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

This longitudinal observational study aimed at characterizing calf exposure to waste milk from cows treated with antimicrobials (WMA) on Swiss dairy farms, and at identifying factors associated with the exposure incidence. Data on WMA feeding to calves were collected on 17 small Swiss dairy farms over a nine-month period using producer-maintained records. Across the participating farms, data on 666 instances (days) on which WMA was fed to calves and on 209 exposed calves were analyzed. Most WMA feeding events involved a combination of drugs from two antimicrobial classes (63.1%), with penicillin-gentamicin used mostly for intramammary (IMM) treatment being the most common drug combination (38.0%). The median exposure incidence among exposed calves was 3.8 days per 30 days at risk of exposure to WMA, with wide variability across herds (range 0.4-30). In a mixed-effects negative binomial model, two herd-level variables were significantly associated with higher exposure incidence: higher annual IMM treatment incidence (incidence rate ratio [IRR] 1.2 per additional treatment day/cow-year, 95% CI: 1.1-1.3), and feeding WMA during the course of IMM treatments (IRR 1.9, 95% CI: 1.2-3.0). The latter practice may have been more frequent in the participating farms than in the overall population of Swiss dairy farms, which might have led to an overestimation of the WMA exposure incidence. Nevertheless, these findings suggest that calf exposure to WMA on small dairy farms is rather infrequent. However, considering the importance of additional characteristics of antimicrobial exposure (e.g. drug classes and dose), the relevance of this finding with regard to outcomes in calves, including the carriage of antimicrobial-resistant bacteria, should be interpreted with caution. While antimicrobial treatments in cows contribute to WMA production, WMA calf exposure additionally depends on farm-specific WMA feeding practices and criteria.