Early weaning plus BRD and monensin toxicity

Time and location: March 2018, western Queensland.

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.

Disease mapping: The weaners were taken ‘off early from cows in poor condition’ (due to a failed wet season) and held in yards for 2 weeks or more, fed on hay and a custom weaner mix. A reconstructed timeline of events is shown here.

Gross findings: 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. A variety of fresh and fixed tissues (lung, heart, liver, spleen, rumen, small intestine and abomasum), a lung swab and bloods were sent to the lab.

Laboratory findings: The property manager had collected lungs from 3 animals when they started dying at two weeks 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 tissue samples submitted by the vet 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, they will eat 1-3% of body weight each day, aim to gain 0.1kg/day.
• They 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.
• They 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.