Inflammatory Myopathy in Horses With Chronic Piroplasmosis.

This research investigates the pathological and immunopathological aspects of muscle disease occurring in horses affected by chronic piroplasmosis. They found that inflammatory muscle disease associated with equine piroplasmosis may involve an autoimmune response with increased regulation of inflammatory proteins that may cause muscle breakdown and degeneration.

Study Design and Implementation

• The study examined 16 horses that tested positive for equine piroplasms and were showing clinical signs of a muscle disease, including poor performance and muscle atrophy.
• Several evaluation methods were used on the horse’s skeletal muscle including histopathology (microscopic examination), immunohistochemistry (localization of antigens or proteins in tissues), indirect immunofluorescence, and molecular detection of piroplasms and inflammatory proteins in skeletal muscle.

Results and Observations

• Upon histological examination, it was found that all of the horses had muscle fiber atrophy. Most of them (81%) also showed degenerative changes and perivascular perimysial and endomysial lymphocytic infiltrates (type of white blood cells).
• Through immunostaining, it was established that muscle fibers expressed strong positive staining for major histocompatibility complex classes I and II in 15 out of 16 horses, indicating an immune response.
• T lymphocyte populations were primarily composed of CD3+, CD8+, and CD4+ cells in equal proportions, with lesser amounts of CD79α+ cells. This provides a marker of the type of immune response being mounted against the muscle tissue.
• The serum from affected horses was evaluated for binding of IgG, IgM, or IgA to sections of normal equine muscle to detect circulating autoantibodies, indicating an autoimmune response. In all cases, distinct sarcolemmal staining was seen in sections treated with serum from affected horses. This contrasted with sections treated with phosphate-buffered saline or control equine sera.

Interpretation and Potential Implications

• RT-PCR testing of muscles from affected animals revealed a significant increase in expression of genes for interferon-γ, interleukin-12, and tumor necrosis factor-α, compared to healthy controls. These proteins are typically involved in the inflammatory response.
• Neither Theileria equi nor Babesia caballi, common pathogens in equine piroplasmosis, were detected in muscle samples from the affected animals. This suggests that the observed myopathy is likely not directly caused by an active infection with these organisms, but rather some other factor, possibly an autoimmune response.
• The findings propose that inflammatory muscle disease associated with equine piroplasmosis may involve an autoimmune pattern with increased regulation of inflammatory proteins that may cause muscle cell atrophy and degeneration.