Characteristics of calves' exposure to waste milk from cows treated with antimicrobials on Swiss dairy farms.

Calf WMA exposure is infrequent but varies widely; herd IMM treatment rates and feeding practices are key drivers.

This longitudinal observational study conducted on 17 small Swiss dairy farms over nine months characterized calf exposure to waste milk from antimicrobial-treated cows (WMA) and identified herd-level factors influencing exposure incidence. Data encompassing 666 WMA feeding events and 209 exposed calves were analyzed. The majority of WMA feeding events involved combinations of drugs from two antimicrobial classes (63.1%), with penicillin-gentamicin used for intramammary (IMM) treatment representing the most frequent combination (38.0%). The median exposure incidence among exposed calves was 3.8 days per 30 days at risk, though variability across herds was substantial (range: 0.4–30 days). Mixed-effects negative binomial modeling identified two herd-level factors significantly associated with higher WMA exposure incidence: greater annual IMM treatment incidence (IRR 1.2 per additional treatment day/cow-year) and the practice of feeding WMA during active IMM treatment courses (IRR 1.9). The authors note that the latter practice may have been overrepresented in participating farms, potentially inflating exposure estimates. Overall, WMA calf exposure was considered relatively infrequent on small Swiss dairy farms; however, the authors caution that drug class, dose, and duration of exposure remain critical variables when assessing clinical outcomes in calves, including antimicrobial resistance carriage. Farm-specific WMA feeding practices and decision criteria were identified as important modulators of calf exposure beyond antimicrobial treatment rates in cows.

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