Blackleg is a rapidly fatal bacterial infection in cattle that destroys muscle tissue from the inside out. The case fatality rate in fully susceptible animals approaches 100%, making it one of the deadliest diseases in livestock. It’s caused by a spore-forming bacterium that cattle ingest from contaminated soil or pasture, and it primarily strikes young, fast-growing animals. The term “blackleg” also refers to a separate fungal disease in canola and other crops, though the livestock disease is what most people mean when they search for it.
How Cattle Get Infected
Blackleg is not contagious. Animals don’t catch it from each other. Instead, cattle pick up bacterial spores by grazing on contaminated pasture or drinking contaminated water. The spores are found naturally in soil and can persist in the environment for years.
Once swallowed, the spores take a surprisingly indirect route to cause disease. Immune cells in the gut lining absorb the spores and carry them through the bloodstream into muscle tissue, where they sit dormant. This is the critical detail: the spores can remain inactive in an animal’s muscles indefinitely, waiting for the right conditions to activate.
The trigger is typically some kind of muscle injury or bruise. When tissue gets damaged, oxygen levels in the muscle drop and lactic acid builds up. This oxygen-deprived environment is exactly what the dormant spores need to germinate, multiply, and begin producing toxins. Those toxins destroy muscle tissue rapidly, causing the massive tissue death that gives blackleg its name. Because the infection starts from spores already inside the animal’s body, blackleg is classified as an endogenous infection, not something picked up from the environment at the moment symptoms appear.
Which Animals Are Most at Risk
Blackleg most commonly affects cattle between 4 months and 2 years old, with animals aged 4 to 6 weeks also vulnerable. Ironically, the best-conditioned, fastest-growing calves on a farm tend to be hit hardest. Rapidly growing muscle may be more susceptible to the small tissue injuries that trigger spore activation. Other domestic and wild ruminants, including sheep, can also develop the disease, though cattle are by far the most commonly affected.
Recognizing the Signs
Blackleg moves fast. An animal that looked healthy in the morning can be dead by evening. The classic signs include sudden lameness, high fever, and visible swelling over large muscle groups, particularly in the upper legs, hip, and shoulder. The swelling feels distinctive: pressing on it produces a crackling sensation under the skin, caused by gas bubbles that the bacteria generate as they destroy muscle tissue.
Affected animals are often severely depressed and reluctant to move. The disease gets its name from what happens to the muscle itself. If the skin over the swollen area is cut away, the tissue underneath is dark red to black, dry, and spongy-looking. Gas bubbles separate the muscle fibers, giving the tissue a porous texture. Squeezing the muscle produces a crackling sound and releases a small amount of thin, blood-tinged fluid. The overall appearance has been described as resembling half-cooked meat.
Animals that die from blackleg bloat and swell rapidly after death, though the smell is typically less putrid than the carcass appearance would suggest.
Why Treatment Rarely Works
The core problem with blackleg is speed. The toxins cause such rapid muscle destruction that by the time an animal shows visible symptoms, the damage is usually too extensive to reverse. Antibiotics exist that can kill the bacteria, but they struggle to penetrate the dead, gas-filled tissue where the infection is concentrated. In practice, most animals with clinical blackleg die despite treatment. The case fatality rate in susceptible populations approaches 100%.
This is why prevention, not treatment, is the standard approach to managing blackleg.
Vaccination Is the Primary Defense
Blackleg is almost entirely preventable with routine vaccination. Clostridial vaccines (commonly sold as 7-way or 8-way products that cover multiple related diseases) are inexpensive and highly effective. The standard protocol calls for an initial dose followed by a booster 2 to 6 weeks later for animals that haven’t been previously vaccinated. After that, an annual booster is typically sufficient for adult cattle.
Timing matters for calves. Vaccination in animals younger than two weeks is less reliable because antibodies passed from the mother can interfere with the calf’s immune response to the vaccine. Most producers vaccinate calves at branding or weaning age and follow up with a booster. In herds with a history of related clostridial diseases, twice-yearly vaccination with an 8-way product may be recommended.
Because the spores persist in soil for so long, there’s no practical way to decontaminate a pasture. Once a farm has had a case of blackleg, the land should be considered permanently contaminated, making consistent vaccination the only reliable long-term strategy.
Blackleg in Plants
The same name refers to a completely unrelated fungal disease that affects canola, rapeseed, and other crops in the cabbage family. Plant blackleg is caused by a fungus that survives on crop stubble left in the field after harvest. In spring, the fungus releases spores that land on growing plants and cause grayish, round lesions on the leaves, often speckled with tiny black dots.
As the infection progresses, it moves down into the stem base, creating dark cankers that girdle the stem and cut off the plant’s ability to transport water and nutrients. Severely affected plants ripen prematurely and topple over, causing significant yield losses. In seedlings, the fungus can constrict the stem just above the soil line, killing plants before they establish.
Managing plant blackleg relies on crop rotation (avoiding planting canola in the same field in consecutive years), planting resistant varieties, and managing stubble to reduce the amount of fungal material carried over between seasons. Unlike the livestock disease, plant blackleg develops gradually over weeks and can be scouted for during the growing season.