Suspected immune-mediated myositis in horses.

The research investigates the occurrence of immune-mediated myositis (IMM), a condition typically characterized by rapid muscle atrophy and inflammation, in horses. The evidence points to this condition being a common cause of muscular issues in horses, particularly those of Quarter Horse bloodlines, with possible treatment methods identified.

Study Design and Methods

• The researchers reviewed the Neuromuscular Diagnostic Laboratory database and identified 37 horses with muscle biopsies marked by lymphocytic infiltrates, an indicator of inflammation and potential IMM.
• A standard retrospective questionnaire was used to gather information about the clinical signs observed in these horses and their responses to treatments applied. This questionnaire was filled out by the horse owners.

Study Findings

• The majority of the horses with suspected IMM were Quarter Horses and were either less than or equal to 8 years old or more than or equal to 17 years old.
• Typical clinical signs included quick muscle loss, particularly in the epaxial and gluteal muscles, along with indications of depression and stiffness.
• Horses showed significantly high Creatine kinase (CK) activity and aspartate transaminase (AST) activity, both of which are indicators of muscle damage.
• The questionnaires revealed that exposure to horses with infectious respiratory diseases occurred in 39% of the horses prior to the onset of IMM symptoms.
• It was observed that in 47% of the cases, there was a recurrence of muscle atrophy.
• On examining the muscle biopsies, increased numbers of Macrohpages and CD4+ T lymphocytes were found. Lesser numbers of CD8+ cells and small clusters of B lymphocytes were also present.

Treatment Effectiveness and Conclusions

• Assessment of treatment efficacy was challenging due to variations in corticosteroid dosage and timing of administration.
• However, the presence of lymphocytic infiltrates found in atrophied muscles is a strong indicator of IMM, assisting in diagnosis.
• The study concludes that IMM seems to be a distinct cause of rapid muscle atrophy, especially in Quarter Horses.
• Although the research did not provide definitive conclusions on treatment, the use of corticosteroids was seen as potentially feasible treatment method.