Immune-mediated myopathies in horses (Proceedings)


Purpura hemorrhagica is an immune mediated vasculitis associated with sensitization and cross-reactivity to proteins in the cell wall of Strep equi.

Infarctive Purpura Hemorrhagica:

Purpura hemorrhagica is an immune mediated vasculitis associated with sensitization and cross-reactivity to proteins in the cell wall of Strep equi. It is type three hypersensitivity, and is characterized by leukocytoclastic vasculitis of the skin. Infarctive purpura hemorrhagica is a severe form of purpura hemorrhagica that is characterized by severe leukocytoclastic vasculitis in numerous tissues (not just skin) that progresses to infarction. Infarctive purpura hemorrhagic in horses has a high mortality. Infarctive purpura hemorrhagica may be mediated by IgA, and is similar to Henoch-Schonlein purpura in humans. Henoch-Schonlein purpura is an immune complex disease, and is most common in children.

Clinical signs and History:

History and clinical signs include some of the typical signs that occur with classic purpura in horses. Previous exposure or infection with Strep equi is usually reported. Clinical signs consistent with purpura include edema (limbs or ventral abdomen) and petechial or ecchymotic hemorrhages on mucous membranes. Clinical signs of infarctive purpura include severe lameness, stiffness, and swelling of muscles. Colic can be an initial complaint. Once clinical signs develop, the clinical course is rapid, and a high mortality rate is reported.


CBC- most horses have leukocytosis and mature neutrophila, with a high fibrinogen. Chemistry profile: high muscle enzymes (CK ranges from 50,000 to > 200,000 U/L). Conformation of purpura can be further supported via a skin biopsy (for evidence of leukocytoclastic vasculitis) and/or a serum titers of antibodies against M protein of Strep equi (ELISA SeM > 1:6,400).


Early recognition and treatment with antibiotics and corticosteroids (dexamethasone 0.1 mg/kg IV SID to BID) with weaning doses over 1-2 weeks. Some clinicians will than switch to prednisolone for an additional 1-2 weeks. Antibiotic therapy should be directed against Strep equi. Other recommended treatments would include intravenous fluids, NSAID, and gastroprotectants.

Immune Mediated Myositis- IgG Mediated

Signalment and History:

Predominately Quarter horses. Age includes two groups (less than 8 years and greater than 17 years. Exposure to horses with infectious respiratory disease before clinical signs is common.

Clinical signs:

Clinical signs include rapid muscle atrophy (days to 1-2 weeks duration). The muscle atrophy can be symmetric to asymmetric and most commonly involves the epaxial and gluteal muscles. Depression, stiffness and fever are reported in some horses.


CBC and serum chemistry is normal, other than the exception of slightly- to moderate increase in CK (mean 9,700 U/L, range of 260-140,000 U/L) and AST (mean 2,880 U/L, range of 350-9,000 U/L). Histology of the affected muscles reveals a lymphocytic inflammatory myopathy. Histology reveals macrophages and lymphocytes within myofibers as well as surrounding blood vessels, atrophied fibers, and regenerative myofibers.


Horses respond favorably to corticosteroid therapy (dexamethasone or prednisolone). The duration of steroids usually ranges between 4 to 8 weeks (with decreasing doses and alternate day therapy). Horses should be evaluated for ongoing or subclinical respiratory infections (Strep equi or zooepidemicus). Antibiotic therapy should be considered in cases in which there is a suspected nidus (abscess). Recurrence of muscle atrophy is reported, which may result in further treatment.

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