Pathologic findings in equine muscle (excluding polysaccharide storage): a necropsy study.

This research examined 229 animals, including horses, ponies, donkeys, and mules, for muscle lesions. The majority of the animals exhibited such lesions, with the most common being chronic myopathic change. The study also uncovered various other muscle abnormalities and emphasized the importance of postmortem examinations in equine muscle evaluation.

Examination Methods and Animal Samples

• Researchers conducted an extensive study of 229 equids, consisting of 217 horses, 8 ponies, 3 donkeys, and a mule, at Oregon State University over a period of 2.5 years. Only animals that were one year of age or older were considered for the study.
• The research made use of necropsy (post-mortem examination) to evaluate the gross and histopathological condition of the equid skeletal muscle. This involved looking for visually apparent muscle lesions and sampling muscle tissues for further analysis.
• The muscle samples were preserved in formalin, processed as per standard procedures, and stained with hematoxylin and eosin (HE) and periodic acid-Schiff (PAS) for glycogen to highlight any changes or abnormalities.

Findings

• Of the examined animals, 65% (149 animals) showed evidence of muscle lesions. The most frequently observed condition was chronic myopathic change – excessive fiber size variation and internal nuclei – seen in 36 animals. This represents 15.7% of the total sample, and was more prevalent in older animals.
• The research team also found evidence of generalized muscle atrophy in 30 animals (13.1%) and other muscle damage due to a variety of causes, including endotoxic injury, bone fractures, bacterial infections, muscle ruptures, exertional rhabdomyolysis, and selenium deficiency.
• Notably, in some cases, the cause of the muscle damage remained unknown.
• Protozoa inside muscle fibers were found in 8.3% of the animals, predominantly horses and ponies. Meanwhile, denervation atrophy was observed in 14 animals, or 6.1% of the sample.
• The data revealed instances of neoplasia (cancer) involving muscle, injection site reactions, and local lymphocytic infiltrates, a sign of immune response.
• Lesscommon findings included ring fibers, fiber splitting, and fat infiltration.

Implications

• The research highlights that skeletal muscle lesions are a frequent occurrence in equids, as demonstrated by the examination data.
• It also underscores the invaluable role of hematoxylin and eosin (HE) and periodic acid-Schiff (PAS) staining in evaluating equine muscle damage, providing a visual contrast to easily distinguish pathology.
• The varied range of findings underlines the variety of conditions equids can suffer from, and thus the importance of regular and detailed postmortem examinations as a key tool for veterinary pathologists and equine health professionals.