Protective Efficacy of an Inactive Vaccine Based on the LY02 Isolate against Acute Haemophilus parasuis Infection in Piglets

Haemophilus parasuis can cause Glässer's disease characterized by fibrinous polyserositis, polyarthritis, and meningitis. The current prevention of Glässer's disease is mainly based on the inactive vaccines; however, the protective efficacy usually fails in heterogeneous or homologous challenges. Here, the predominant lineage of H. parasuis (LY02 strain) in Fujian province, China, characterized as serovar 5, was used to evaluate the protective immunity against acute H. parasuis infection in piglets after inactivation. Following challenging with H. parasuis, only mild lesions in the pigs immunized with the killed vaccine were observed, whereas the typical symptoms of Glässer's disease presented in the nonimmunized piglets. A strong IgG immune response was induced by the inactive vaccine. CD4(+) and CD8(+) T lymphocyte levels were increased, indicating the potent cellular immune responses were elicited. The significantly high levels of IL-2, IL-4, TGF-β, and IFN-γ in sera from pigs immunized with this killed vaccine suggested that the mixed Th1 and Th2 immune responses were induced, associated with the high protection against H. parasuis infection compared to the nonimmunized animals. This study indicated that the inactivated LY02 strain of H. parasuis could serve as a potential vaccine candidate to prevent the prevalence of H. parasuis in Fujian province, China.