What is a black leg?: Understanding its causes in livestock and rare human cases

September 30, 2025 •

4 min read

According to research published in the Journal of Clinical Microbiology, cases of human infection with the blackleg bacterium Clostridium chauvoei are extremely rare and often lethal. So, what is a black leg? This term most commonly refers to a fatal bacterial disease in livestock, though it is also used colloquially to describe other serious forms of leg discoloration in people.

Quick Summary

Blackleg is a deadly bacterial disease caused by Clostridium chauvoei that affects livestock, causing muscle swelling and necrosis. While very rare, it can also infect humans. The term can also describe other conditions, like gangrene or venous insufficiency, that cause leg discoloration.

Key Points

Blackleg is a deadly livestock disease: Caused by the bacterium Clostridium chauvoei, it leads to rapid, fatal muscle infection in cattle and sheep.
Human infection is extremely rare but serious: Only a handful of human cases with Clostridium chauvoei have been reported, most with lethal outcomes, often involving gas gangrene.
Survival is possible with early treatment: In 2023, an Australian woman became the first documented survivor of C. chauvoei infection, highlighting the importance of rapid medical intervention.
Leg discoloration has multiple human causes: In general health, a black leg can signify conditions like gangrene, venous insufficiency, peripheral artery disease, or severe bruising.
Immediate medical attention is vital: Any persistent or unexplained dark discoloration of the leg, especially with other symptoms like pain or fever, requires immediate medical evaluation.

What Is Blackleg as a Veterinary Disease?

Blackleg, also known as black quarter, is a highly fatal, non-contagious disease that primarily affects cattle and sheep. It is caused by the anaerobic, spore-forming bacterium Clostridium chauvoei, which can remain dormant in the soil for years. Outbreaks often occur after soil disturbance, such as flooding or excavation, which exposes the spores. The bacteria are typically ingested and travel through the bloodstream to muscle tissue, where they can lie dormant.

Once activated, usually by an injury or muscle trauma that creates low-oxygen conditions, the bacteria multiply rapidly and produce a lethal toxin. This causes severe inflammation, gas gangrene, and muscle death (necrosis). The gas buildup produces a crackling sound, or crepitation, when the affected area is pressed.

Typical Symptoms in Livestock

Sudden onset of lameness
High fever, followed by a normal or subnormal temperature shortly before death
Hot, painful swelling in the large muscles of the upper leg, hip, shoulder, or chest, which rapidly becomes cold and insensitive
Crepitation, or a crackling sensation upon touch, due to gas buildup under the skin
Characteristic rancid odor from affected tissue
Dark red to black discoloration of the muscle upon examination, giving the disease its name

Can Humans Get Blackleg?

Infection with Clostridium chauvoei in humans is exceedingly rare, with only a few documented cases worldwide. However, when it does occur, it is a life-threatening medical emergency. It is considered a zoonotic disease, meaning it can be transmitted from animals to humans, but most cases involve contamination from soil. The bacteria can enter through a wound, and, similar to the process in livestock, multiply in low-oxygen environments to cause gas gangrene.

Historically, human cases of C. chauvoei infection were considered lethal. These cases were often associated with significant pre-existing conditions or severe trauma. However, in 2023, an Australian woman made history by becoming the first documented person to survive the infection after contracting it while gardening. Her survival was attributed to early and aggressive treatment with antibiotics and surgery. Given its rarity, differentiating C. chauvoei from other more common pathogenic Clostridia species can be challenging for laboratories. For more information, you can read the case report on human fulminant gas gangrene caused by Clostridium chauvoei from the National Institutes of Health Human Fulminant Gas Gangrene Caused by Clostridium chauvoei.

Other Causes of a 'Black Leg' in Humans

While a true Clostridium chauvoei infection is extremely rare in humans, leg discoloration that presents as black or dark is a serious symptom that can indicate other underlying medical conditions requiring immediate attention.

