Phytogenic interventions at the heat stress-bacterial infection interface in chickens: a critical One Health review and research agenda for antimicrobial stewardship

Heat stress affects chickens by reducing feed intake, disrupting redox balance, weakening immune function, and damaging intestinal barriers. These effects increase the risk of enteric infections, complicate treatment decisions, and encourage empirical antimicrobial use. This review examines phytogenic solutions at the intersection of heat stress and infection, assessing whether their advantages support antimicrobial stewardship and One Health goals. Evidence is categorized as direct, proximal, or supportive, with quantitative comparisons extracted when study designs permit. Currently, the strongest evidence supports antioxidant, anti-inflammatory, barrier-protective, and pathogen-reducing effects. Evidence is considerably weaker for reduced antimicrobial use, lower abundance of resistance genes, reduced food-chain contamination, or limited environmental dissemination. Fermented Allium preparations are presented as an illustrative case from hot-climate production systems, not as proof of One Health benefit. We propose a staged evaluation framework: phytogenics should (i) demonstrate animal health and performance benefits; (ii) demonstrate reduced antimicrobial use under defined treatment protocols; (iii) quantify effects on resistance, food safety, and environmental burden; (iv) confirm these effects at commercial scale using standardized outcome reporting. Phytogenics are best positioned as preventive, adjunctive tools alongside climate management, diagnostics, vaccination, biosecurity, veterinary care, and necessary antimicrobial treatment. The objective is not to eliminate antibiotics but to remove unnecessary treatment without compromising bird welfare or clinical outcomes.