Osteomyelitis

Localised osteomyelitis plus sequestrum (Image sourced from Bristol Biomed Image Archive with permission)

• Mainly due to haematogenous spread in young animals
• Often as an extension from suppurative arthritis
• Usually isolated: Actinomyces pyogenes, Salmonella, E.coli, Klebsiella, Streptococci
• Metaphyses and epiphyses mostly affected
• Most infections are exacerbated by host response
• Pathogenesis:
  • Prostaglandin and cytokines stimulate osteoclastic bone resorption
  • Blood vessel occlusion and tissue necrosis -> lack of removal of agent and cartilage -> cartilage thickening + persistance of infection
• Affected area may be surrounded by fibrous inflammatory tissue
• Metaphyseal abscesses may develop
• Sequestra may also develop -> surrounded by granulation tissue
  • Isolated from osteoclastic resorption
  • May persist for long time
  • Obstruct repair

• Vertebral osteomyelitis
  • Lesions may affect the spinal cord
  • Usually due to Actinomyces pyogenes
  • Suppurative inflammation may cause fracture of vertebral body -> dorsal displacement -> damage to spinal cord
  • May form encapsulated abscess protruding into spinal canal -> spinal cord compression

• Actinomycosis
  • = Lumpy jaw in cattle
  • Caused by Actinomyces bovis
    • Gram positive, branching, filamentous
    • Obligatory parasite of mucous membranes of oropharynx
    • Surface tissue is usually injured for infection to occur
    • Osteomyelitis of mandible -> destruction of bone -> replacement by fibrous tissue with pyogranulomatous nodules cotaining 'club colonies' of bacteria
    • Suppurative osteomyelitis
    • Fistulous tracts may discharge onto skin and into oral cavity
    • Excessive periosteal bone formation
    • Greatly enlarged mandible