Incidence of antimicrobial resistant fecal Escherichia coli in Tennessee swine units

Sows and pigs from ten Tennessee swine units from various regions of the state were sampled to determine the incidence of resistant fecal Escherichia coli (E. coli) to selected antimicrobial agents. At each site 5 sows and 5 pigs per sow were randomly identified. At 7 days post partem, selected individuals were rectally swabbed to obtain samples for culture. Additional samples were collected from pigs at 35 and 63 days of age. Fecal samples were transferred to lactose MacConkey agar and cultured overnight. Ten lactose fermenting colonies, consistent with E. coli in color and morphology, were randomly chosen and subjected to apramycin, carbadox, gentamicin, neomycin, and oxytetracycline. Isolates were determined to be sensitive, intermediately resistant, or resistant according to the standardized diffusion test established by the National Committee for Clinical Laboratory Standards (NCCLS). Statistical comparisons were made using a Chi-square analysis to determine changes in resistance percentages of antibiotics over time, compare incidence between pigs and sows, and compare incidence between farms classified as low or high antimicrobial drug use. Changes in the percentage of resistant isolates over time were observed to be significant for all five antibiotics (P < .0001). Isolates expressed the greatest sensitivity to apramycin and carbadox, and expressed the greatest percentage of resistance to oxytetracycline. Resistant isolates were observed to be at their highest level in pigs at 35 days of age. Differences (P < .0001) in the percentage of isolates resistant to apramycin, carbadox, gentamicin, and neomycin as compared between sows and pigs were observed. Differences (P < .0001) in the incidence of resistance were observed between farms of high and low antimicrobial drug use in pigs at 35 days of age for all antibiotics tested. These results indicate the need for limited and/or selective antibiotic use in order to preserve future effectiveness.