Molecular epidemiology and genomic analysis of bulbul coronavirus in Guangdong, China

Bulbul coronavirus (BuCoV), a delta coronavirus recently identified in passerine birds, remains poorly characterized regarding its ecology and evolutionary dynamics. This study aimed to determine the prevalence of BuCoV in wild avifauna, clarify its evolutionary relationship with other delta coronaviruses, and identify genetic signatures potentially relevant to host adaptation and cross-species transmission in southern China.

From 2023 to 2024, we conducted molecular surveillance across 12 regions in Guangdong Province, China. A total of 2,145 avian fecal samples were collected and screened for BuCoV using real-time quantitative PCR. The complete genomes of representative strains were obtained using next-generation sequencing. Subsequent analyses included phylogenetic reconstruction using maximum likelihood methods, recombination detection using RDP4 and SimPlot, and comparative amino acid analysis.

BuCoV was detected exclusively in Shenzhen (3/168, 1.78%), with all positive samples originating from bulbuls (Pycnonotus spp.). The Shenzhen strain GD2411 exhibited the highest nucleotide identity with BuCoV strains HKU11-796 (97.26%) and HKU11-934 (96.79%), but far lower similarity (78.9%-82.4%) to other delta coronaviruses. Phylogenetic analysis placed GD2411 in a monophyletic clade with HKU11 strains. Recombination analyses revealed mosaic structures within the spike (S) gene, involving multiple coronavirus lineages. Thirty-one amino acid substitutions were detected in the S protein, together with mutations in RdRp, 3CLpro, and nucleocapsid.

These findings suggest that BuCoV GD2411 emerged through inter-lineage recombination and is undergoing adaptive evolution, particularly in the spike protein. The detection of BuCoV exclusively in Shenzhen, a critical node in the East Asian-Australasian Flyway, suggests that migratory birds may facilitate viral dissemination. The identified mutations may affect viral replication, host adaptation, or immune evasion. These findings provide essential baseline genomic and epidemiological data critical for understanding BuCoV diversity and assessing potential zoonotic risks in southern China.