Increased levels of standardized ileal digestible threonine attenuate intestinal damage and immune responses in Escherichia coli K88+ challenged weaned piglets

Abstract

This study was conducted to investigate the effects of graded levels of standardized ideal digestible threonine (SIDT) on the small intestinal morphology and immune responses of Escherichia coli K88+ challenged weaned piglets. Thirty-six crossbred (Large White×Landrace) barrows, with an initial BW of 7.34±0.55kg and weaned at 21d, were assigned to 1 of 6 treatments according to a 3 (3.7, 7.5 or 11.1g/kg dietary SIDT content)×2 (non-challenged or challenged) factorial arrangement. On d 16, half of the piglets in each SIDT treatment were orally challenged with 10mL E. coli K88+ culture (1×109CFU/mL), while the other half were orally inoculated with the same amount of phosphate buffer (PBS). On d 18 (on 2d after E. coli challenged), the pigs were killed and blood and intestinal samples were collected. The average daily gain (ADG) and feed conversion ratio (FCR) of piglets receiving the 7.5 and 11.1g/kg SIDT diets were significantly improved (P<0.01) during pre-challenge (0–16d), post-challenge (16–18d) and over the entire experiment (0–18d) compared with pigs fed the 3.7g/kg SIDT diet. E. coli K88+ challenge markedly decreased (P<0.05) ADG and FCR of piglets compared with PBS. Increasing dietary SIDT increased the relative thymus body weight of non-challenged piglets while the relative body weight of challenged piglets was not affected by SIDT. Increasing of dietary SIDT significantly increased (P<0.05) lymphocyte proliferation under challenge. Compared with pigs fed the 3.7g/kg SIDT diet, the interleukin-1β concentration significantly (P<0.05) decreased, and the interleukin-6 concentration significantly (P<0.05) increased in the jejunum of piglets fed the 7.5 and 11.1g/kg SIDT diets. It was observed that an interaction between dietary SIDT and E. coli challenge on the levels of mucosal IgA and interleukin-1β (P<0.05). Increasing SIDT increased the duodenal villus width in non-challenged piglets and there was no difference among challenged piglets. Duodenal villi height decreased (P=0.05) after E. coli challenge. Piglets receiving either the 7.5 or 11.1g/kg SIDT diets had increased villi heights and villi height to crypt depth ratio (P<0.05), and decreased crypt depth (P<0.05) in jejunum compared with the 3.7g/kg SIDT diet. Once challenged with E. coli K88+, villus height and villus height to crypt depth ratio of jejunal decreased (P<0.05). The interaction between challenge and dietary SIDT was observed in jejunal crypt depth and villus height to crypt depth ratio. Increasing dietary SIDT decreased the jejunal crypt depth and increased villus height to crypt depth ratio in non-challenged piglets. Moreover, the ileal crypt depth increased (P<0.05) and ileal villus height to crypt depth ratio decreased (P<0.05) after E. coli challenge. Collectively, these results indicate that appropriate dietary SIDT improved mucosal immune function and intestinal morphology of non-challenged pigs and also had beneficial effects in maintaining jejunal integrity and repairing villous damage caused by E. coli challenge. Our findings indicate that appropriate dietary SIDT levels need to be considered when formulating diets for weaned piglets in different physical status.