Clostridial myositis, also known as blackleg, is a severe, rapidly fatal infectious disease of livestock that remains a significant concern for veterinarians and producers.
The disease is caused by Clostridium chauvoei, a spore-forming, toxin-producing anaerobic bacterium that is widely distributed in soil.
“These spores are commonly found in soil,” said Alexis Thompson, DVM, Ph.D., DACVPM (Epidemiology). “Livestock typically ingest these spores while grazing, which results in no clinical signs. However, the spores do not cause problems until specific conditions occur (trauma/hypoxia), which allows the organism to multiply and invade the tissues.Due to its aggressive nature and high fatality rate, blackleg is most often diagnosed postmortem, making the service provided by the Texas A&M Veterinary Medical Diagnostic Laboratory, TVMDL, critical for producers and veterinarians seeking to uphold herd health.
Susceptibility and clinical signs
Blackleg primarily affects cattle, with the highest risk observed in young calves between 6 months and 2 years of age. Affected cattle are often in excellent body condition, which can make losses particularly surprising and distressing for producers.
“The disease progresses very quickly, often leading to sudden death without prior clinical signs,” Thompson said.
When present, clinical signs may include:
- Fever
- Lameness, stiffness, or altered gait
- Lethargy and decreased appetite
- Painful, hot, swollen muscle groups
- Palpable crepitus — gentle pressure on swollen muscle produces a crackling or popping sensation (similar to “bubble wrap”) due to gas accumulation within the affected tissue.
Death typically occurs within 12–48 hours of onset. Necropsy examination often reveals severely damaged, dark, gas-filled, foul-smelling muscle tissue. Lesions most commonly involve large muscle groups including the hindquarters, forequarters, and trunk. The infection may also spread to the heart.
Traveling from soil to muscle tissue
Transmission of blackleg is closely linked to the unique life cycle of C. chauvoei:
- Highly resistant endospores persist in soil for long periods under a wide range of environmental conditions.
- Animals ingest spores while grazing contaminated pastures.
- Spores cross the intestinal mucosa, enter the bloodstream, and localize in skeletal muscle tissue, where they may remain dormant for extended periods.
- Muscle trauma (e.g., rough handling, exercise, bruising, or even routine procedures such as vaccination) creates localized hypoxic conditions.
- Anaerobic conditions trigger spore germination, rapid bacterial proliferation, and exotoxin production (including alpha toxin), leading to myonecrosis, systemic toxemia, and death.
Importantly, bacterial replication and toxin production occur only within the living host. After death, vegetative cells sporulate upon environmental exposure.
“When an infected animal dies, its carcass becomes the source of millions of new spores, especially if it’s left in the pasture,” Thompson said. “These spores can spread and lead to a deadly outbreak in the herd, which makes disposal very important in these cases.”
Diagnostic recommendations
Blackleg is most often diagnosed after death based on a combination of clinical history, gross lesions, and laboratory testing. At TVMDL, the primary diagnostic test for confirmation is fluorescent antibody (FA) testing performed on affected muscle tissue. This assay is rapid and highly specific for detection of C. chauvoei directly within tissue sections.
“In some cases, an anaerobic culture of the muscle can be attempted to isolate the bacteria directly,” said Thompson, “but this is oftentimes more time consuming and less commonly used because of the rapid decomposition that can happen in that tissue after death.”
Sample/specimen collection guidelines
Proper sample collection and handling are critical for accurate diagnosis:
- Submit fresh skeletal muscle from affected areas, especially tissue that is dark, swollen, or gas-filled.
- Include both abnormal and adjacent normal tissue when possible.
- Heart muscle may also be submitted if cardiac involvement is suspected.
- Store samples chilled (refrigerated), not frozen.
- Submit samples promptly to the laboratory to minimize decomposition.
- Clearly label samples and indicate blackleg as a suspected diagnosis on the submission form.
Final thoughts for herd health
Blackleg is a rapidly fatal clostridial disease most commonly affecting young, healthy cattle. Administration of multivalent clostridial vaccines is an effective tool for reducing losses from sudden death. When cases arise, TVMDL provides speedy diagnostics and practical guidance to support veterinarians and producers — helping protect herds and prevent future losses.
To learn more about TVMDL’s various test offerings, visit tvmdl.tamu.edu or call the College Station laboratory at 888.646.5623 or the Canyon laboratory at 888.646.5624.
