Antimicrobial Resistant Escherichia coli in Faeces from Preweaned Dairy Calves

Antimicrobial resistant (AMR) bacteria are increasing threats for human and veterinary medicine. Faecal Escherichia coli (E. coli) from preweaned dairy calves is often resistant to multiple antimicrobials and calves may therefore serve as reservoirs for these bacteria and their resistance genes. This thesis investigated the prevalence, risk factors, and spread of resistant E. coli on Swedish dairy farms, with special emphasis on quinolone resistant E. coli (QREC). Faecal samples from preweaned calves and post-partum cows were analysed for resistant E. coli and set in relation to potential risk factors. The farm environment was sampled to study the occurrence and spread of QREC. The occurrence of faecal resistant E. coli in calves was strongly age-dependent, but was also associated with herd size, milking system, calf housing, and geographic location of the farm. Treatment with some broad-spectrum antimicrobials in cows or calves increased the occurrence of resistant E. coli in calves. Feeding waste milk from cows treated with antimicrobials during lactation to calves increased the proportion of streptomycin and quinolone resistant E. coli in calves, but feeding waste colostrum from cows treated with antimicrobials at drying off had no effect on AMR E. coli. Feeding such colostrum or milk to calves was a common practice on Swedish dairy farms, in particular on farms in southern Sweden, on non-organic farms, and on farms with tie stall housing. On farms where QREC is common in faeces of calves, these bacteria were also widespread in the farm environment. In particular, the calf feed and water trough contained QREC. The same QREC genotype was found throughout the same and on different farms, suggesting contagious spread of QREC within and between farms. Fluoroquinolone treatment, WM feeding, group calving, poor farm hygiene, purchasing cattle or shared animal transports were some risk factors for increasing the occurrence of QREC on the farm. Altogether, the results indicate that proper biosecurity and improved hygiene, less exposure to broad-spectrum antimicrobials, and restrictive waste milk feeding may be important factors to reduce the burden of AMR E. coli on dairy farms.