Black Leg

Blackleg myositis (Image sourced from Bristol Biomed Image Archive with permission)

• Causative agent: Clostridium chauvoei
• May affect the fattest cattle at pasture in the summer
• Rapid toxaemia -> death
• If clinical signs observed:
  • Toxaemia -> pyrexia, depression, pulmonary oedema, circulatory collapse
  • Muscle lesions -> lameness, swollen hot muscles later becoming cool as necrosis sets in
• Pathogenesis:
  • Spores gain entry to GI tract -> blood -> muscle -> lie latent
  • Under right conditions (usually anaerobic following injury) they germinate and bacilli grow
  • Toxin damages capillaries -> serosanguinous exudate
  • Muscle necrosis due to gas producing bacteria -> emphysaema and crepitus
• Grossly:
  • Early stages
    • At muscle periphery
    • Dark red
    • Distended by serous or serosanguinous exudate
    • Wet cut surface
  • Old stages
    • Centre of lesion is full of gas bubbles, porous, dry, reddish black
    • Rancid odour
• Histologically:
  • Early stages
    • Separation of myofibres by exudate
    • Coagulative necrosis
    • No nuclei
  • Old stage
    • Fragmented muscle fibres separated by gas bubbles
    • Gram positive bacilli may be found in clumps