Clostridium septicum is the causal agent of a number of wound and tissue infections of livestock including malignant oedema, gas gangrene, 'false blackleg' and swelled head in rams. However, as Cl. septicum is a post-mortem invader, its isolation from tissues alone is insufficient to establish a diagnosis (Parkinson et al 2010). In this case the history, necropsy and histopathology findings all support a diagnosis of false blackleg (malignant oedema) as the cause of death in five of 78 Angus weaner calves.
Five of 78 mixed sex Angus weaners were found dead in a paddock in the Bathurst district in early May 2014. The calves, running on improved pastures, were immediately vaccinated with 5:1 on May 10. A calf observed to be sick late on May11 was necropsied on May 12. Two more calves died on about May 13 but no deaths occurred subsequently.
The calves, vaccinated at marking but not subsequently, were yard weaned over a two week period in early April. On the 9 April the calves escaped from the yards destroying them in the process. The owners presumed they had been panicked by some dogs known to be in the area at the time.
A well grown 200 kg steer was necropsied. There was blood from nose and anus and an extensive area of wet red emphysematous subcutaneous tissue and musculature especially over the back and rump.
Fluorescent antibody testing of a skeletal muscle smear (sample ref: M14-06604-0003) was negative for Cl. Chauvoei, Cl.novyi B and Cl.sordelli and positive for Cl. septicum.
Histopathology of a sample of muscle tisse revealed that many myofibres were hypereosinophilic and myofibrils were separated by clear spaces. Eosinophilic material was noted multifocally within the connective tissue. Large numbers of relatively large bacilli were noted in association with necrosis, haemorrhage and proteinaceous fluid deposition. There was a light scattering of predominantly mononuclear cells within the interstitium.
The vaccination history supports a diagnosis of clostridial disease. The calves had only been vaccinated at calf marking and had not received a booster vaccination at weaning. While three calves died within two days of a second vaccination, no further losses occurred subsequently.
While the gross and histopathology findings of myonecrosis and oedema support a diagnosis of blackleg, only Clostridium septicum was detected on fluorescent antibody testing. Cl. septicum is a common post mortem invader which, unlike Clostridium chauvoei, proliferates rapidly after death. However, considering the presence of muscle lesions in this case, it may be significant.
One possibility is that some of the calves, trampled while panicked into escaping from the yards, sustained tissue damage, inactivating latent spores of Cl. septicum, causing 'pseudo blackleg.' However, the possible injury occurred 3-4 weeks before the first deaths, a longer than expected time interval between tissue damage then activation and germination of spores and toxin production.
In regards to Cl. septicum myositis, Jubb et al (2007) distinguish between malignant oedema and gas gangrene as wound infections and pseudo blackleg due to activation of latent spores. Pseudo blackleg is characterised by lack of detectable wounds, deep multiple lesions in widely separated muscles, extensive blood-stained gelatinous exudate in connective tissues with only small gas bubbles, and demonstration of Cl. septicum in lesions shortly after death. While history, gross and microscopic muscle lesions and immunofluorescence are suggestive of Clostridium septicum as the causative agent in this case, the pathogenesis is uncertain.