Blackleg in Animals

Overview

Blackleg is an acute, febrile, and highly fatal worldwide disease affecting cattle and sheep, caused by Clostridium chauvoei. The disease is characterized by emphysematous swelling and necrotizing myositis, typically impacting large muscles, also referred to as clostridial myositis. The etiological agent, C. chauvoei, is naturally found in the intestinal tracts of animals, with spores capable of surviving in soil for years, potentially acting as a source of infection. Outbreaks often occur on farms where recent excavations or flooding have taken place, leading to the organisms being ingested. Once in the bloodstream, these bacteria are deposited in muscles and various other tissues, where they may remain dormant.

Infection in Cattle

In cattle, blackleg infections are endogenous and can occur without any history of wounds, although excessive exercise or bruising may act as precipitating factors. Animals most commonly affected are beef breeds in excellent health and gaining weight, typically aged between 6–24 months, although cases have been reported in calves as young as 6 weeks and cattle up to 10–12 years old. The disease tends to occur during summer and fall, being rare in winter. Sheep generally contract blackleg through wound infections related to injuries from activities such as shearing or surgical procedures, with a nearly 100% case fatality rate. Notably, blackleg is more prevalent in sheep in New Zealand.

Clinical Presentation

The clinical presentation of blackleg usually begins suddenly, with some animals found dead without prior symptoms. Common signs include acute, severe lameness—primarily in the hind legs—and marked depression. Initially, a fever may be present, but by the time more evident clinical signs arise, the body temperature might be normal or subnormal. Characteristic edematous and crepitant swellings develop in areas such as the hip, shoulder, chest, back, or neck. These swellings are initially focal, hot, and painful; however, as the disease progresses rapidly, the swelling increases, crepitation is noted upon palpation, and skin over the swellings becomes cold and insensitive due to reduced blood supply. General symptoms may include prostration and tremors, with death often occurring within 12–48 hours. In certain cases, lesions are limited to the myocardium and diaphragm, manifesting as abnormal breathing and pericardial friction rub.

Diagnosis

Diagnosis of blackleg can be suggested by the presence of crepitus and swelling in the large muscles. Investigative steps may include ultrasonographic examination of affected areas and postmortem analysis featuring anaerobic cultures of the tissues along with biochemical identification of organisms. A rapidly fatal and febrile condition in well-nourished young cattle, particularly typically beef breeds, with signs of crepitant swellings suggests blackleg. Affected muscles often appear dark red to black, dry, spongy, and have a sweet odor, with infiltration by small gas bubbles. In sheep, shallower lesions may be easily overlooked. While both C. chauvoei and other clostridial species can occasionally present overlapping tissue changes, laboratory confirmations are essential for a definitive diagnosis.

Control Measures

To control blackleg in animals, administration of multivalent vaccines containing C. chauvoei, C. septicum, and C. novyi (when necessary) is advised. Cattle should be moved away from affected pastures, and calves aged 2 months should receive two vaccinations, with annual boosters before the high-risk season. In outbreaks, all susceptible cattle should be vaccinated and prophylactically treated with penicillin to prevent further cases. Naive ewes should also receive two vaccinations, with the last being a month prior to lambing, followed by yearly boosters. For flocks experiencing blackleg outbreaks, prophylactic penicillin and antiserum treatments are recommended. Young sheep ought to be vaccinated before pasture access, noting that clostridial vaccines may elicit a weaker immune response in sheep and goats compared to cattle. Disposal of carcasses must be conducted properly—either via burning or deep burial—to limit spore contamination of the soil.