Muscles Inflammatory - Pathology

Myositis

• Inflammation of muscle

Bacterial myositis

• Bacteria gain entry via:
  • Direct penetration
  • Blood stream
  • Extension
• Lesions caused:
  • Suppurative
  • Serosanguinous
  • Granulomatous

Abscesses

• Caused by pyogenic bacteria
• Usually:
  • Arcanobacterium pyogenes in sheep and cattle
  • Streptococcus equi in horses
  • Corynebacterium pseudotuberculosis in sheep and goats
  • Pasteurella multocida causes cellulitis in cats
• Usually localised suppuration and necrosis of myofibres
• Occasionally my spread via the fascia

Actinomyces bovis

• "Lumpy jaw"

Viral myositis

• Rare in the UK

Parasitic myositis

• Rare in domestic animals

Nematodes

• Trichinella larvae

• Ancylostoma larvae
  • Cause inflammation and necrosis
• Ascarid larvae
• Dirofilaria immitis in dogs causing ischaemic myopathy due to infartcs

Cestodes

• Taenia solium, - cysticercosis in swine and humans
• T. ovis - cysticercosis in sheep
• T. saginata in cattle cysticercosis
• Multiceps serialis in rabbits, primates coenurosis
• Echinococcus granulosus hydatid disease in herbivores and humans

Protozoa

• Toxoplasma gondii

• Neospora caninum

• Sarcocystis

• Hepatozoon americanum

• Trypanosoma

• Babesia bovis

Arthropods

• Dipterious larvae causing myiasis
• Hypoderma spp. in cattle
  • May occasionally burrow into muscle

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:
    • Mainly eosinophilic infiltrate, few lymphocytes, monocytes and plasma cells
    • In recurrent disease plasma cells predominate
    • Necrotic myofibres, atrophy and attempted regeneration
  • Chronic stage:
    • Atrophy predominates
    • Fasciculi are shrunken
    • Condensation of stroma -> enlarged endomysium
    • Focal plasma cell and lymphocyte infiltration

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