[Persistance of resistant Escherichia coli in a calf herd]

In an experimental station for calf feeding the occurrence and distribution of drug resistant E. coli in faeces have been investigated among selected groups of calves. Samples were obatined just after the calves arrived 1--2 weeks old in January--February and at intervals until an age of 7--11 months. Daily from arrival to the age of 8 weeks all calves at the station received antibiotics in their feed. In 1971 it was chlortetracycline and in 1972 zincbacitracin. In addition, from 3.5 percent to 60 percent of all calves were every month given therapeutic doses of antibiotics, mainly penicillin + streptomycin and neomycin. Both years the highest number of treatments were administered in the months of February and March (cf. fig. 1). In each of the years studied, a high prevalence of calves harbouring resistant E. coli (total resistance: greater than 88 percent) and R-+E. coli (1971: 83 percent and 1972: 50 percent) was demonstrated at the first sampling. As the calves grew older a general tendency to exchange the resistant strains with sensitive ones was recognized. The prevalence of calves with R-+ E. coli generally devreased at a faster rate than that of total resistance. A discussion is given of the epidemiological significance of cross infection and of selection pressure of antibiotics for the establishment of a resistant flora in individuals as well as for replacement of this flora with sensitive strains.