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.