Patterns of antimicrobial resistance in pathogenic Escherichia coli isolates from cases of calf enteritis during the spring-calving season.

Neonatal enteritis is a common condition of young calves and can be caused by pathogenic strains of Escherichia coli. We hypothesised that on-farm antimicrobial use would result in an increased frequency of resistance in these strains during the calving season. We also sought to determine if the frequency of resistance reflected on-farm antimicrobial use. Faecal samples were collected from cases of calf enteritis on 14 spring-calving dairy farms during two 3 week periods: Period 1 - February 11th through March 2nd 2008 and Period 2 - April 14th through May 5th 2008. E. coli were cultured from these samples, pathogenic strains were identified and antimicrobial susceptibility testing was carried out on these pathogenic isolates. Antimicrobial prescribing data were collected from each farm for the previous 12 months as an indicator of antimicrobial use. The correlation between antimicrobial use and resistance was assessed using Spearman's correlation coefficient. Logistic regression analysis was used to investigate the relationship between resistance, sampling period and pathotype. Penicillins and aminopenicillins, streptomycin, and tetracyclines were the most frequently prescribed antimicrobials and the greatest frequencies of resistance were detected to these 3 antimicrobial classes. A strong correlation (ρ=0.879) was observed between overall antimicrobial use and frequencies of antimicrobial resistance on farms. Sampling period was significant in the regression model for ampicillin resistance while pathotype was significant in the models for streptomycin, tetracycline and trimethoprim/sulphamethoxazole resistance. The frequencies of resistance observed have implications for veterinary therapeutics and prudent antimicrobial use. Resistance did not increase during the calving season and factors other than antimicrobial use, such as calf age and bacterial pathotype, may influence the occurrence of resistance in pathogenic E. coli.