blackleg

Blackleg is an acute bacterial disease that kills cattle within 24 to 48 hours of symptom onset. The causative organism, Clostridium chauvoei, lives dormant in soil and enters the animal through wounds, minor cuts, or the digestive tract. Once established, it produces gas in muscle tissue and releases powerful toxins. The disease strikes hardest in animals aged 6 months to 2 years, particularly those in good condition on rich pasture. Mortality approaches 100 percent once clinical signs appear.

The name comes from the visible discoloration of the hindquarters and hind legs, which turn dark purple or black as blood stagnates in affected tissue and overlying skin dies. The subcutaneous gas pockets create a characteristic "crackling" sensation under the skin when handled. An animal may show only lameness or reluctance to move before rapid systemic collapse. Post-mortem examination reveals emphysematous muscle lesions, particularly in the hindquarters, thighs, and shoulder.

Prevention and vaccination

Blackleg is readily preventable through vaccination. Clostridium chauvoei is one of the organisms included in common clostridial combination vaccines (often labeled as "7 in 1" or "8 in 1" formulations, referring to the number of clostridial types covered). A primary course of two injections, typically given 4 weeks apart, protects calves from around 12 weeks of age. Booster shots are given annually or every 12 to 18 months depending on local disease pressure and the product used. In herds with no vaccination history, a single outbreak often prompts immediate mass vaccination of all susceptible stock.

The disease occurs sporadically rather than in epidemics. Cases cluster in herds with poor vaccination records or in pastures where large numbers of young cattle graze over contaminated soil, particularly after heavy rain or flooding that brings dormant spores to the surface. Once a paddock has hosted cases, the risk remains for years; this is why vaccination rather than pasture management forms the primary control strategy.

Treatment of blackleg with antibiotics is rarely successful once clinical signs develop, making prevention the only economically sound response. The disease does not spread between animals, so isolating a sick animal offers no benefit to herd mates. Some regions require blackleg notification to veterinary authorities or to herd health programs, since vaccine failure or poor compliance in one herd can signal risk across a district.