Mass screening for antimicrobial resistant Escherichia coli in dairy cows in northern New South Wales

Objective To describe aspects of the epidemiology of antimicrobial resistance in Escherichia coli shed in the faeces of milking cows in a dairying region of New South Wales. Design A survey based on multi‐stage sampling with repeated measures made within herds for estimating within‐herd correlation of resistance status, and with repeated measures made on identical specimens for estimating test‐retest reliability. Procedure From a population of 110 dairy herds, 30 were selected at random and from each herd between 5 and 10 faecal specimens were obtained from fresh manure pats. E coli from faecal specimens were grown on hydrophobic grid membrane filters (HGMF) and replicated onto chromogenic agar and agar containing antimicrobials (gentamicin, ampicillin, tetracycline and sulfamethoxazole). Image analysis was used to assess colony growth. Data were analysed descriptively, by generalised linear mixed models and by Taylor series linearisation to account for attributes of the survey design. Results Of the 10,279 E coli isolates assessed, 91% expressed no resistance, 7.3% were resistant to sulfamethoxazole, 3.6% to tetracycline, 2.2% to ampicillin and 0.09% to gentamicin. The most common multiple resistance phenotype was ampicillin‐tetracycline‐sulfamethoxazole (1.8% of isolates). Most multiple resistant isolates appeared clustered within particular herds but were too rare to obtain valid estimates of variance, confidence intervals or intra‐herd correlation. The estimated proportion of isolates in the population that were susceptible to all four antimicrobials was 97% (95% CI: 91% to 100%) and 55% of cows had no resistance detected in faecal E coli (95% CI: 27% to 83%). Within‐herd correlation of shedding status (any resistance pattern) was absent and test‐retest reliability of the measurement system was estimated to be at the lower end of good (0.40) but increased to excellent (0.89) after excluding sulfamethoxazole resistance, which had a greater measurement error. Conclusion Antimicrobial resistance was uncommon in E coli in the population of dairy cows studied. HGMF and image analysis is an effective tool for detecting rare forms of resistant E coli that are not uniformly distributed in livestock populations.