Feed supplemented with three different antibiotics improved food intake and reduced the activation of the humoral immune response in healthy weaned pigs but had differing effects on intestinal microbiota

The aim of this study was to determine the effects of 3 antibiotics used for pulmonary pathologies added in the feed of weaned pigs on growth performance, commensal microbiota, and immune response. At weaning, a total of 72 pigs were randomly assigned by live weight and litter to one of the following diets: control (typical weaning diet), control + 400 mg tilmicosin/kg, control + 600 mg amoxicillin/kg, and control + 300 mg doxycycline/kg. Individually penned pigs were sacrificed after 3 wk (12 pigs/treatment) or 4 wk (6 pigs/treatment). During the fourth week, all pigs received the control diet to test the residual effect of the antimicrobial supplementation. The antibiotic supplementation increased growth and feed intake during the first week (P < 0.01) and over the first 3 wk combined (P < 0.05). Gain to feed ratio tended to improve during the first week (P = 0.076) by the antibiotics compared with the control. Among the antibiotic treatments, no difference was observed in ADG and feed intake, which were also unchanged by the diet on the fourth week. The fecal enterobacteria counts were increased by amoxicillin on d 14 and 21 (P < 0.05 and 0.01, respectively) and were reduced by tilmicosin (P < 0.001) compared with the control. Amoxicillin reduced lactic acid bacteria (P < 0.01) compared with the control. The antibiotic supplementation tended to reduce total bacteria variability in jejunum (Shannon index, P = 0.091) compared with the control. The antibiotic treatment reduced mean total serum IgM (P = 0.016) after 3 wk and did not change the mucosal histomorphometry of the small intestine. For tilmicosin, the observed positive action on piglet performance and feed intake can originate by the reduced costs of immune activation determined by the action on intestinal microbiota. For amoxicillin and doxycicline, the observation on intestinal and faecal microbiota seems to be not sufficient to explain their growth promoting effect