Dietary tryptophan regulated performance, intestinal morphology and inflammation, and disease resistance of coccidia-challenged broiler chickens

The objective of the study was to evaluate tryptophan’s functionally essential role in regulating performance, intestinal inflammation, and disease resistance during a coccidia challenge. A total of 300 two-day-old Cobb 500 broiler chickens (54.29 ± 0.284 g body weight [BW]) were group housed (5 chickens/pen; 10 pens/treatment) in battery brooders and had ad libitum access to a basal diet and water. At 7 d of age, pens were randomly assigned to 1 of 6 levels of dietary tryptophan: 0.15%, 0.185%, 0.22%, 0.32%, 0.42%, or 0.52% of the total diet. Following a 3-d adaptation period, chickens received an inoculum in the feed, consisting of Eimeria acervulina (7.2 × 103), Eimeria maxima (1.7 × 103), and Eimeria tenella (2.8 × 103). Weight gain and feed intake were recorded at 5 and 10 d post-inoculation (DPI). On 5 and 10 DPI, feces were collected for oocyst enumeration by flow cytometry, and jejunum was sampled from 1 bird per pen to analyze morphology and cytokine gene expression. Data were analyzed by one-way ANOVA, and orthogonal polynomial contrasts were used to evaluate the linear and quadratic effects of tryptophan on the dependent variables. Dietary tryptophan increased (linear and quadratic, P < 0.01) BW and feed intake and decreased (linear and quadratic, P < 0.01) feed conversion ratio during the entire experimental period. Chickens fed more tryptophan had increased (linear and quadratic, P < 0.05) shedding of E. acervulina at 5 and 10 DPI and E. tenella at 5 DPI. Supplemental tryptophan increased (linear and quadratic, P < 0.01) crypt depth at 5 and 10 DPI and decreased (linear and quadratic, P < 0.01) the ratio of villus height to crypt depth at 5 DPI. Tryptophan also decreased (linearly, P = 0.03) interferon-γ expression at 5 DPI and increased (quadratically, P < 0.01) interferon-γ, FOXP3, and interleukin-10 expression in the jejunum at 10 DPI. Current results suggest that dietary tryptophan levels above 0.22% improved growth performance, although concentrations between 0.22% and 0.52% may exacerbate intestinal inflammation and damage associated with coccidia infection.

Key Words: Tryptophan, Eimeria, broiler.