Dose-dependent contribution of Eimeria tenella to Salmonella infection in broilers: impacts on growth, immunity, and cecal microbiota.

In broiler production, various pathogens coexist, with Salmonella risking public health and coccidiosis reducing productivity; therefore, understanding their interaction is essential. Therefore, this study investigated Eimeria tenella challenge doses on growth, Salmonella colonization, cecal lesions, gut permeability, immune responses, and cecal microbiota in broilers challenged with Salmonella Typhimurium (ST). A total of 392 1-day-old broilers were randomly assigned to four treatments (seven replicates) in battery cages for 20 d. Four treatments comprised a non-challenge control (NC) and three ST-challenged groups (challenged on d 1) that received different E. tenella doses on d 8: 0 (ST-challenged control: STC), 25,000 (coinfection, C25k), or 50,000 (coinfection, C50k) oocysts. One-way ANOVA with Tukey's post hoc test was used for comparing all groups, and the orthogonal polynomial contrast was used to evaluate E. tenella challenge dose effect in ST-challenged groups. Coinfection impacted growth (C25k and C50k vs. NC) most evidently from d 8 to 14, with body weight gain reduced by 27 % and 41 %, feed intake reduced by 11 %, and feed conversion ratio increased by 0.23 and 0.56, respectively (P < 0.05). Similarly, among the ST-challenged groups, these parameters also worsened linearly with increasing E. tenella challenge (Linear, P < 0.05). The highest mortality was in the C50k group (16 %; P < 0.05). Cecal ST load on d 14 increased linearly with increasing E. tenella dose, peaking at the C25k group on d 20 (Linear and quadratic, P < 0.05). Eimeria tenella dose linearly increased gut permeability, and upregulated the expression of interferon γ and interleukin 10 on d 14 (Linear, P < 0.05). The coinfection induced cecal microbial shifts, increasing the abundance of Enterobacteriaceae (P < 0.05). The results demonstrate that coinfection with 25,000 oocysts of E. tenella produced outcomes comparable to higher doses, without excessive mortality, exacerbating pathological, microbial, and immune responses, and increasing Salmonella burden. Therefore, this study supports the feasibility of a practical coinfection model for evaluating interventions against Salmonella in broilers.