The emergence of bacterial pathogens that are resistant to medically important antimicrobial drugs is a current public health concern. Some scientists as well as government officials believe that the agricultural use of antibiotics, particularly at sub-therapeutic doses, is inappropriate and is contributing to the development of antimicrobial resistance with public health implications. There is also a question about the appropriateness of using certain human drugs, such as fluoroquinolones and expanded-spectrum cephalosporins, in animals. One such drug that has received considerable attention is ceftiofur, an expanded spectrum cephalosporin which is labeled for use in cattle and other food animals. Ceftiofur is similar to ceftriaxone, an expanded spectrum cephalosporin used in human medicine. Ohio dairy herds and finishing swine barns at a single farm in Michigan were studied to determine the prevalence of the ESBL producing E. coli in food animal populations and to quantify the association between the use of ceftiofur and the dissemination of ESBL producing E. coli in these animals.
We found that ceftiofur was reportedly used to treat some health conditions in 11 (61%) of the herds and none of the swine barns included in this study. The mean percentage of animals treated with ceftiofur was 12.4% in herds using ceftiofur, ranging from 0 to 36% during the six month observation period. ESBL producing E. coli were recovered animals in 67% of the dairy herds and from 34% of the individual dairy samples. These ESBL producing E. coli were recovered from 40% of the swine barns and nine of the 864 individual finishing swine. Dairy herds in which ceftiofur use was reported were approximately twenty-five times more likely to have animals with ESBL producing E. coli than those that did not use ceftiofur (P<0.01). Our data suggest that use of ceftiofur on farms leads to the dissemination of these bacterial resistance genetics in the intestinal flora of food animal populations. At the individual level, there was no association between administering ceftiofur and the presence of ESBL producing E. coli. Our observation of a herd level association but not an individual level association suggests that the treatment of an individual with ceftiofur leads to the dissemination of ESBL producing E. coli to the entire population of animals. This suggests that interventions designed to reduce the dissemination of antimicrobial resistance genetics should be applied at the herd level and not at the individual animal level.