LAUSR

The experiment was conducted to investigate the effects of trace amounts of antibiotic on growth performance, diarrhea, systemic immunity, and intestinal health of weaned pigs experimentally infected with an enterotoxigenic Escherichia coli. Weaned pigs (n = 34, 6.88 ± 1.03 kg body weight [BW]) were individually housed in disease containment rooms and randomly allotted to one of three dietary treatments: nursery basal diet (CON) and 2 additional diets supplemented with 0.5 or 50 mg/kg carbadox to the nursery basal diet (TRA or REC), respectively. The experiment lasted 18 d with 7 d before and 11 d after the first E. coli inoculation. The E. coli F18 inoculum was orally provided to all pigs with a dose of 10 10 cfu/3 mL for 3 consecutive days. Fecal and blood samples were collected on d 0 before inoculation, and d 2, 5, 8, and 11 post-inoculation (PI) to test the percentage of β-hemolytic coliforms in total coliforms and complete blood cell count, respectively. Sixteen pigs were euthanized on d 5 PI, whereas the remaining pigs were euthanized at the end of the experiment to collect jejunal and ileal mucosa and mesenteric lymph node for gene expression and bacterial translocation, respectively. Pigs in REC had greater (P < 0.05) final BW and lower (P < 0.05) overall frequency of diarrhea, compared with pigs in CON and TRA groups. Pigs in TRA had the lowest (P < 0.05) average daily gain (ADG) and feed efficiency from d 0 to 5 PI, highest (P < 0.05) percentage of β-hemolytic coliforms in fecal samples on d 2 and 5 PI, and greatest (P < 0.05) bacterial colonies in mesenteric lymph nodes on d 11 PI, compared with pigs in CON and REC. Pigs in TRA had the greatest (P < 0.05) neutrophils on d 5 PI and higher (P < 0.05) white blood cell counts and lymphocytes than other groups on d 11 PI. Pigs in TRA had the greatest (P < 0.05) serum C-reactive protein on d 2 and 5 PI and serum TNF-α on d 5 PI, compared with pigs in the CON and REC. Pigs were fed REC had increased (P < 0.05) mRNA expression of ZO-1 and OCDN, and reduced (P < 0.05) IL1B, IL6, and TNFA in ileal mucosa on d 5 PI, compared with the CON, whereas TRA up-regulated (P < 0.05) mRNA expression of IL1B, IL6, and PTGS2 in ileal mucosa on d 11 PI, compared with the REC. In conclusion, trace amounts of antibiotic may exacerbate the detrimental effects of E. coli infection on pig performance by increasing diarrhea and systemic inflammation of weanling pigs.