Host Physiology and Environmental Stress as Potential Modifiers of Mycoplasma synoviae Disease Severity in Chickens

Mycoplasma synoviae (MS) is a poultry pathogen associated with respiratory disease, infectious synovitis, eggshell abnormalities, and reduced production performance. Clinical expression varies substantially among farms, flocks, and individual birds, ranging from subclinical infection to respiratory, articular, reproductive, or systemic disease. This variation suggests that pathogen presence alone may not fully explain differences in disease severity. This narrative review synthesizes evidence relevant to the hypothesis that MS disease severity may reflect interactions between pathogen colonization and host physiological state. Experimental and field studies demonstrate that MS infection involves respiratory colonization, lateral transmission, strain-dependent differences in infectivity and virulence, adhesion to host ligands, and tissue tropism. Studies specifically investigating MS infection also document inflammatory, oxidative, apoptotic, and metabolic responses in infected chickens. The review then evaluates host physiological pathways and environmental stress responses that may modify these outcomes, including corticosterone signaling, the growth hormone–insulin-like growth factor 1 (GH–IGF-1) axis, melatonin and circadian regulation, and cytokine networks. These systems are known to influence immune competence, redox balance, epithelial integrity, tissue repair, and metabolic allocation in chickens. The available evidence supports a mechanistic framework in which pathogen factors and host physiology may interact to modify MS disease expression. Evaluation of the available evidence indicates that MS-specific support is strongest for pathogen factors and tissue responses; however, none of the proposed environmental, endocrine, or circadian modifiers has been causally tested in an MS challenge model, and genetic, microbiota, and omics-based biomarker evidence remains preliminary. This framework therefore provides testable hypotheses and candidate measurements for future studies.