Blackleg is an acute, febrile, highly fatal disease of cattle, sheep and goats caused by Clostridium chauvoei characterized by swellings with pockets of trapped air that produce crepitation (crackling) over affected areas. Although blackleg can affect any muscle in the body, including the heart and diaphragm, heavy muscles of the front and hind limbs are most commonly involved. Spores of C. chauvoei exist in the intestinal tract of animals and are widely distributed wherever cattle have been pastured and housed. It’s safest to assume that spores exist everywhere. Spores are extremely resistant and persist in the environment for very long periods of time. Spring is an opportune time for producers to eliminate risk by vaccinating calves before being turned out to pasture.
Blackleg can be associated with drought. Cattle when grazing ingest C. chauvoei, a soilborne pathogen. As they graze on shorter and shorter forage the risk of ingesting spores increases. Anything that disturbs the soil (flooding, excavation) can also precipitate infections.
While blackleg is an infectious disease, it is not considered contagious in that cattle with infection will not directly transmit the disease to other cattle. Ingested C. chauvoei spores enter the bloodstream and are deposited in muscle tissue throughout the animal’s body. Spores lie dormant in the muscle tissue until stimulated to sporulate and cause disease. They are activated in low-oxygen environments, like those created in damaged or bruised tissue. Any activity that causes bruising promotes the disease. Once this stimulating event occurs (transport, handling, injection sites, rough/rigorous pasture activity), spores germinate to produce disease-causing bacteria that quickly multiply. The real killer becomes a highly potent toxin produced by replicating bacteria.
Although blackleg has occurred in calves as young as two months, the disease generally affects animals between six months and two years. Blackleg typically occurs in beef breeds, in excellent health, and prime condition. The pattern of mortality in an outbreak is generally a few deaths each day, over several days. Most cases are seen in cattle from six to 24 months old, but thrifty calves as young as six weeks and cattle as old as 12 years can be affected. The disease usually occurs in summer and fall and is uncommon in winter. In sheep, blackleg is almost always the result of wound infections (shearing injuries, tail docking, crutching, castration).
The onset of blackleg is sudden. In the beginning, a few cattle are found dead without any previous signs — eating and normal in the morning, dead by evening. Acute, severe lameness and marked depression are common. There may be a fever initially, but by the time clinical signs are observed, body temperature falls to normal or subnormal. Characteristic fluid-filled, crackling swellings develop over the hip, shoulder, chest, back, neck, or elsewhere. At first, swellings are small, hot, and painful. As the disease rapidly progresses, the swelling enlarges, become crepitant, and the skin becomes cold and insensitive as blood supply to affected areas shuts down. Animals become very depressed, may exhibit tremors and ultimately become prostrate. Death occurs within 12 to 48 hours. In some cattle, lesions are restricted to heart muscle and diaphragm so no external signs are evident.
Some calves may recover if treated very early with high doses of an antibiotic, like penicillin. However, since blackleg progresses so rapidly, most affected animals fail to respond to treatment; those that do are often too debilitated to recover. In short, treatment is rarely successful. Carcasses should be destroyed by burning or buried deeply in a fenced-off area to limit heavy spore contamination of the soil.
Success comes with ensuring that cattle are vaccinated against blackleg and the other major clostridial diseases that might occur in a particular area. Veterinarians can advise clients on the prevalence of other clostridial diseases and the necessity to include them in vaccine protocols. While it is not possible to prevent clostridial spores from being present in soil, affordable, high-quality vaccines will prevent the occurrence of blackleg.
Vaccination safeguards and rules regarding blackleg vaccine:
- Users of agricultural and veterinary pharmaceuticals must always read labels and strictly comply with label directions and conditions of use.
- Cold chain safeguards, personal safety and sound biosecurity practices need to be followed with all vaccines.
- Calves require two doses of blackleg vaccine, ideally one month apart, but at least in the spring and at weaning. It is desirable to give the initial two doses of vaccine before young cattle reach their most susceptible age of six months. First-calf heifers should receive a third vaccination for blackleg the following spring to provide the best protection. The third vaccination should provide lifelong immunity to blackleg.
- To await the occurrence of blackleg before vaccinating is unwise, as vaccines take 10 to14 days before they begin to provide immunity.
- It is wise to vaccinate all newly bought young cattle.
- The vaccine should be delivered just under the skin — not into the muscle. Draw up a pinch of skin and insert the needle between the skin and the muscle. The loose skin of the neck is convenient for this.
- Do not save unused parts of bottles or containers of vaccines for future use, as they can become contaminated with undesirable organisms and/or lose their potency. Destroy any vaccine not used within 24 hours of opening.
- If in doubt about the cause of deaths in cattle, consult your veterinary practitioner.
- Remember: Blackleg is almost entirely preventable by vaccination.
Dr. Ron Clarke is a consulting veterinarian living in Alberta.