Immune-Mediated Myopathies in Horses

Purpura hemorrhagica is an acute, immune-mediated, necrotizing vasculitis (type III hypersensitivity) associated with sensitization and cross-reactivity to proteins in the cell wall of Streptococcus equi subsp equi. Purpura hemorrhagica usually occurs as a sequela to previous streptococcal respiratory infection (eg, strangles) but occasionally follows other antigenic stimuli (eg, S equi bacterin vaccination, equine influenza, or chronic suppurating wounds).

Purpura hemorrhagica is often associated with mild increases in serum CK activity; however, horses vaccinated for, or exposed to, S equi within the past 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 activities. 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, potassium or sodium penicillin (22,000–44,000 U/kg, IV, every 6 hours for 14 days), and prolonged high doses of dexamethasone (0.07–0.1 mg/kg, IV, every 12–24 hours for at least 10 days), followed by tapering doses of prednisolone (at an initial dosage of 2 mg/kg, PO, every 12–24 hours for an additional 1–2 weeks). Without aggressive steroid treatment, the condition progresses to intestinal infarction and death.

Immune-mediated myositis, horse

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Courtesy of Dr. Stephanie Valberg.

Immune-mediated myositis is a form of myosin heavy chain myopathy that is characterized by rapidly developing atrophy of the epaxial and gluteal muscles. It primarily occurs in Quarter Horse–related breeds and is highly associated with carrying heterozygous or homozygous genotypes for a mutation in MYH1 that encodes the fast-twitch type 2X myosin heavy chain. The condition shows a bimodal age distribution, affecting horses < 8 years or > 16 years of age. Both heterozygous and homozygous MYH1 cases develop atrophy; however, homozygous horses seem to be more severely affected. A triggering factor appears to be exposure to S equi or a respiratory disease or IM vaccination against S equi or equine herpesvirus (rhinopneumonitis) and influenza. 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. Hematologic abnormalities are usually restricted to mild to moderate increases in serum CK and AST activities. However, in some chronic cases, serum muscle enzyme activities are normal.

A diagnosis can be made via genetic testing for the MYH1 mutation. Muscle biopsies of epaxial and gluteal muscles taken in the acute stages can show lymphocytic vasculitis, anguloid atrophy, myofiber infiltration with lymphocytes, fiber necrosis with macrophage infiltration, and regeneration. Biopsies of semitendinosus or semimembranosus muscles may show some evidence of atrophy and vasculitis, but notable inflammatory infiltrates may be absent. Rarely, immune-mediated myositis can progress to systemic calcinosis, at which point the product of serum calcium (mg/dL) multiplied by serum phosphorus (mg/dL) is > 65 mg²/dL². Systemic calcinosis presents in a variety of ways with ventral edema, dyspnea, renal disease, or laminitis and has a poor prognosis.

Less frequently, horses of a variety of breeds can develop focal regions of atrophy not associated with the MYH1 mutation. Focal symmetric atrophy of cervical muscles has been reported in a pony with immune-mediated polymyositis.

Another clinical form of myosin heavy chain myopathy is acute nonexertional rhabdomyolysis. A stiff gait is the initial clinical sign. This progresses rapidly to severely painful, firm, swollen epaxial and gluteal muscles. Many horses become recumbent with unrelenting pain that may warrant euthanasia. A diagnosis is made via genetic testing for the MYH1 mutation. This form of rhabdomyolysis can be triggered by exposure to or concurrent infection with Streptococcus equi or Anaplasma phagocytophilum. Muscle tissue from these cases often lack any evidence of inflammatory cells.

Treatment of myosin heavy chain myopathy involves antimicrobial treatment for horses with concurrent signs of infection and administration of dexamethasone (0.05 mg/kg, IV, every 24 hours for 3 days), followed by prednisolone (1 mg/kg, PO, every 24 hours for 7–10 days, then tapered by 100 mg/week for 1 month). Dantrolene (4 mg/kg, PO, every 6–8 hours until serum CK activity begins to decline) is recommended for horses with nonexertional rhabdomyolysis. For horses with immune-mediated myositis, dantrolene is not usually necessary, and serum CK activity often normalizes after 7–10 days of corticosteroid treatment. For horses with atrophy, generally muscle mass gradually recovers throughout 2–3 months even without corticosteroid treatment. Recurrence of atrophy in susceptible horses is common, especially in homozygotes, and may require reintroduction of corticosteroid treatment. In horses with the MYH1 mutation, avoiding strangles vaccination and using the minimum possible number of IM vaccinations spaced 4–6 weeks apart is recommended, in addition to avoiding irritating IM injections.