Epidemiology and pathogenicity of Haemophilus parasuis in eastern China

<i>Haemophilus parasuis</i> (<i>H. parasuis</i>), the causative agent of Glässer's disease (polyserositis), exhibits considerable serotype diversity and is globally distributed. To investigate the epidemiological characteristics of <i>H. parasuis</i> in China, 810 suspected infection samples, including lung tissues and pleural effusions, were systematically analyzed. These samples were collected between 2022 and 2024 from six major pig-producing provinces in China. The analysis revealed a national infection rate of 52.10% (422/810), with Jiangxi Province exhibiting the highest prevalence (71.76%). Seasonal analysis indicated significantly higher incidence rates during winter (66.29%) and spring (60.45%) compared to summer (34.81%) and autumn (46.46%). Serotyping of 56 <i>H. parasuis</i> isolates demonstrated that serotype 5 (42.86%) and serotype 12 (19.64%) were predominant, while 10.71% of the strains were nontypeable. Notably, Jiangxi Province displayed a single-serotype profile, whereas other provinces exhibited multiple-serotype cocirculation patterns. Virulence gene analysis revealed the universal absence of <i>HPM-1370</i> and the consistent presence of <i>vta3</i> across all isolates. Serotypes 4 and 8 exclusively harbored the <i>vta</i> gene cluster, while serotype 5 retained other target genes despite lacking <i>HPM-1370</i>. The <i>HPM-1371</i> gene was detected only in serotypes 5 and 14. Serotypes 1, 11, 12, and 13 exhibited a combination of conserved <i>wza-vta1-vta2 -vta3</i> genes, although 36.37% (4/11) of serotype 12 isolates lacked the <i>wza</i> gene. Serotypes 2 and 7 carried only <i>wza</i> and <i>vta3</i> genes. Animal challenge experiments demonstrated marked differences in strain pathogenicity: the H5-1 strain induced 100% mortality with acute septicemia, widespread alveolar destruction, and fibrinous exudation; the H12-1 strain caused 50% mortality accompanied by severe pleural adhesions and hemorrhagic lesions; and the H7-1 strain resulted in 33.33% mortality with localized pulmonary damage. These findings provide essential evidence for the development of targeted prevention and control strategies against <i>H. parasuis</i> infection.