Evidence for Genotype-Associated Differences in Disease Severity and Limitations of Serotype-Based Classification in Glaesserella parasuis Revealed by Whole-Genome Sequencing in Japan

isolates from diseased pigs in Japan, focusing on underrecognized aspects of disease epidemiology and control. Although Glässer's disease is well recognized in swine production, its epidemiology remains incompletely understood, particularly regarding the relationship between serotype, genotype, and pathogenicity. Serotypes 5 or 12 (5/12) (28.9%) were predominant, followed by serotype 7 (10.8%). Phylogenetic analysis based on core-genome single nucleotide polymorphisms and cluster analysis classified the isolates into three genetic groups, with no clear association between serotype and genetic grouping. One genetic group tended to exhibit a lower proportion of severe clinical cases compared with the others, with a statistically significant difference observed in one comparison but not in the other. These findings provide evidence suggesting genotype-associated differences in disease severity, indicating that pathogenic potential may be more closely linked to genetic background than to serotype. These findings suggest a potential limitation of serotype-based vaccine strategies. Although 86.7% of isolates lacked antimicrobial resistance genes, resistance determinants were identified on contigs predicted to be of plasmid origin. These results indicate that antimicrobial resistance, while not widespread, may be underestimated and could disseminate. Overall, our findings highlight underexplored aspects of Glässer's disease relevant to improving control and prevention.