Blackleg is an acute infection of heavy muscles caused by Clostridium chauvoei and malignant oedema is an acute infection of wounds usually caused by Clostridium septicum though other clostridia may be involved. Clostridia are present in the soil and gut of farm animals and are a constant threat to unvaccinated animals in the live export process. Once ingested spores may be absorbed and then spread in the blood to various tissues in the body.
In blackleg, ingested spores may lodge in muscle and remain dormant there for a long time. Injury to muscle following vaccination, transport or rough handling may trigger germination and extremely rapid bacterial multiplication with production of toxin and gas. Multiple animals are usually affected. Most cases are found dead.
There is a risk of black leg occurring in unvaccinated livestock in the live export process as a result of the intensive animal handling associated with export processes.
In malignant oedema, skin wounds may be followed by bacterial proliferation and production of toxins. Specific risk factors may include shearing cuts, head butting or fighting behaviour in penned animals (particularly intact males), and possibly intramuscular injections.
Affected animals are usually found dead or dying. Infected animals found alive will be severely depressed and have obvious swelling or lameness. The typical lesions are readily seen but may be obscured by rapid post mortem decomposition.
In blackleg, there may be an area over the shoulder, back or rump that is swollen, and has darkened skin, and gas bubbles may be detected by gentle finger pressure. At necropsy, the affected muscle is black and dry, and gas bubbles will have spread between the muscles. The lesion may be in any muscle including tongue or heart and may be quite small.
In malignant oedema, tissues around the inciting wound will be soft, swollen and pit on finger pressure. At necropsy, tissues surrounding the wound will be wet and large quantities of dark brown exudate will have spread out under the skin.
Differential diagnoses include bloat and anthrax.
Specimens for laboratory confirmation must be taken from freshly dead animals. These include smears of affected muscle or wound exudate for bacteriology; sections of affected muscle, collected aseptically, and submitted chilled for bacteriology; and sections of affected muscle or tissue adjacent to an affected wound, in buffered formalin for histology.
Treatment with antibiotics (procaine penicillin, or oxytetracycline) may be attempted but infection is likely to be overwhelming. To stop an outbreak, all susceptible animals in the mob should be vaccinated and treated with antibiotics (procaine penicillin, erythromycin, or oxytetracycline).
Vaccination provides effective protection against clostridial diseases. Handle animals gently to avoid bruising and overexertion.