Musculoskeletal System
March 2019

Signalment (Slide A, JPC# 75-36):  3-year-old German shepherd dog     

HISTORY:  This dog had a two week history of hematuria, bilateral exophthalmos, conjunctivitis, and protrusion of the nictitating membranes.

HISTOPATHOLOGIC DESCRIPTION:  Skeletal muscle:  Multifocally expanding the epimysium, perimysium, and endomysium and separating, surrounding and replacing myocytes there are numerous eosinophils and plasma cells with fewer lymphocytes and macrophages, which are often perivascular, admixed with bands of fibrous connective tissue. Myocytes exhibit polyphasic degenerative and necrotic changes characterized by: a pale, swollen, and vacuolated sarcoplasm with disrupted myofibrils (degeneration); a hypereosinophilic, shrunken, and fragmented or hyalized sarcoplasm with loss of cross striations, and a pyknotic or karyorrhectic nucleus (necrosis); or a lightly basophilic sarcoplasm and multiple internalized, linearly arranged, vesiculate nuclei with prominent nucleoli (regeneration). There is multifocal hemorrhage, edema, and few hemosiderin-laden macrophages and vessels are lined by hypertrophic (reactive) endothelial cells.

MORPHOLOGIC DIAGNOSIS:  Skeletal muscle:  Myositis, eosinophilic, lymphoplasmacytic and histiocytic, chronic, multifocal and polyphasic, marked, with myofiber degeneration, necrosis, loss, and regeneration, German shepherd dog, canine.

ETIOLOGIC DIAGNOSIS:  Immune-mediated myositis

CONDITION:  Masticatory myositis

SYNONYMS:  Eosinophilic myositis, atrophic myositis (now grouped together as masticatory myositis)

Signalment (Slide B, JPC# 75-37):  18-month-old heifer 


HISTOPATHOLOGIC DESCRIPTION:  Skeletal muscle:  Diffusely infiltrating and markedly expanding the epimysium, perimysium and endomysium and separating, surrounding, replacing, and resulting in fragmentation of myofibers are large numbers of eosinophils, fewer macrophages, and rare lymphocytes, which are often perivascular, admixed with bands of fibrous connective tissue. There is diffuse atrophy of myofibers and multifocally myocytes exhibit polyphasic degeneration and necrosis characterized by: a pale, swollen, and vacuolated sarcoplasm (degeneration) or a hypereosinophilic, shrunken, hyalinzed and fragmented sarcoplasm with loss of cross striations, and a pyknotic or karyorrhectic nucleus (necrosis). Multifocally macrophages infiltrate individual necrotic myofibers. Multifocal myocytes are expanded by protozoal cysts up to 30-50 um in diameter that contain many basophilic, oval to crescent-shaped 1 X 3 um zoites. Vessels are lined by hypertrophic (reactive) endothelial cells.  Multifocally edema expands the tunica media and perivascular areas of vessels within areas of inflammation. Within one focal area, myofibers are replaced by mature adipocytes.

MORPHOLOGIC DIAGNOSIS:  Skeletal muscle:  Myositis, eosinophilic and lymphocytic, chronic-active, diffuse, marked, with myofiber atrophy, degeneration, necrosis, loss and replacement with adipose tissue, and multiple intracellular sarcocysts, breed unspecified, bovine.

ETIOLOGIC DIAGNOSIS:  Sarcocystic myositis

CAUSE:  Sarcocystis sp.

CONDITION:  Eosinophilic myositis


Immune-mediated myositis:


Cattle and sheep:



Cattle and sheep:



Cattle and sheep: 



Cattle and sheep: 



Cattle and sheep:




Atrophy of muscles of mastication in dogs:

Infectious myositis: 

Granulomatous myositis in cattle:



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