Determinants of Antibiotic Resistance in Escherichia coli Isolated from Calves

The development of bacterial antibiotic resistance is a public and veterinary health problem. It is not known what conditions or management strategies minimize the occurrence of resistance in animal agriculture. The purpose of this project is to collect Escherichia coli from from calves raised under a variety of management systems, link the patterns of resistance to management and environmental attributes, define the economics of antibiotic use, and develop educational modules to describe approaches that minimize the occurrence of antibiotic resistant bacteria.
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<ol> <li>Cohort study of calves raised as heifer replacements, fed beef, and veal animals. Animals will represent the geographic diversity of operations in California.
<li>Clinical trial evaluating the impact of environment and management on the emergence and persistence of antibiotic resistance in commensal organisms isolated from calves.
<li>Molecular characterization of the population structure of the commensal organisms isolated from calves.
<li>Developement of educational strategies to minimize the occurrence of antibiotic resistance in calves.</ol></p>
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Thirty different types of calf rearing facilities (veal, fed-beef, and heifer replacement) are included in the study. The farms enrolled have been located in Humboldt, Orland, Merced, Fresno, and Tulare counties all in California. Animal Selection-Newborn calves or those newly arrived at the facility are enrolled into the study. At each facility, 10-15 calves have been enrolled. Each calf enrolled in the study is uniquely identified. In the small dairies we have enrolled the 10-15 youngest calves on the first visit and on the follow-up visit additional calves that had been born since the previous visit have been enrolled. During the first visit a questionnaire has been administered and a fecal sample and blood sample from all calves entered into the study cohort. Two, four and six-week samples are thereafter collected from the cohort. History on treatments of the calves are collected. On the second sampling period, a new set of calves have been enrolled. In addition animals from the first sampling period (if available) have been selected and fecal swab obtained. Fecal samples have been cultured for E. coli and Salmonella. Antibiograms have been performed on 3 E. coli isolates per sample and on any Salmonella isolate found. After each sampling period the farm receives a report of the results of the study including antibiotic resistance, treatment and colostrum status information The significant outcomes include: clear age related increases in antibiotic resistance, the effect is stronger in herds that supplement milk with antibiotics, and resistance increases following therapy but rapidly disappears.
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Objective 2 & 3: This is a 2-part clinical trial evaluating the influence of environment and antibiotic use on the dynamics of enteric E. coli isolated from calves. In the trial, the use of antibiotis in the milk as well as individual antibiotic therapy promoted the emergence of more resistant fecal E. coli population in the calves. The environment does not to the same degree influence the resistance patterns of the enteric E. coli of the calf. The calves not directly exposed to antibiotics also carried E. coli resistant to several antibiotics. Outreach and teaching describing study outcomes and defining approaches for calf rearing to minimize the occurrence and transfer of antibiotic resistance in calves has been initiated. The investigators have participated in several dairy production days organized by farm advisors. These data have been incorporated in web pages on market animal production and has appeared in several lay journals.</p>
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We have acquired knowledge on challenges of raising bull calves without antibiotics in the present calf ranch system in California. The study has shown that without the use of antibiotics for therapy or as prophylaxis, it is extremely difficult and labor-intensive to raise pre-weaned calves. Even though the mortality and morbidity does not directly reflect the extent of problems that the calf ranches will face, the study could indicate the difficulties that a calf ranch would encounter when the animals are under unusual stress due to environmental conditions and a high disease pressure with the presence of calf pathogenic salmonella. On the herds that we have worked with there have been changes in management.</p>