True blackleg is a myositis of the skeletal and sometimes cardiac muscles with an associated toxaemia and a high case fatality rate. It is primarily a disease of young cattle and is caused by the bacterium Clostridium chauvoei. Blackleg is a common cause of death on the coast of NSW (Hart 2011, Poad and Ball et al 2012) but is rarely diagnosed west of the mountains. This report is considered to be of interest because it describes an outbreak of blackleg in unvaccinated calves on three properties between Megalong and Lowther (15-20 km south of Lithgow and west of Katoomba) in the upper Coxs river catchment, west of the Blue Mountains.
The outbreak occurred on three beef cattle properties between 10 October and 8 December 2012. In all cases, the affected calves were in good body condition, about 3 months of age, unvaccinated and from cows that were also not vaccinated (Table 1). In case 1, 20 calves were sired by a Limousin bull over Limousin cross cows (with a mortality rate of 30%) and 28 were by the same Limousin bull over Hereford Angus cross cows (none of which died). All cases had histopathology consistent with blackleg and all were confirmed positive for Cl. chauvoei by fluorescent antibody (FA) test and negative for Cl. septicum, Cl. sordelli and Cl. novyi type B.
|Case 1||Case 2||Case 3|
|Time of outbreak||10-16 October 2012||10-15 November 2012||8 December 2012|
|Mortality rate||7 of 40 (17.5%)||12 of 330 (3.6%)||1 of 350 (0.3%)|
|Breed||Limousin cross||Murray Grey||Hereford cross|
|Approx. age of calves||3 months||3 months||3 months|
|Diagnosis||FA +ve||FA +ve||FA +ve|
|Vaccination status calves||No||No||No|
|Vaccination status dams||No||No||No|
A sick calf and a dead calf were presented to the Lithgow Veterinary Hospital on 15 October 2012. The sick calf, a well-grown three quarter Limousin was alert and responsive but dull. While it could be encouraged to stand, it preferred to sit or lie down. It had diarrhoea, a rapid heart rate and laboured breathing but was not febrile. The calf was also observed to bleed excessively from the venipuncture site. Despite treatment including approximately 45,000 IU/kg of procaine penicillin, meloxicam and oral fluids and electrolytes, the calf died overnight. Blood tests from this calf were generally unremarkable except that the CK levels were 1995 U/L (normal 0-300 U/L) and GSH PX were 9 Ug/Hb (normal 40-300 U/gHb).
The calf presented dead to the Lithgow Veterinary Hospital (case 1) and calves from the second and third properties (cases 2 and 3) were autopsied. All were remarkably well grown with bloody fluid exuding from the mouth and sub-cutaneous emphysema palpable over the upmost trunk and sometimes upper hind limb. In cases 1 and 2 the muscle of one hind leg was most affected (as shown in figures 2 and 3) whereas in case 3 the primary site appeared to be the myocardium (please see separate case report in this edition of Flock and Herd).
Histopathology in all three cases revealed a severe acute necrotising and haemorrhagic myositis. Gas bubbles and bacilli were seen within the tissues.
Blackleg is routinely diagnosed in young rapidly growing cattle on the coast of NSW. However, it is rarely diagnosed west of the mountains. Given that widely separated outbreaks of blackleg were first reported in NSW in 1872 (Seddon and Albiston 1965) and that Cl. chauvoei spores are believed to lie dormant in muscles it seems likely that the causative organism has had considerable time and opportunity to spread across the cattle raising districts of Australia. This suggests that environmental factors are behind the current distribution of clinical blackleg in cattle in NSW. The most obvious feature of the distribution of blackleg in NSW is its association with high rainfall. It is therefore of interest that while this outbreak occurred just west of the Blue Mountains (but east of the Great Divide), it is also a high rainfall area. Lowther averages 960 mm pa, Lithgow 830 mm and Katoomba 1402 mm (Bureau of Meteorology, 2012).
The available veterinary literature mentions that while blackleg can occur in younger calves and older cows, cattle between 6 months and 2 years of age are most susceptible (Seddon and Albiston 1965, Beveridge 1983, Radostits et al 2010, Parkinson et al 2010). Hart's analysis (2010) however shows that seven of 24 outbreaks (29%) and 22 of 59 deaths (37%) occurred in calves under 6 months of age. This is likely to be an underestimate because he presented his data in overlapping age groups. Poad and Ball (2012) analysed data from outbreaks of blackleg on the NSW north coast and showed that 18 of 46 outbreaks (39%) occurred in calves under 6 months of age (although they consider that the owners may have underestimated the age of affected calves). In the three cases reported here, all occurred in calves approximately 3 months of age. The combined data suggest that calves are highly susceptible to blackleg from 3 months of age onwards. This has practical significance meaning that the initial vaccination, usually given at calf marking, should not be delayed.
While the available literature also reports that blackleg can occur in older cattle, no animals older than two years of age died in the combined 95 outbreaks reported by Hart (2010), Poad and Ball (2012) and here.
Of interest, the six calves that died were all three quarter Limousin (6 of 20 or 30%), while none of the 28 Limousin calves from the Hereford Angus cows died, although one became sick and responded to treatment. This difference is significant (p=0.016). Perhaps the increased muscularity of the Limousin calves rendered them more susceptible.