Common causes for a black or dark-looking leg include:

Gangrene: The death of body tissue due to a lack of blood flow or a serious bacterial infection. Types include dry gangrene (often in people with diabetes or blood vessel disease) and wet gangrene, which involves bacterial infection and spreads quickly.
Venous Insufficiency: A condition where the leg veins' valves are weakened, causing blood to pool in the lower legs. As pressure builds, capillaries leak, and the iron from red blood cells stains the skin, leading to dark, brown, or black discoloration known as hemosiderin staining.
Peripheral Artery Disease (PAD): This condition involves narrowed arteries restricting blood flow to the limbs. Severe cases can result in critical limb ischemia, causing the skin to turn blue or black due to a lack of oxygen.
Severe Trauma and Bruising: A significant injury can cause extensive internal bleeding under the skin. As the blood clots and breaks down, it can cause the skin to appear black or dark purple before fading.

Comparison of Leg Discoloration Causes in Humans


Condition	Cause	Onset	Symptoms	Appearance	Urgency
*Blackleg (C. chauvoei)*	Rare bacterial infection from soil spores entering a wound	Rapid, typically within 12-48 hours	Fever, systemic shock, gas gangrene	Black, spongy, often with crepitation	Extreme Emergency
Gangrene	Lack of blood flow or severe bacterial infection	Rapid (wet) or gradual (dry)	Pain, swelling, foul-smelling discharge, blisters	Pale gray, blue, black, or bronze skin	Extreme Emergency
Venous Insufficiency	Weakened vein valves and blood pooling	Gradual, chronic	Swelling, pain, itching, skin ulcers	Brown or purplish patches, especially on lower legs/ankles	Requires medical attention
Peripheral Artery Disease	Narrowed arteries due to plaque buildup	Gradual, chronic; worsened by exertion	Pain (claudication), numbness, changes with elevation	Pale, bluish, or black skin in severe cases	Requires medical attention
Severe Bruising	Trauma or injury	Rapid, immediate	Pain, swelling, tenderness at injury site	Dark purple or black, fading over time	Varies depending on severity

Why Medical Diagnosis is Crucial

Any unexplained, sudden, or persistent discoloration of the leg, especially when accompanied by other symptoms like pain, swelling, fever, or a foul odor, should be evaluated by a healthcare professional immediately. While a severe bruise can be harmless, symptoms of gangrene, PAD, or venous insufficiency can worsen significantly if left untreated, potentially leading to serious complications, including amputation. A doctor can perform a proper diagnosis through a physical examination and additional tests to determine the correct cause and treatment plan.

Conclusion

The term what is a black leg? can refer to several distinct and serious health concerns. In its most definitive form, it is blackleg disease caused by Clostridium chauvoei, a lethal bacterial infection in cattle that very rarely affects humans. However, in human health, a dark or black-colored leg is a critical symptom that can point to other dangerous medical conditions, including various forms of gangrene, venous insufficiency, or peripheral artery disease. Early and accurate medical diagnosis is the single most important step for anyone experiencing these symptoms to ensure the best possible outcome. Ignoring such a warning sign could have severe, and potentially fatal, consequences.

Frequently Asked Questions

No, blackleg disease in humans is not contagious from person to person. The infection is typically caused by spores from the soil entering a wound, not through direct contact with another infected individual.

Since human infection is so rare, there is no common progression. However, similar to gas gangrene, the first signs can include a sudden onset of severe, unexplained pain, swelling, and redness at the site of a trauma, possibly accompanied by fever and systemic symptoms like low blood pressure.

Treatment requires aggressive medical intervention, including high-dose antibiotics and surgical removal of the infected, necrotic tissue. Hyperbaric oxygen therapy may also be used in some cases.

Hemosiderin staining is the brown or dark discoloration of the skin that occurs when red blood cells leak from veins and break down, releasing iron into the surrounding tissue. It is a symptom of conditions like chronic venous insufficiency.

Yes, meat from infected animals should not be consumed. The presence of lesions, gas formation, and discoloration indicates the disease, and slaughtering infected animals is prohibited.

If you notice sudden, severe, or persistent dark discoloration on your leg, especially with accompanying symptoms like pain, swelling, fever, or a foul odor, you should seek immediate medical help.

Wet gangrene involves a bacterial infection in dead tissue and can be caused by various pathogens. Gas gangrene, a form of wet gangrene, is specifically associated with gas-producing bacteria, like Clostridium perfringens and, in rare cases, C. chauvoei, that infect deep muscle tissue.