Vet's vision with Phil Rennie Tauranga Vets

Blackleg is an acute, febrile, highly fatal disease of cattle and sheep caused by Clostridium chauvoei (C chauvoei).

It is characterized by emphysematous swelling, commonly affecting heavy muscles and is found worldwide.

‘Blood poisoning’ background

C chauvoei is found naturally in the intestinal tract of animals. Spores remain viable in the soil for years and are purported to be a source of infection. Outbreaks of blackleg have occurred in cattle on farms in which recent excavations have occurred or after flooding. The organisms probably are ingested, pass through the wall of the GI tract, and after gaining access to the bloodstream, are deposited in muscle and other tissues (spleen, liver and alimentary tract) and may remain dormant indefinitely.

In cattle, blackleg infection is sourced from bacteria already within the animal. Lesions develop without any history of wounds, although bruising or excessive exercise may precipitate disease in some cases. Commonly, the animals that contract blackleg are of the beef breeds, in excellent health and gaining weight. Outbreaks occur in which a few new cases are found each day, sometimes for 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 10–12 years may be affected.

The disease usually occurs in summer/ autumn and is uncommon during winter. In the case of sheep, disease is almost always the result of a wound infection and often follows some form of injury such as shearing cuts, docking, crutching, or castration. The case fatality rate approaches 100 per cent. In New Zealand, blackleg is seen more frequently in sheep.

Clinical presentation

Usually, onset is sudden and a few cattle may be found dead without any warning signs. Acute, severe lameness and marked depression are common. Initially, there is a fever but, by the time clinical signs are obvious, body temperature may be normal or subnormal. Characteristic swollen and crepitant (gas pockets) of muscles develop in the hip, shoulder, chest, back, neck, or elsewhere. At first, the swelling is small, hot and painful. As the disease rapidly progresses, the swelling enlarges, there is crepitation on palpation, and the skin becomes cold and insensitive with decreased blood supply to affected areas. General signs include prostration and tremors. Death occurs within 12–48 hours. In some cattle, the lesions are restricted to the myocardium and the diaphragm.

Diagnosis & Control

Typically this is a rapidly fatal, feverish course of disease in well-nourished young cattle, particularly in beef stock, with crepitant (gas filled) swellings of the heavy muscles suggests blackleg. The affected muscles are dark red to black and dry and spongy, have a sweetish odour and are infiltrated with small bubbles but little swelling. The lesions may be seen in any muscle, even in the tongue or diaphragm. In sheep, lesions of the spontaneously occurring type are often small and deep, they may be overlooked. Field diagnoses are confirmed by laboratory demonstration of C chauvoei in affected muscle. The samples of muscle should be taken as soon after death as possible to get a reliable result.

Multivalent vaccines containing the C chauvoei antigen are normally safe and reliable for cattle and sheep. Calves three to six months old should be vaccinated twice, four weeks apart, followed by annual boosters before the anticipated danger period (usually spring or early summer). In an outbreak, all susceptible cattle should be vaccinated and treated prophylactically with penicillin (antibiotic) to prevent new cases for as long as 14 days. Cattle should be moved from affected pastures.

Naive ewes should be vaccinated twice one month before lambing and then with yearly boosters. In outbreaks in flocks of ewes, prophylactic penicillin is also recommended. Young sheep should be vaccinated before going to pasture. Immunity in young sheep is relatively short. Clostridial vaccines are reported to create a weaker immune response in sheep and goats than in cattle. Carcasses should be destroyed by burning or buried deeply in a fenced-off area to limit heavy spore contamination of the soil.