THE MERCK VETERINARY MANUAL
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Immune-Mediated Myopathies in Horses

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Infarctive Hemorrhagic Purpura

Purpura hemorrhagica is often associated with mild increases in serum CK activity; however, horses vaccinated for, or exposed to, Streptococcus equi within the last month rarely may develop extremely high serum CK activity, variable edema, acute colic, firm swellings within muscle and under the skin, and unilateral lameness. Clinical signs are due to painful infarctions of skeletal muscle, subcutaneous tissue, focal areas of the GI tract, and lungs resulting from a severe immune-mediated vasculopathy. Hematologic abnormalities include leukocytosis, hyperfibrinogenemia, hypoalbuminemia, and marked increases in serum CK and AST. A diagnosis is often established based on clinical signs, a leukocytoclastic vasculitis in skin and affected tissues, and a very high S equi M protein titer.

Successful treatment requires early detection, penicillin for 14 days, and prolonged high doses of dexamethasone (0.12–0.2 mg/kg) for at least 10 days, followed by tapering doses of prednisolone at an initial dosage of 2 mg/kg. Without aggressive steroid treatment, the condition progresses to intestinal infarction and death.

Immune-Mediated PolyMyositis

Immune-mediated polymyositis is characterized by rapidly developing atrophy of the epaxial and gluteal muscles. It is seen in Quarter Horses, although other breeds may be affected. The condition shows a bimodal age distribution, affecting horses <8 yr or >16 yr of age. In approximately one-third of affected horses, a triggering factor appears to have been exposure to S equi or a respiratory disease. Rapid onset of atrophy of the back and croup muscles is accompanied by stiffness and malaise. Atrophy may progress to involve 50% of the horses' muscle mass within a week and may lead to generalized weakness. Focal symmetric atrophy of cervical muscles has been reported in a pony with immune-mediated polymyositis. Hematologic abnormalities are usually restricted to mild to moderate increases in serum CK and AST. However, in some chronic cases serum muscle enzyme activities are normal. Muscle biopsy of epaxial and gluteal muscles shows lymphocytic vasculitis, anguloid atrophy, myofiber infiltration with lymphocytes, fiber necrosis with macrophage infiltration, and regeneration in acute stages. Biopsies of semitendinosus or semimembranosus muscles may show some evidence of atrophy and vasculitis, but significant inflammatory infiltrates may be absent. The extent of the inflammatory infiltrates in epaxial muscles is such that a diagnosis can often be established from several formalin fixed-core needle biopsy samples. The reason why specific muscle groups are affected is unclear.

Treatment involves antibiotic therapy for horses with concurrent signs of infection and administration of dexamethasone (0.05 mg/kg for 3 days), followed by prednisolone (1 mg/kg for 7–10 days) tapered by 100 mg/wk throughout 1 mo. Serum CK often normalizes after 7–10 days of treatment. Generally, muscle mass gradually recovers throughout 2–3 mo even without corticosteroid treatment. Recurrence of atrophy in susceptible horses is common and may require reintroduction of corticosteroid therapy.

Last full review/revision February 2014 by Stephanie J. Valberg, DVM, PhD, DACVIM, ACVSMR

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