Blackleg in unweaned calves in good condition

Above: Haemorrhages were visible on the cardiac surface.

Time and location: March to April 2022, central Queensland​​.

Case definition: Six-month-old, unweaned calves in good condition found dead or seen to die within a few hours of becoming lethargic and recumbent.

Disease mapping: Two calves were found dead, five died within a few hours of being noticed lethargic and becoming recumbent without signs of struggle, one calf seen salivating and lame before death. Another 7 calves were missing at mustering. All the deaths and missing calves were from the same paddock. Following a very good wet season there was abundant grass and forbs. The deaths were near the tracks or in the yards after mustering. The calves in that paddock were the only mob on the property that had not yet been vaccinated with 5-in-1.

Gross findings: Post mortem of a 6 month old calf in excellent body condition that died about two hours previously found 5-10 mL of dark blood in the pericardial sac and paintbrush haemorrhages on the right side surface, auricle and apex of the heart. There was a small paintbrush haemorrhage on the lumbar muscle but no evidence of swelling, discolouration or emphysema in the large muscle groups. Sparse ecchymotic haemorrhages were seen on the muscosal surface of the abomasum. The brain, rumen, liver, spleen and intestines appeared normal. Lungs and kidneys were congested but no visible lesions. Fresh and fixed samples of heart, lung, liver, kidney, spleen, skeletal muscle and brain were submitted, along with aqueous humour, joint fluid, peritoneal and pericardial fluid.

The differential diagnoses were: Clostridium chauvoei or other Clostridial species; poisoning by toxic plants – cyanide (Eremophila maculata, Eucalyptus cladocalyx), cardiac glycosides leading to sudden death with exertion (oleander, rubber vine, mother of millions, Adonis microcarpa [pheasants eye],  Digitalis purpura [foxglove] or Corchorus olitorius; arsenic from an old dip (although none was recorded in the paddock involved); and Bovine Ephemeral Fever.

Laboratory findings: Analysis of the aqueous humour ruled out nitrate/nitrate and cyanide poisoning. Culture of the liver was negative for Haemophilus spp. Fluorescent antibody tests were positive for Clostridium chauvoei in the heart and skeletal muscle and Clostridium novyi B in the skeletal muscle. Histopathology of the heart showed acute, severe myocarditis, epicarditis and endocarditis with numerous scattered gram-positive bacilli within affected areas. Skeletal muscle also showed multifocal, acute, severe degeneration and necrosis with haemorrhage and clumps of Gram-positive bacilli. There was moderate expansion of fascial tissues by clear spaces (putative emphysema). The diagnosis was blackleg.

Animal / management / environment risk factors: The unvaccinated status and good season contributed to the risk of blackleg.

Recommendations: The owners were starting prophylactic vaccination of the calves for clostridial disease at the time of the last deaths. The recommended preventative measure in the longer term was to ensure the vaccination program has initial vaccine and booster completed by 5 months so the calves are protected during weaning.