Molecular serotyping and virulence assessment of Avibacterium paragallinarum strains circulating in Chinese poultry flocks

Background Infectious coryza, caused by Avibacterium paragallinarum ( A. paragallinarum ), is a highly contagious respiratory disease that poses a significant threat to poultry production in China. Updated information on the epidemiology, serotype distribution, and virulence of circulating strains is essential for formulating effective regional control and vaccination strategies. Methods A total of 200 clinical samples from nine provinces were analyzed to determine the prevalence, serotype distribution, and seasonal dynamics of A. paragallinarum . The bacterial isolation, characterization, species-specific PCR, and molecular serotyping based on the hmtp210 gene were performed. Phylogenetic relationships were inferred using the neighbor-joining method. The pathogenicity of representative isolates (serotype A and C) was assessed using SPF chickens. Results A. paragallinarum prevalence varied geographically, with the highest isolation rates in Anhui, Guangxi, and Guangdong provinces, and no detection in Sichuan or Hubei. A distinct seasonal pattern was observed, with peaks in late autumn and winter. 22 isolates were recovered and serotyped: serotype A predominated ( n = 15), followed by serotype C ( n = 5), and serotype B ( n = 2). Phylogenetic analysis of hmtp210 revealed clear clustering congruent with serotype classification. Both serotypes induced typical infectious coryza, with directly inoculated birds exhibiting more severe clinical signs than contacts. Conclusion This multi-provincial study provides contemporary insights into the epidemiology, serotype distribution, and virulence of A. paragallinarum in China. These findings underscore the necessity for region-specific surveillance and support the development of targeted vaccines based on locally prevalent, virulent strains.