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Early weaning plus BRD and monensin toxicity

Unusual presentation:

Separation, changes in diet and increases in stocking density are stressors at weaning.

This disease event, which occurred after an early weaning, was regarded as significant due to:

  • The large number affected – 100 becoming sick and 40 dying in a mob of 450
  • Many with high temperatures and respiratory signs.

Time and location: In March 2018, in yards in western Queensland. A reconstructed timeline of events is shown above.

Case definition:  80-100 kg Wagyu-cross weaners of mixed sex in poor body condition

  • With high rectal temperatures, depression, listlessness, coughing and nasal exudate
  • In yards for 2 weeks or more, fed on hay and a custom weaner mix
  • Taken ‘off early from cows in poor condition’ (due to a failed wet season).

Gross post mortem findings: Gross findings were not available for 3 weaners sampled post mortem by the property manager when they started dying at two weeks. The main autopsy findings by the vet for a bull weaner calf with a high temperature (over 41oC), three weeks after the mob had been yarded, were ventral lung consolidation and a good gut fill. At this point BRD was considered to be likely and antibiotic treatment started.

Laboratory findings: The property manager had collected lungs from 3 animals and all showed evidence of moderate to severe pulmonary oedema and haemorrhage. Pathology in the heart, lung and liver collected from 1 animal (myocardial necrosis and hepatic ischaemia) were consistent with a cardiac toxin. This raised the possibility of exposure to monensin, either as sudden access or in amounts beyond the recommended daily intake. Feed samples sent for monensin testing showed levels ranging from 51-310 mg/kg.

The vet had submitted a variety of fresh and fixed tissues (lung, heart, liver, spleen, rumen, small intestine and abomasum), a lung swab and bloods. This showed evidence of bronchopneumonia and rumenitis. BHV-1 and Mycoplasma were identified but their significance was unable to be determined.

Animal / management / environment risk factors: The circumstances contributing to this disease event appeared to be ‘a perfect storm’ of tough seasonal conditions, low bodyweight, little immunity, variable feed quality and consumption and a rain event. The problem was regarded as multifactorial, including monensin toxicity, bovine respiratory disease and some degree of rumenitis.

Recommendations to the producer: Weaners less than 100 kg are a high risk group.

  • Good quality hay is a must for weaners
  • Will eat 1-3% of body weight each day
  • Aim to gain 0.1kg/day
  • Cannot get enough protein from roughage and will require protein meals to make up the deficits
  • Divide weaner into weights ranges and allow sufficient access for all animals to feed
  • Require 15-20cm per head trough space for concentrates
  • Avoid feeding off the ground use trough and hay feeders, clean water troughs daily
  • Ensure adequate and even mixing of concentrates, avoid situations where concentrates ‘may settle out’ leading to toxic levels of rumensin or urea
  • Monitor consumption of concentrates
  • Seek input from a veterinarian and suitably qualified nutritionist before early weaning
  • Review MLA publication Weaner management in northern beef herds.
Histo revealed the heart muscle pathology in this SDI (slide from Berrimah Vet Lab)

Histo revealed the heart muscle pathology in this SDI (slide from Berrimah Vet Lab)

Disease mapping:  Regularly tightening the case definition as information becomes available provides clarity when more than one disease process is in play (eg for this SDI where there was a combination of respiratory disease, rumenitis and death). The case definition helps direct the lines of enquiry including:

  • the selection of ‘typically affected’ animals for sampling
  • what risk factors to assess (eg attack rates for biggest animals / time spent feeding etc).