Abstract
A number of studies have reported that there is a high prevalence of antimicrobial-resistant faecal bacteria excreted by dairy calves. Although faecal shedding is influenced by a variety of factors, such as the environment and calf age, feeding milk with antimicrobial residues contributes significantly to an increased prevalence of antimicrobial-resistant (AMR) bacteria, such as extended spectrum beta-lactamase (ESBL)-producing E. coli. As a follow-up to the European Food Safety Authority (EFSA) Scientific Opinion on the risk of AMR development in dairy calves published in January 2017, this review aims to illustrate more recent research in this area, focusing on the period 2016 to 2020. A total of 19 papers are reviewed here. The vast majority assess the commensal faecal bacteria, E. coli, isolated from dairy calves, in particular its antimicrobial-resistant forms such as ESBL-producing E. coli and AmpC-producing E. coli. The effect of waste milk feeding on the prevalence of pathogens such as Salmonella spp. has also been investigated. Current research findings include positive effects on daily liveweight gain and other advantages for calf health from feeding waste milk compared to milk replacer. However, the negative effects, such as the demonstrable selection for antimicrobial-resistant bacteria, the shift in the intestinal microbiome and the possible negative consequences that these could have on global public health, should always be taken into consideration.
Highlights
The use of antimicrobials and the subsequent selection for antimicrobial resistance in commensal and pathogenic bacteria is a global health problem affecting both human and veterinary medicine as well as the wider human and animal populations and the environment [1,2].On a global scale, dairy cows are frequently treated with antimicrobials, with mastitis being the most common indication for the use of such veterinary medications [3,4,5,6]
The continuous feeding of small amounts of antimicrobial residues in waste milk is a potential cause for concern with respect to the development of antimicrobial resistance, and for this reason, in 2017, the European Food Safety Agency (EFSA) published a “Scientific Opinion” report on the possible risks posed by such feeding [3]
The overall conclusions of the ESFA report were that while feeding calves with colostrum containing certain antimicrobial residues did not increase faecal shedding of AMR bacteria, the feeding of waste milk to calves has a transient effect on the faecal shedding of AMR bacteria, which reduces over time, and measures should be taken to inactivate antimicrobial residues and/or AMR bacteria prior to feeding [3]
Summary
The use of antimicrobials and the subsequent selection for antimicrobial resistance in commensal and pathogenic bacteria is a global health problem affecting both human and veterinary medicine as well as the wider human and animal populations and the environment [1,2].On a global scale, dairy cows are frequently treated with antimicrobials, with mastitis being the most common indication for the use of such veterinary medications [3,4,5,6]. Whenever dairy cows are treated with medicinal products that are licensed with a withdrawal period (during which time the milk cannot be delivered to the commercial dairy for human consumption), waste or discard milk is produced. This waste milk is sometimes dumped into watercourses, drains or onto manure heaps but is more commonly fed to calves [3]. The continuous feeding of small amounts of antimicrobial residues in waste milk is a potential cause for concern with respect to the development of antimicrobial resistance, and for this reason, in 2017, the European Food Safety Agency (EFSA) published a “Scientific Opinion” report on the possible risks posed by such feeding [3]. As the EFSA Scientific Opinion was published in January 2017, the aim of this narrative literature review is to provide an updated overview of the scientific research in this area published since 2016
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