Immunogenicity and protective efficacy evaluation of an inactivated Lawsonia intracellularis vaccine candidate developed on the basis of a Chinese isolate in pigs

Abstract Porcine proliferative enteropathy (PPE), caused by the obligate intracellular bacterium Lawsonia intracellularis (LI), is an important enteric disease of pigs that impairs intestinal function and growth performance. Vaccination is the most effective measure for the prevention and control of PPE. However, it is extremely difficult to isolate and cultivate LI strains in vitro, and Enterisol®Ileitis is the only commercially available LI vaccine in China. Thus, the isolation of local LI strains and the development of LI vaccines represent urgent priorities for the Chinese swine industry. In this study, the immunogenicity and protective efficacy of an inactivated vaccine developed from the previously isolated Chinese strain LI-HuB23 were evaluated. Thirty-five LI-negative weaned pigs were initially assigned to one of three vaccination groups: those receiving the LI-HuB23-inactivated vaccine, those receiving Enterisol®Ileitis, or those receiving sucrose-phosphate-glutamate buffer (SPG). Vaccinated pigs were then challenged with either LI-HuB23 pure culture or JX703 tissue homogenate. Protection efficacy was assessed on the basis of serum LI-specific antibody responses, serum cytokine levels, average daily gain, feed conversion ratio, fecal LI shedding, fecal scores, gross ileal lesions and histopathology. The results showed that the LI-HuB23-inactivated vaccine induced rapid and sustained serum LI-specific antibody responses, with 100% seropositivity by 21 days post-immunization. Serum LI-specific antibody titers were significantly higher in the inactivated vaccine groups than in the Enterisol®Ileitis groups at 21 and 35 days post-immunization. Vaccination also increased serum IFN-γ, TNF-α and IL-6 levels. Vaccination with the LI-HuB23-inactivated vaccine significantly improved growth performance, reduced fecal scores and fecal LI shedding, and attenuated gross and histopathological ileal lesions. Overall, the LI-HuB23-inactivated vaccine induced serum LI-specific antibody and cytokine responses and reduced the adverse effects of LI challenge on growth performance, fecal shedding and ileal pathology. These findings support the further development of injectable inactivated LI vaccines.