PhD Thesis Abstract

Spatial, Temporal and Management-Specific Factors Influencing Antibiotic Resistance and Carbohydrate Fermentation Patterns in Bovine Enteric Escherichia coli and the Clinical Consequences of Limiting Antibiotic Use in Pre-weaned Calves.

Antibiotics are used in animal agriculture for prophylactic and therapeutic purposes. Recently, increasing levels of antibiotic resistance have been identified in pathogenic and commensal bacteria. This dissertation describes temporal, animal, and farm specific factors that govern the emergence of multiple resistant enteric Escherichia coli (E. coli) in pre-weaned calves and assesses effects on calf health when minimizing use of antibiotics in commercial systems.

Antibiotic resistance patterns of fecal E. coli from calves were described using cluster analysis techniques. The outcome clusters were then used in multi-level logistic regression models to identify factors associated with increasing levels of resistance. A study of thirty-three farms in California assessed management factors associated with antibiotic resistance in fecal E. coli of pre-weaned calves and indicated that therapeutic and prophylactic antibiotic use was associated with increasing levels of multiple resistance as were calf age, geographical location of the farm, and hygiene protocols.

A clinical trial evaluated the influence of prophylactic and therapeutic use of antibiotics on antibiotic resistance of fecal E. coli from pre-weaned calves and the effects on calf health and performance. Both prophylactic antibiotic use in the milk replacer and individual antibiotic therapy increased resistance in the fecal E. coli. However, prophylactic and therapeutic uses of antibiotics were important to maintain the health of immuno-compromised calves during their first four weeks of life.

Antibiotic resistance patterns and carbohydrate fermentation patterns of E. coli were used to investigate potential clonal dissemination of antibiotic resistant bacteria across several farm types in three western states. The result was that phenotypic techniques, such as antibiotic resistance typing and biotyping, were demonstrated to be valuable techniques for studies of the dissemination of antibiotic resistance in commensal enteric E. coli.