Trends in antimicrobial resistance of Salmonella isolated from animals, foods of animal origin, and the environment of animal production in Canada, 1994-1997

The purpose of our study was to determine the occurrence, magnitude, trends , and relationships regarding antibiotic resistance of Salmonella isolated from animals, animal food products, and the environment of animals. We exam ined 621 strains of 67 different serovars isolated in 1994, 721 strains of 75 different serovars isolated in 1995, 1,219 strains of 83 different serov ars isolated in 1996, and 1,336 Salmonella strains of 92 different serovars isolated in 1997, for resistance to 17 antibiotics at one to three differe nt concentrations with the agar dilution method. The overall resistance mag nitude regressed from 9.2% in 1994 to 8.1% in 1997. Resistance to streptomy cin (30.4% of 3,897 isolates), tetracycline (27.3%), and sulfisoxazole (23. 7%) was highest. Resistance to streptomycin, tetracycline, kanamycin, and g entamicin declined during the 4-year period. Notable increases in resistanc e to ampicillin, chloramphenicol, and neomycin occurred during the 1994-199 7 years. None of the isolates was resistant to amikacin. None of the isolat es was resistant to ciprofloxacin at 1, 2, and 4 mug/ml. Salmonella bredene y isolates from turkeys showed a decreased sensitivity to ciprofloxacin and were resistant at the low level of 0.125 mug/ml, but none of these isolate s was resistant at 1 mug/ml. Resistance to nalidixic acid correlated signif icantly with decreased sensitivity to ciprofloxacin; 122 of 127 (96%) isola tes resistant to nalidixic acid at 32 mug/ml were resistant to ciprofloxaci n at 0.125 mug/ml but sensitive at 1 mug/ml. Resistance to S. typhimurium t o each of the seven antibiotics ampicillin, chloramphenicol, kanamycin, neo mycin, streptomycin, sulfisoxazole, and tetracycline increased persistently during each of the years 1994-1997, but none of the S. typhimurium isolate s showed decreased sensitivity to ciprofloxacin. Clinical isolates of Salmo nella were twice as frequently resistant to the antimicrobials in the test panel than isolates obtained during surveys. Salmonella isolates from turke ys were more frequently resistant than isolates from pigs, cattle, and chic kens.