Muscles Inflammatory - Pathology

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Myositis

Bacterial Myositis


Viral Myositis

Parasitic Myositis

Immune-Mediated myositis

Masticatory muscle myositis (MMM)

  • Occurs in dogs
  • Tends to affect the masseter and temporalis muscles
  • Autoantibodies selectively attack muscles of mastication (type IIM fibres)
  • Acute stage = eosinophilic myositis
    • Extremely swollen, painful, hard masticatory muscles
  • Chronic stage = atrophic myositis
    • Atrophy may occur without previous acute stage
  • Grossly:
    • Bilateral but not necessarily symmetrical
    • At first, enlarged and oedematous muscles becominf dark red, firm with reddish streaks and yellowiss foci
  • Histologically:
    • Acute lesions:
    • Chronic stage:
      • Atrophy predominates
      • Fasciculi are shrunken
      • Condensation of stroma -> enlarged endomysium
      • Focal plasma cell and lymphocyte infiltration


Canine dermatomyositis

Idiopathic myositis

Canine polymyositis

  • DOES NOT include MMM above
  • Rare
  • Clinical signs:
    • Muscle pain
    • Fever
    • Weakness, particularly in hind limbs
  • Grossly:
    • Inflammation is accompanied by muscle necrosis and regeneration
  • Histologically:
    • Inflammatory foci with lymphocytes, plasma cells and occasionally eosinophils
  • Possibly immunological cause
  • Single biopsy may not be enough to make the diagnosis as it tends to be multifocal and disseminated
  • Lesions similar to some infectious causes (such as Toxoplasma gondii)


Canine bilateral extraocular polymyositis

  • In Golden retrievers
  • Very rare
  • Clinical signs:
    • Acute bilateral exophthalmos
  • Involves only extraocular muscles except for retractor blbi
  • Grossly:
    • Swollen and pale extraocular muscles
  • Histologically:
    • Sterile myositis
    • Multifocal necrosis of myofibres
    • Lymphocytic, histiocytic and plasmacytic infiltrate with few neutrophils


Bovine and ovine eosinophilic myositis

  • Unknown aetiology
  • Major cell involved is eosinphil
  • Usually noticed at slaughter - green muscle discoloration
  • May be caused by degenteration of Sarcocystis spp.
  • Grossly:
    • Green discolored areas, well demarcated
    • Mostly in muscles of back and thighs
  • Histologically:
    • Massive eosinophil infiltration
    • Muscle fibres may degenerate
    • In old lesions - endimysial fibrosis, myofibre atrophy, some regeneration, lymphocytes, plasma cells, macrophages and few eosinophils
    • Capsule may form