ABSTRACT: Eimeria (E.) maxima parasite infects the midgut disrupting the jejunal and ileal mucosal lining causing high morbidity and mortality in chickens. Heat stress (HS) is a seasonal stressor that affects biological functions leading to poor performance. Our objective was to elucidate how HS, E. maxima infection, and their combination affect the ileum transcriptome. Two-hundred and forty 2-week-old males Ross708 chickens were randomly allocated into four treatment groups: thermoneutral control (TNc), thermoneutral infected (TNi), heat stress control (HSc), and heat stress infected (HSi), with 6 replicates each of 10 birds. Infected groups received 200x103 sporulated E. maxima oocysts/bird, and heat treatment groups were raised at 35oC. At 6-day post-treatment, five chickens per group were randomly sampled, and ileum tissues were collected for RNA extraction and sequencing using NGS Illumina sequencing platforms. A total of 413, 3377, 1908, and 2304 DEGs were identified when applying the comparisons: TNc vs HSc, TNc vs TNi, HSi vs HSc, and TNi vs HSi, respectively, at cutoff ≥1.2-fold change (FDR: q<0.05). HSc vs TNc showed upregulation of lipid metabolic pathways and degradation and metabolism of multiple amino acids, and downregulation of most immune-related and protein synthesis pathways. TNc vs TNi displayed upregulation of most of the immune-associated pathways and eukaryotic mRNA maturation pathways, and downregulation of fatty acid metabolism and multiple amino acid metabolism pathways including tryptophan. Comparing HSi versus HSc and TNi revealed that combining the two stressors restored some normal cellular functions, e.g., oxidative phosphorylation and protein synthesis, and reduced the intensity of immune response associated with E. maxima infection. Tryptophan metabolism was upregulated, including genes that contribute to catabolizing tryptophan through serotonin and indole pathways; which might result in reducing the cytoplasmic pool of nutrients and calcium available for the parasite to scavenge and consequently might affect the parasite’s reproductive ability.
