fibers
- •Case 1 developed disseminated, grossly visible granulomas throughout all examined striated muscles at necropsy, with severe refractory muscle pain and recurrent esophageal dysphagia
- •Nested PCR of 18S rRNA gene confirmed >99% homology with Sarcocystis fayeri, establishing this as a rare cause of granulomatous eosinophilic myositis in horses