Dairy cows are often treated with antimicrobials, most often due to mastitis requiring the use of such veterinary medicines. When dairy cows are treated with medicines that have a licensed withdrawal period (during which the milk cannot be transported and sold for human consumption), waste or discard milk is produced. This milk, which is unfit for human consumption, is sometimes simply poured out, but more often used to feed calves.
Several studies have already reported a high prevalence of antimicrobial resistance in faecal bacteria shed by dairy calves. Although faecal shedding is influenced by many factors, such as the environment and the age of the calf, feeding milk containing antimicrobial residues contributes significantly to the increase in the prevalence of antimicrobial resistant (AMR) bacteria, such as the extended-spectrum beta-lactamase (ESBL)-producing E. coli.
The European Food Safety Authority (EFSA) publsihed its scientific opinion on the risk of AMR development in dairy calves in 2017. The authors of a new study presented recent research in this area, focusing on the period 2016-2020.
A total of 19 studies were evaluated, reporting different results. Feeding waste milk containing antimicrobial residues appears to increase the shedding of antimicrobial resistant bacteria in calves, but this process is often short-lived and transient. Changes in the microbiome of calves have generally been reported following feeding of waste milk, but it is not possible to confirm clearly whether these changes have a positive or negative effect on calf health.
Likewise, the transfer of antimicrobial resistant bacteria from waste milk to calves appears to be a very complex phenomenon. It is important to note that the majority of the publications reviewed in this study examined only the pattern of E. coli resistance. Although E. coli is one of the most common bacteria found on dairy farms, these results should be verified in the future by investigating other bacterial pathogens and commensal bacteria, such as Enterococcus spp.