Dynamics of Fecal Antimicrobial Resistance in the Young Dairy Calf
Open Access
- Author:
- Springer, Hayley
- Graduate Program:
- Pathobiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 28, 2022
- Committee Members:
- Jasna Kovac, Outside Unit & Field Member
Ernest Hovingh, Chair & Dissertation Advisor
Bhushan Jayarao, Major Field Member
Suresh Kuchipudi, Major Field Member
Anthony Paul Schmitt, Professor in Charge/Director of Graduate Studies - Keywords:
- antimicrobial resistance
antibiotic resistance
dairy
calf - Abstract:
- The development and spread of antimicrobial resistance is a threat to both human and animal health. Identifying reservoirs of antimicrobial resistance genes (ARG) is an important aspect of understanding where to target stewardship efforts most effectively and to locate likely sources of ARG that could spread to humans or the environmental. Dairy calves are more likely to carry antimicrobial resistant fecal Escherichia coli than are other dairy animals on dairy farms, suggesting they may play a role as a reservoir of ARG on dairies. This project aims to understand more about the dynamics of AMR in the dairy calf and to assess the impact of management factors and potential interventions to mitigate the development of resistant fecal E. coli. A field study of E. coli recovered from paired pre- and post-weaning samples from waste-milk-fed commercial dairy calves identified more frequent recovery of phenotypic resistant and multi-drug resistant (MDR) E. coli from pre-weaning samples than from post-weaning samples. This same pattern was also seen in a resistome analysis of the fecal metagenome of these samples. Further, it identified an inverse relationship between recovery of MDR E. coli and age of calf through the post-weaning period among randomly isolated E. coli, but not E. coli isolated under conditions to select for β-lactamase and extended-spectrum β-lactamase producing E. coli. This suggests that although frequency of isolation declines, highly resistant E. coli are still present through the postweaning period, albeit at low levels. An experimental study evaluating the impact of weaning calves at age at 7 weeks or11 weeks, and low levels of antibiotics in the diet, or no antibiotics in the milk, identified the presence of antibiotics as a greater driver of antimicrobial resistance than weaning age. Following identification of a positive association between iron scavenging genes and MDR E. coli recovered from veal calves and dairy farms from previous studies, a hypothesis was developed that the low iron levels of milk could select for E. coli carrying iron scavenging genes, and thereby potentially co-select for MDR E. coli. Calves were fed either unsupplemented milk replacer or milk replacer supplemented with ferrous iron in an effort to reduce the selective advantage of iron scavenging proteins. No differences between treatments were identified based on phenotypic resistance. These findings can be utilized to build a more effective on-farm steward program through altering management practices and support the need for further evaluation of strategies to mitigate antimicrobial resistance in the young dairy calf.