Antimicrobial resistance in Salmonella enterica subsp. enterica serovar typhimurium from humans and production animals

We have studied the frequency of antimicrobial resistance and epidemiological relatedness among 473 isolates of Salmonella enterica subsp, enterica serovar typhimurium (S. typhimurium) from human and veterinary sources. The human strains were clinical isolates from patients with diarrhoea sent to the State Serum Institute during August 1993 (228 isolates). The animal strains were isolated from clinical or subclinical infections in cattle (48 isolates), pigs (99 isolates) or poultry (98 isolates), all from 1993. All strains were tested against 22 different antimicrobial agents used in both human and veterinary medicine with the tablet diffusion method. Strains were also phage-typed and the plasmid content determined in all resistant strains. Ribotyping was performed on selected strains. Of 228 human isolates tested, 19.3% of the strains were resistant to one or more antimicrobial agent compared with 10.4% of strains from cattle, 11.1% of strains from pigs and 9.2% of strains from poultry. Multiple resistance, i.e. resistance against at least four antimicrobial agents, was found in 9.2% of the human strains, but in only two of the cattle isolates, The majority of the multi-resistant strains in humans were from infections contracted outside Denmark, most often in southern Europe or south-east Asia. Resistance in human strains was most common against tetracycline (13%), ampicillin (12%), sulphonamide (12%), streptomycin (10%) and chloramphenicol (8%). The resistance pattern differed somewhat in animal isolates: Poultry strains were usually resistant only to ampicillin, white pig and cattle isolates were most often resistant to sulphonamide, tetracycline and streptomycin. Typing of the strains showed that some animal strains and human strains were indistinguishable. In conclusion, while antimicrobial resistance was present in S. typhimurium isolated from humans and animals in Denmark, multiple resistance was most often acquired outside Denmark.