Immune-Mediated Myositis
Stephanie J. Valberg
The majority of horses with immune-mediated myositis (IMM) are of Quarter Horse bloodlines and are either 8 years of age or younger or 16 years of age or older.26,100-102 In approximately 40% of horses with IMM, a triggering factor appears to have been exposure to S.
equi or a respiratory disease. The most prominent clinical sign of IMM in Quarter Horses is rapid onset of muscle atrophy, particularly affecting the back and croup muscles (Fig. 42.11), accompanied by stiffness and malaise. Atrophy may progress to involve 50% of a horse's muscle mass within a week and may lead to generalized weakness.Hematologic abnormalities are relatively minor in affected Quarter Horses unless a concurrent infection is present and are usually restricted to mild to moderate elevations in serum CK and AST activity. However, in some cases with chronic atrophy, serum muscle enzyme activities are normal. Muscle biopsy of epaxial and gluteal muscles shows lymphocyte vasculitis, angular atrophy, lymphocyte myofiber infiltration, fiber necrosis with macrophage infiltration, and regeneration. Biopsies of semitendinosus or membranous muscles may show some evidence of atrophy and vasculitis, but significant inflammatory infiltrates may be absent in these tissues. The extent of the inflammatory infiltrates in epaxial muscles is such that a diagnosis can often be established from several formalin fixed Tru-Cut samples.
The lymphocytic infiltrate seen in muscle samples from horses with IMM contains a high CD4/CD8 ratio with no evidence of IgG binding to myofibers and some upregulation of major histocompatibility Class I and II staining on muscle cell membranes.26,103 Recently, a mutation in the gene encoding type 2X myosin heavy chain was found to be associated with IMM in Quarter Horse-related breeds.104 Genetic testing for IMM will soon be available.
Horses with concurrent evidence of streptococcal infection should be treated with antibiotics.
It is likely prudent to avoid intramuscular injections. Administration of corticosteroids appears to immediately improve signs of malaise and inappetence and prevented further progression of muscle atrophy. Recommended dosages are dexamethasone (0.05 mg/kg IM or IV) for 3 days, followed by prednisolone (1 mg/kg PO) for 7 to 10 days, tapered by 100 mg/week over 1 month. Serum CK activity often normalizes after 7 to 10 days. Muscle mass will usually gradually recover over 2 to 3 months. Horses that are not treated with corticosteroids may develop extensive muscle atrophy, but in many cases muscle mass will gradually recover. Recurrence of atrophy in susceptible horses is common and may require reintroduction of corticosteroid therapy. Some horses develop focal residual muscle atrophy.Equine Inflammatory Myopathies
Other causes of inflammatory myopathies exist in horses apart from IMM.105 Piroplasmosis, for example, can produce chronic muscle atrophy characterized by muscle fiber atrophy, degeneration, and perivascular perimysial and endomysial lymphocytic infiltrates.106 Focal symmetric atrophy of cervical muscles and lymphocytic infiltrates has also been reported in a pony with IMM.107