Blackleg (myositis caused by Clostridium chauvoei) is rarely reported in sheep in South Australia. In this case five lot-fed lambs in 5,000 were found dead while another four were found with swollen hind limbs and in one case, a swollen face. Although the lambs had characteristic lesions of blackleg and Clostridial-type organisms were seen on slide preparations, the diagnosis could not be confirmed with specific antibody tests. However, the other findings suggest that it is a case of clostridial myositis, or blackleg. In sheep, clostridial myositis typically develops from a wound infection following procedures such as castration, docking, crutching and shearing, or secondary to bruising during transport.
In March 2020 a producer from South Australia reported that 5 ex 5,000 lambs were found dead, and a further four lambs were sick with swollen back legs, and one with a swollen face. The affected lambs were from two pens. One pen contained lambs from Western Australia that had been on the farm for 2-3 weeks of induction and one week on a full feedlot ration. The other affected pen were local Mallee lambs that had been on feed for two weeks. At no time did the affected pens have any direct contact, and the pens were not adjacent. Adjacent pens appeared unaffected.
Lambs for the feedlot are obtained from a wide range of sources, and vary in age and breed. All lambs are vaccinated with Glanvac 3-in-1 + B12, and drenched with a triple combination drench on induction. Lambs are only shorn if they are carrying long wool. They are inducted on hay and grain trail feeding for 1-3 weeks, before transitioning to an ad lib. diet of barley, lupins and feedlot pellets, fed via feeders. The owners are experienced in managing and feeding lambs, and as such normally have a very low disease incidence and mortality rate. They were extremely concerned by this 'outbreak' and claim to have never seen anything similar.
Examined lambs showed marked, diffuse, firm, hot swelling of an affected limb (or face in one case). They were lethargic, pyrexic and inappetent. Death occurred within 48-72 hours, presumably from toxaemia. The remaining lambs in the involved pens appeared unaffected.
On necropsy two euthanised lambs were found to be grossly normal, apart from an affected hindlimb. The lambs were in good body condition and appeared to have been doing well prior to becoming ill. The affected limbs showed marked cellulitis, as well as myositis. The swelling and inflammation was most notable in the upper limb, but continued distally towards the hoof, as well as along the ventral abdomen. The affected subcutis was exudative and extremely swollen, while the affected muscle showed purple discolouration and loss of structure.
Histopathology revealed a necro-haemorrhagic, multifocal to coalescing, moderate to marked myositis with oedema and intra-lesional cocco-bacilli. Muscle histopathology is suggestive of clostridial myositis. Gram's stain applied to the muscle sections revealed scant clusters of Gram positive cocco-bacilli within necrotic areas. However, culture was unrewarding and the fluorescent antibody test did not detect Clostridium spp. The scant Gram positive bacteria noted histologically were probably contaminants. Based on these results, other possible differentials to consider here include Vitamin E / Selenium deficiency or monensin toxicity.
It is possible that the lambs arrived in the feedlot with dormant spores in muscle but it seems more likely that they picked up clostridial spores from the soil in the feedlot. However, it is difficult to explain why only two pens (of approx. 10-15 pens total) were affected, and why this clinical presentation has never been observed at this site previously. There has been nothing notable about environmental conditions that is hypothesised to have increased soil-borne transmission.
Unfortunately fluorescent antibody tests in this case failed to confirm clostridial myositis, although symptoms, presentation and history are highly suggestive. The post-mortem was performed on a sacrificed lamb, so post-mortem bacterial invasion of muscle tissue is considered unlikely.
Clostridial myositis (blackleg) is very rarely diagnosed in South Australia, but sporadic cases may occur and some may remain undetected. Clostridial diseases are characterised by rapid onset and death, followed by rapid decomposition of bodies, so it is possible the condition is under-recognised in SA. PIRSA has recorded only two cases (both in 2020) in many years in SA.
Many sheep producers use a 3-in-1 vaccination regime in lambs containing antigens against for cheesy gland (Corynebacterium pseudotuberculosis), pulpy kidney (Clostridium perfringens type D), and tetanus (Cl. tetani). However, the more expensive 6-in-1 vaccine (which also contains Cl. septicum, Cl. chauvoei and Cl. novyi) may be more appropriate if more cases of blackleg are diagnosed.
In this case the owner was advised to burn or deeply bury the carcasses of affected lambs as clostridial spores may survive for long periods in soils. In addition all lambs coming to the property should be vaccinated with at least a 3-in-1 vaccine one month or more prior to arrival (usual practice) and consideration may be given to using a 6-in-1 vaccine for a couple of years at induction into the feedlot. The producers who supplied the lambs affected could also be advised that they might consider using a 6-in-1 vaccine on their properties.