Black leg

• Blackleg is an acute, highly fatal disease of cattle and sheep caused by Clostridium chauvoei
• In cattle, characteristic lesions of emphysematous swelling of the musculature often develop without a history of wounds
• In sheep, the disease is almost always the result of a wound infection after shearing cuts, docking, castration, and similar
• Control is via administration of a multivalent vaccine containing antigens for multiple clostridial organisms
• C. chauvoei is found naturally in the intestinal tract of animals
• Spores remain viable in the soil for years and are a source of infection
• Outbreaks of blackleg have occurred in cattle on farms with recent excavations or after flooding
• In cattle, blackleg infection is endogenous
• In sheep, the disease is almost always the result of a wound infection
• Proposed pathogenesis of blackleg
• Clostridium chauvoei spores are initially ingested from contaminated soil and spread to multiple tissues, primarily skeletal muscle
• Spores become dormant in the muscle and later germinate due to a low oxygen environment, producing exotoxins causing local necrosis and inflammation
• Proliferating bacteria and toxins enter the bloodstream causing exotoxemia and ultimately death
• The main virulence factor of C. chauvoei is toxin A (CctA), a β-barrel, pore-forming hemolysin
• CctA oligomerizes on the cell surface to form a polymer pore that perforates the cell membrane, resulting in disruption of membrane permeability and cell lysis
• Clinical findings and lesions
• Initially, there is a fever, but by the time clinical signs are obvious, body temperature may be normal or subnormal
• Characteristic edematous and crepitant swellings develop in various muscles
• Necrohemorrhagic and emphysematous myositis is present in skeletal muscles, mainly in the hindlimbs
• Swelling enlarges, there is crepitation on palpation, and the skin becomes cold and insensitive
• Diagnosis
• Presence of crepitus and swelling of large muscles is suggestive
• Ultrasonographic examination of affected areas
• Postmortem examination with anaerobic culture of affected tissues and biochemical identification organisms in tissue samples
• Control
• Move animals from affected pastures
• Vaccination with a multivalent vaccine containing C chauvoei, C septicum, and, where needed, C novyi antigens
• Calves should be vaccinated twice, 4 weeks apart, followed by annual boosters
• Naive ewes should be vaccinated twice, with the second dose 1 month before lambing and then with yearly boosters
• In outbreaks, all susceptible cattle should be vaccinated and treated prophylactically with penicillin
• Carcasses should be destroyed by burning or deep burial to limit spore contamination of the soil