Immune - Mediated Myositis and Emerging Myopathies

Immune Mediated Myopathies

There are 3 newly recognized myopathies with an apparent immune-mediated origin that have recently been recognized in horses. The first myopathy manifests as acute, severe rhabdomyolysis1 the second presents as focal severe muscle swelling due to infarction and the third myopathy is characterized by rapid muscle atrophy. Many, but not all, of the cases of immune-mediated myositis appear to be a sequella to infection with Streptococcus equi subspecies equi.

Acute rhabdomyolysis due to S. equi

The small numbers of cases described in the literature are Quarter Horses less than 7 years of age. This may not reflect the prevalence of the disease, however, as many cases may not present to university hospitals or diagnostic laboratories.

Clinical signs: Affected horses usually have evidence of submandibular lymphadenopathy and/or guttural pouch empyema due to S. equi. Owners notice that horses develop a stiff gait which progresses rapidly to markedly firm, swollen, painful epaxial and gluteal muscles. Muscle pain becomes severe in spite of aggressive antimicrobial and anti- inflammatory treatment. The majority of reported cases became recumbent, was unable to rise, and developed unrelenting pain necessitating euthanasia within 24-48 hours of hospitalization.

Hematological abnormalities include mature neutrophilia, hyperfibrinogenemia, and marked elevations in creatine kinase (115,000 – 587,000 U/L), and aspartate aminotransferase activities (600- 14,500 U/L).Titers to the M protein of S. equi are low in affected horses, unless horses are recently vaccinated for strangles. Titers to another protein called myosin binding protein were high in a small number of horses that were tested.

At postmortem examination large, pale areas of necrotic muscle are evident in hindlimb and lumbar muscles. The histopathologic lesions are characterized by severe acute myonecrosis with a degreed of macrophage infiltration. Sublumbar muscles often show the most severe and chronic necrosis as indicated by greater macrophage infiltration of myofibers. [...]