Transcriptional Cytokine Responses Linked to Pathological Outcomes in Broiler Chickens Fed Residual Doses of Antimicrobials

Cross-contamination of poultry feed with antimicrobials may cause unintended exposure to residual antibiotic doses, but the immunological consequences remain unclear. We tested whether six antimicrobials (colistin, doxycycline, flumequine, thiamphenicol, tiamulin, and tilmicosin) delivered in feed at 2% of the maximum approved dose modulate immune-related gene expression and splenic histology in broiler chickens. Female Ross 308 chicks received antibiotic-contaminated diets throughout the rearing. Spleen and caecal mucosa were sampled on days 7, 21, and 35, and analyzed by RT-qPCR. Spleen was sampled on day 35 for histopathology. Responses were strongly time- and tissue-dependent, with the largest effects on day 21. In spleen, selected antimicrobials showed selective down-regulation of IL-2, IL-4, IL-6, IL-8, IL-12p40, and IFN-ß, while IFN-γ was significantly up-regulated (p ≤ 0.05). In caecal mucosa, transcriptional modulation was weaker and more limited. Significant changes were restricted to MUC2 on day 21 and to IL-2 and IL-8 on day 35 (p ≤ 0.05). Splenic histopathology showed unchanged lymphatic nodule counts, but increased lymphoid atrophy, necrosis, and inflammatory infiltrates, most frequently in colistin- and doxycycline-fed birds. Overall, residual antimicrobials in feed are not biologically neutral and are associated with distinct structural changes in immune-related gene expression and splenic tissue development.