Infectious bronchitis virus exhibits pan-mucosal tropism and ascends via the cloacal hub to infect the urogenital system

Infectious bronchitis virus (IBV), a prominent respiratory pathogen of chickens that incurs substantial economic losses to the global poultry industry, exhibits extensive genetic heterogeneity. Highly virulent strains manifest broad multi-organ tropism and elicit severe renal and oviductal lesions. Nevertheless, the precise tissue tropism profiles of different IBV genotypes, as well as their in vivo dissemination routes from respiratory mucosa to distal urogenital tissues, remain incompletely characterized. Here, we systematically elucidated the actual tissue tropism and systemic dissemination routes of IBV using in situ immunofluorescence assays, in vivo fluorescence tracking of a recombinant reporter virus, and an estrogen-induced physiological oviduct model. Our findings reveal that all tested IBV strains exclusively target mucosal epithelial cells. Classical Mass-type and emerging genotype VI strains are limited to respiratory tissues and cranial glands, while virulent QX-like isolates show pan-mucosal tropism across respiratory, lower intestinal, and urogenital tracts. Multi-route challenge and spatiotemporal antigen tracking exclude hematogenous viremia as the spread mechanism. Instead, QX-like IBV colonizes the gut and cloaca, a unique avian anatomical junction, then migrates retrogradely along continuous epithelia to the kidneys and oviducts. This work clarifies tissue tropism of major IBV genotypes and identifies a cloaca-dependent ascending mucosal pathway for urogenital infection of QX-like IBV. These results advance our mechanistic understanding of IBV pathogenesis and support the design of precise disease control and mucosal vaccine strategies. IMPORTANCE Infectious bronchitis virus (IBV), a highly contagious avian gammacoronavirus, causes huge economic losses in global poultry farming. While primarily infecting respiratory mucosa, virulent strains trigger severe renal and reproductive damage. It has long puzzled researchers how respiratory-entry IBV reaches distant urogenital organs. This work delineates IBV’s in vivo spread and rules out typical hematogenous dissemination. IBV possesses inherent pan-mucosal tropism restricted to surface epithelia. Virulent strains colonize the cloaca—the avian junction of digestive, urinary, and reproductive tracts—then ascend retrogradely along connected mucosa to kidneys and oviducts. Immature pullets are shielded by sealed oviduct outlets, while mature hens develop severe genital lesions. Resolving this historical dispute, our findings revise IBV pathogenic theories and highlight mucosal immunity as a key target for innovative vaccine design.