Heat stress, respiratory dysbiosis, and poultry pathobiome: A comprehensive review of chronic respiratory disease in chickens.

The poultry respiratory microbiome is increasingly recognized as a critical determinant of poultry health, mucosal immunity, and host resistance to opportunistic pathogens. Chronic Respiratory Disease (CRD) exerts a severe economic toll on the global broiler industry. While conventionally attributed to specific etiological agents such as Mycoplasma gallisepticum, disease progression is fundamentally tied to broad ecological dysbiosis within the respiratory tract. This comprehensive review synthesizes current knowledge on the functional roles and compositional dynamics of the poultry respiratory microbiome in both healthy and diseased states. Notably, we acknowledge the emerging threat of climate change, elucidating how heat stress physically and immunologically disrupts the respiratory mucosal microenvironment, acting as a primary catalyst for microbiome destabilization and CRD susceptibility. By shifting the paradigm from a single-pathogen model to a comprehensive pathobiome perspective, we detail the microbial shifts that characterize CRD. Furthermore, because investigating this low-biomass environment presents unique analytical challenges, we critically evaluate current technical methodologies. We address critical bottlenecks in sample collection, DNA extraction biases, and the comparative efficacy of 16S rRNA amplicon versus shotgun metagenomic sequencing. Ultimately, this synthesis provides a foundational framework for optimizing diagnostic methodologies and developing microbiome-targeted interventions -such as next-generation probiotics- to mitigate CRD in an era of escalating environmental stressors.