Josepha DeLay, Olivia Stone, Julie Hansford, Đurđa Slavić, Cynthia Miltenburg

Animal Health Laboratory, University of Guelph, ON (DeLay, Slavić); Springer Animal Hospital, Sturgeon Falls, ON (Stone); Ontario Veterinary College class of 2020 (Hansford), Guelph, ON; Ontario Ministry of Agriculture, Food, and Rural Affairs, Guelph, ON (Miltenburg)

AHL Newsletter 2019;23(4):6.

Over a 24-hour period in early August 2019, 8 of 30 beef calves died unexpectedly, without premonitory signs except mild weakness in 1 calf. The calves were all approximately 5 months of age, were at pasture with their dams, and received no supplementary feed. The cows were clinically normal. The calves and cows were unvaccinated for clostridial or other diseases. No toxic plants or other toxin source was identified in the pasture. Field postmortems were carried out on 3 calves and lesion identification was challenging due to tissue autolysis and decomposition. Pulmonary edema and fibrinous pleuritis were identified in 2 calves, and rumens were full in all calves. Clinical differential diagnoses included blackleg (despite the absence of gross lesions in skeletal muscle); lead or plant toxicity (pasture exposure); and blue-green algae, sulfur, or nitrate toxicity (water exposure).

Formalin-fixed and fresh tissue samples and blood samples from the calves were submitted to the Animal Health Laboratory for evaluation. Histologically, severe necrotizing and fibrinosuppurative myositis with low numbers of plump bacilli was identified in diaphragm from 1 calf, consistent with clostridial myositis (blackleg). Mild fibrinosuppurative epicarditis and pleuritis in 2 calves were suspected to have the same etiology. Fluorescent antibody testing for clostridia identified Clostridium chauvoei in sections from diaphragm and heart in 2 calves, confirming the diagnosis of clostridial myositis (blackleg) as the cause of unexpected deaths in this herd. The herd was vaccinated with a multivalent clostridial vaccine while test results were pending. One additional calf died after vaccination, 24 hours after the previous calves; no additional losses were reported. Neighboring herds have since adopted a clostridial vaccination protocol.

Clostridium spp. are anaerobic, spore-forming bacilli found in the intestinal tract of livestock and in soil. Although blackleg is a well-recognized disease in cattle, the exact pathogenesis is not completely understood. Cattle likely ingest C. chauvoei spores from contaminated soil, which are subsequently disseminated to tissues. The spores remain dormant until a poorly-understood triggering event occurs, which may include local tissue hypoxia due to traumatic injury, facilitating proliferation of the spores.  Explanations for outbreak situations are speculative, but exposure to large bacterial loads is suspected. Increased incidence of blackleg has been associated with high rainfall, presumably because water-saturated soil promotes anaerobic conditions and clostridial growth.

Blackleg is most commonly seen in cattle ranging from 6 months-2 years of age, which is slightly older than the calves in this group.  The disease is most commonly described in cattle at pasture, however, it has also been described in cattle housed indoors and fed soil-contaminated silage or haylage. C. chauvoei is not among the many clostridia that are postmortem contaminants. As a result, a diagnosis of blackleg can be confirmed with certainty by identifying the organism in association with compatible histologic lesions using fluorescent antibody testing.

References

Groseth PK et al. Large outbreak of blackleg in housed cattle. Vet Rec 2011;169(13): 339.