Blackleg is a severe, rapidly progressing bacterial disease that primarily affects cattle and sheep globally. This acute infection has a high mortality rate, often approaching 100% in affected livestock. It frequently strikes young, well-nourished cattle between six months and two years of age. Blackleg is classified as a clostridial disease and causes significant economic loss for producers worldwide.
The Pathogen and How It Spreads
The organism causing blackleg is the bacterium Clostridium chauvoei. This microbe is an anaerobe, thriving in environments lacking oxygen, and forms highly resistant spores. These durable spores survive in the soil for years, making pastureland a long-term reservoir for the infection. Outbreaks are often associated with environmental disturbances like flooding or excavation, which bring buried spores to the surface where they are easily ingested by grazing animals.
The primary route of infection for cattle is ingesting spores while eating contaminated feed or grazing on contaminated soil. Once consumed, the spores pass through the gastrointestinal tract wall and enter the bloodstream. They travel throughout the body and become lodged, often remaining dormant within muscle tissue for long periods.
Local tissue damage, such as bruising or trauma, creates the low-oxygen conditions necessary to stimulate dormant spores to germinate. In this anaerobic environment, the bacteria multiply rapidly and produce potent exotoxins. These toxins cause localized tissue death (myonecrosis) and lead to a generalized toxemia that quickly overwhelms the animal. In sheep, infection is commonly linked to external wounds from procedures like shearing or castration, which allow spores to enter the tissue directly.
Recognizing the Clinical Signs
The onset of blackleg is sudden, and many animals are found dead without showing prior signs of illness. When clinical signs are observed, they indicate severe and rapid disease progression. An affected animal may initially exhibit acute, severe lameness, especially if the infection is in the large muscles of the leg or shoulder. Symptoms often include a high fever, followed by depression, lethargy, and refusal to eat.
A hallmark of the disease is characteristic swelling, often in the hip, shoulder, or chest, caused by underlying muscle inflammation. This swelling is initially hot and painful but quickly becomes cold and insensitive as the tissue dies. Palpation of the swollen area may reveal a crackling sensation, known as crepitus, caused by gas accumulating due to bacterial activity. Death usually occurs swiftly, often within 12 to 48 hours of the first noticeable symptoms.
Post-mortem examination is frequently the way the disease is definitively identified. The affected muscle tissue is dark red to black, presenting a dry and spongy appearance, from which the disease derives its name. This necrotic muscle often contains small gas bubbles and may emit a sweet or rancid odor. While classic lesions are found in the large skeletal muscles, the infection can also occur in other muscle groups, including the diaphragm, heart, or tongue.
Preventing Blackleg in Livestock
Since blackleg is rapidly fatal, antibiotic treatment is rarely successful unless administered in the very early stages. Prevention is therefore the most effective strategy. The most practical way to control blackleg is through a comprehensive vaccination program. Most commercial vaccines are bacterin-toxoids, which stimulate the immune system to produce antibodies that neutralize the deadly toxins produced by the bacteria.
Calves should be vaccinated for blackleg around three to four months of age, when temporary immunity from their mother’s colostrum begins to wane. For full protection, this initial vaccination requires a booster dose, typically given three to six weeks later. All vaccinated animals should receive an annual booster to maintain a high level of immunity, especially in areas where blackleg is a known problem.
Environmental management is a significant factor in controlling disease spread. Because Clostridium chauvoei spores persist in the soil for years, it is advisable to avoid grazing young, susceptible animals on contaminated pastures. Proper handling of livestock to reduce bruising and muscle injury helps prevent the activation of dormant spores within an infected animal. Carcasses of animals that have died from blackleg must be disposed of properly, preferably by burning or deep burial, to prevent further environmental contamination.