Muscles Inflammatory - Pathology

()Map MUSCULOSKELETAL SYSTEM (Map) MUSCLES

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

Tuberculosis

• Caused by Mycobacterium bovis
• Grossly:
  • Yellowish spherical nodules
  • Caseous content
  • Fibrous capsule usually
• Histologically:
  • Granuloma
  • Central necrosis
  • Epitheliod and giant cells at periphery

Actinomyces bovis

• "Lumpy jaw"
• May extend to muscles, especially masseters, from osteomyelitis
• Histologically:
  • Caseous or suppurative centres of lesions
  • Epithelioid and giant cells at periphery
  • 'Sulfa granules' at centre - gram-positive branching rods

Viral myositis

• Rare in the UK

Foot and Mouth disease

• Caused by Apthovirus
• Main presentation are vesicles
• May also involve skeletal and heart muscle
• Grossly:
  • Yellow streaks and grey foci
• Histologically:
  • Segmental myofibre necrosis
  • Infiltration of lymphocytes and neutrophils

Bluetongue

• Caused by bluetongue virus of family Reoviridae
• Non-contagious
• Insect-borne
• Causes vasculitis, especially in oral cavity
• Grossly:
  • Infarctions -> necrosis
  • Haemorrhage
• Histologically:
  • Necrosis -> calcification or regeneration (depends on age of lesion)

Parasitic myositis

T. ovis cysticerci (Image sourced from Bristol Biomed Image Archive with permission)

Sarcocyst (Image sourced from Bristol Biomed Image Archive with permission)

• Rare in domestic animals

Nematodes

• Trichinella larvae
  • Particularly in pigs and dogs
  • Public health and economic importance
  • T. spiralis affecting dogs, pigs, cats and humans is most common in hotter climates
  • Larvae distend myofibres
  • May be seen grossly if dead and calcified
  • Predilection for active muscles, e.g. diaphragm, intercostal, masseter and tongue muscles
  • Histologically:
    • Focal myositis - neutrophils, lymphocytes, eosinophils

• 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
  • Cats as final hosts
  • Intermediate hosts, birds and mammals have tachyzoites throughout body, infrequently in skeletal muscle
  • Histologically:
    • Multifocal necrosis of myofibres
    • Lymphocyte and plasma cell infiltration
    • In later stages bradyzoite cysts
• Neospora caninum
  • Necrotising inflammation of muscles
  • Myositis more severe than in toxoplasmosis
• Sarcocystis
  • Histologically:
    • Merozoites causing focal and segmental necrosis
    • May involve mineralisation
    • Non-purulent myositis with plasma cells, lymphocytes and macrophages
  • Diaphragm and masseters most severely affected
• Hepatozoon americanum
  • Develop cysts in skeletal muscle of dogs and cats
  • May be accompanied by pyogranulomatous myositis - macrophages and neutrophils mainly with occasional other cells
• Trypanosoma
  • Infrequent muscle lesions with mononuclear infiltrates
  • Dogs, cats and pigs are affected
  • Parasites lie between myofilaments
  • May cause fibre degeneration
• Babesia bovis
  • In cattle
  • Infrequent muscle necrosis

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