Pathologic Findings in Equine Muscle (Excluding Polysaccharide Storage): A Necropsy Study

Abstract

Abstract. Gross and histopathologic evaluation of skeletal muscle was performed in 229 equids (217 horses, 8 ponies, 3 donkeys, and 1 mule) 1 year of age or older undergoing postmortem examination at Oregon State University in a 2.5-year period. Animals were evaluated for grossly evident muscle lesions, and muscle samples were fixed in formalin, processed routinely, and stained with hematoxylin and eosin (HE) and periodic acid-Schiff (PAS) for glycogen. Muscle lesions were detected in 149 animals (65%). Chronic myopathic change (excessive fiber size variation and internal nuclei) was evaluated in horses without polysaccharide storage myopathy and was the most common finding (36 animals; 15.7%). Chronic myopathic change was more common in older animals. Generalized muscle atrophy was present in 30 animals (13.1%). Myonecrosis was attributed to endotoxic injury (11 animals; 4.8%), bone fracture (8 animals; 3.5%), bacterial infection (5 animals; 2.2%), muscle rupture (3 animals; 1.3%), selenium deficiency (2 animals; 0.9%), and exertional rhabdomyolysis (1 horse; 0.4%); cause was not determined in 9 animals (3.9%). Intramyofiber protozoa were detected in 19 horses and ponies (8.3%). Denervation atrophy was detected in 14 animals (6.1%). Neoplasia involving muscle occurred in 3 animals (1.3%), injection site reactions were detected in 4 animals (1.7%), and focal lymphocytic infiltrates were found in 6 animals (2.6%). Other findings were ring fibers (2 horses; 0.9%), fiber splitting (2 horses; 0.9%), and fat infiltration (1 horse; 0.4%). Skeletal muscle lesions are common in equids examined at postmortem. Transverse sections stained with HE and PAS are invaluable when evaluating equine muscle.

Muscular senescence in cetaceans: adaptation towards a slow muscle fibre phenotype

“…Some of these changes, in particular, increased fibre size variation and internal nuclei, have been traditionally considered as chronic myopathic changes, which are defined as nonspecific findings associated with a variety of neuromuscular disorders32. However, they have been previously associated with increasing age in equids33 and humans13. Fibre type grouping, as was observed in the muscles of the older cetaceans in our study, is the phenomenon of clustering of a specific fibre type being rather than the random distribution observed in younger muscles.…”

Sarcocystis fayeri in skeletal muscle of horses with neuromuscular disease

“…Prevalence of infection could vary with geographic location throughout the world. [16,17,21] In this study, geographic origin of equids with neuromuscular disease included California, Colorado, Indiana, Ohio, Oregon, and the United Kingdom. The majority of the equids with neuromuscular disease in this study (82.9%) and all healthy horses were natives of California.…”

“…[16,21] Diaphragm, heart, tongue, and esophageal muscles have been reported to have higher parasite loads. [16,21] The prevalence of intramyofiber protozoal cysts in a postmortem study of 229 equids was 8% (18 horses, 1 pony) [17], and was the third most common histological finding in skeletal muscle. [17] Identification of intramyofiber-encysted parasites in horses has been possible through histological, ultrastructural, immunohistochemical, and molecular studies.…”

“…[16,21] The prevalence of intramyofiber protozoal cysts in a postmortem study of 229 equids was 8% (18 horses, 1 pony) [17], and was the third most common histological finding in skeletal muscle. [17] Identification of intramyofiber-encysted parasites in horses has been possible through histological, ultrastructural, immunohistochemical, and molecular studies. [14,22,23] The prevalence of intramyofiber encysted protozoans has not been investigated specifically in horses with neuromuscular disease.…”

“…[8,[14][15][16] Similar to other species, the presence of sarcocysts in equine skeletal muscle has been considered an incidental finding since muscle pathology has not been observed. [17] However, there have been sporadic reports of myositis in horses associated with Sarcocystis spp. [18,19] Furthermore, experimental infection of ponies and horses with S. fayeri has resulted in severe myositis and stiff gait.…”