Host Factors Potentially Contributing to Increased Susceptibility in Certain Layer Chicken Lines

Avian influenza (AI) continues to threaten global poultry production, with accumulating evidence suggesting that certain commercial layer lines may exhibit increased susceptibility under specific experimental conditions compared with broiler chickens. This narrative review synthesizes published experimental infection studies identified through a comprehensive PubMed search, focusing on low pathogenic H9N2 and highly pathogenic H5N1, H5N2, H7N7, and H7N9 viruses. Although bird age and production stage varied across studies, consistent disparities in immune regulation and viral replication dynamics have been reported. We critically evaluate host determinants underlying these differences-including microRNAs, major histocompatibility complex polymorphisms, sialic acid receptor distribution, gut microbiota, and hormonal influences-and integrate findings across viral subtypes and pathogenicity classes to inform breed-tailored vaccination, nutritional, and therapeutic strategies.