Effects of antibiotic regimens on the fecal shedding patterns of pigs infected with Salmonella typhimurium

An experiment was conducted to determine (i) the effects of antibiotic regi mens on the shedding patterns of pigs infected with Salmonella Typhimurium and (ii) whether antibiotic resistance increases the incidence of pathogen shedding. The experiment involved 48 50-day-old pigs challenged with Salmon ella Typhimurium and receiving one of four antibiotic regimens including (i ) intramuscular injection of ceftiofur sodium followed by inclusion of oxyt etracycline in the feed; (ii) apramycin in the feed for 14 days followed by oxytetracycline; (iii) carbadox in the feed until pigs reached 35 kg follo wed by oxytetracycline; (iv) no antibiotics (control). Fecal samples were c ollected preinoculation, 2 and 4 days postinoculation (DPI) and at weekly a nd biweekly intervals thereafter to determine shedding patterns. Salmonella Typhimurium isolates from 2, 4, 7, 21, 42, and 70 DPI were analyzed for an tibiotic resistance. A time effect (P < 0.05) was observed, indicating that the proportion of isolates resistant to at least one antibiotic varied ove r time. Overall resistance was determined to be 46% at 2 DPI and increased significantly (P <.05) thereafter. Treatment x time and antibiotic x time i nteractions were also observed (P < 0.05) as the percentage of isolates res istant to each test antibiotic increased over time. In no case did the deve lopment of antibiotic resistance result in an increased incidence of sheddi ng of the original inoculate. The incidence of shedding was reduced in pigs receiving the apramycin-oxytetracycline treatment, when compared to contro l pigs; however, no differences were observed between antibiotic treatments .