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Botulism the prime suspect

Unusual presentation:

Death of nine cattle over a week.

An 18-month-old silage bale

Above: Contamination of silage with botulism toxin was a possibility.

Time and location: September 2021, Far North Queensland​​.

Case definition: Sudden death of cattle being fed a mixed ration.

Disease mapping: Eight weaners (from a mob of 30) and a cow (one of 10) died over a 7-day period.  The cattle only had access to a mixed ration of tropical grass, silage and supplementary feed. Contamination of the silage with Botulism toxin was considered. All cattle had been vaccinated eight months earlier with Longrange Botulinum vaccine.

Gross findings: Several post-mortem examinations were conducted with extensive laboratory workup, without confirmation of a definitive diagnosis. Sera were also collected from three animals that were considered healthy and three that had possibly been unwell and that had recovered.

Laboratory findings: No significant gross or histopathological changes were observed in the post-mortems. Tests for Bovine Ephemeral Fever and Clostridia were negative.

All of the six live animals were ELISA negative for Botulinum toxin (note this does not rule out botulism as a possible diagnosis). Their sera was also tested by ELISA for Botulism antibodies, with only three of the six demonstrating significant titres. Because animals previously vaccinated should have had positive titres, this was a concerning finding.

Animal / management / environment risk factors: The possibility of failure of effective immunization, despite the history of whole herd vaccination, was discussed with the owner.

Recommendations: The old silage was replaced with more recently preserved bales. The herd was re-vaccinated for botulism and it was decided that higher quality maize silage would be sourced in the future.