Bones Inflammatory - Pathology

()Map MUSCULOSKELETAL SYSTEM (Map) BONES

Bone infections in general

• Caused by:
  • Bacteria most commonly (e.g. lumpy jaw in cattle due to Actinomyces bovis)
  • Viruses, fungi and protozoa less commonly

• Routes of infection:
  • “Inoculated” at time of fracture
  • Extension from other infected sites (sinuses, middle ear, joints)
  • Haematogenous (young farm animals)

• Gross lesions
  • Suppurative exudate (in bacterial infection), necrosis, bone proliferation, pathological fractures
  • Dead bone portions may be separated from blood supply and form bone sequestra

• Consequences
  • Extension to adjacent bone
  • Haematogenous spread to other bones and soft tissue
  • Pathologic fractures
  • Sinus tracts to exterior

• Osteitis = inflammation of bone
• Periostitis = inflammation of periosteum, usually develops from infection in adjacent tissues
• Osteomyelitis = inflammation of medullary cavity

Osteitis

• E.g.: Fusobacterium necrophorum in the foot, calf diphteria, bite wounds, extension from paranasal sinuses, compound fractures, extensions from infections in the joints, periodontal disease, Atrophic rhinitis

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

Periostitis

• Chronic periostitis - pockets of exudate and irregular new bone fomation

Panosteitis

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

• Typically young large or giant breeds
• 75% of cases in German shepherd dogs, 5-12 months old
• Clinical signs:
  • Shifting lameness
  • Often eosinophilia in peripheral blood
• Usually self-limiting in one to several months
• Grossly:
  • Foreleg long bone diaphyses affected (multiple bones in 50% of cases)
  • Increased bone formation in the medullary cavity and often in the periosteum
• Contast with hypertrophic osteodystrophy