Establishment of an Infection Model in Pigs for Chinese Clinical Strains of Lawsonia intracellularis

This study aimed to establish a stable porcine proliferative enteropathy (PPE) model using Chinese isolates of Lawsonia intracellularis (LI) and clinical infected intestinal tissue homogenates from different sources, construct a systematic evaluation index system covering fecal bacterial shedding, fecal score, average daily gain, feed conversion ratio, serum antibody level and histopathological changes, and identify the optimal challenge dose and preferred strain, so as to provide the standardized model and data support for the research on LI pathogenic mechanism, vaccine efficacy evaluation and prevention and control strategies.

Twenty-eight-day-old weaned piglets were selected and divided into a negative control group and multiple infection groups. In Experiment 1, the pathogenicity of four LI tissue homogenates (JX703, GX301, HN312, and LI-HuB23) was compared at the dose of 1×10⁹ LI per piglet. In Experiment 2, JX703 tissue homogenate with the strongest comprehensive pathogenicity and the in vitro cultured LI-HuB23 isolate were selected to orally inoculate 28-day-old piglets at gradient doses (10¹⁰ LI per pig, 10⁸ LI per pig, and 10⁶ LI per pig). The piglets were monitored for 28 days after inoculation, during which fecal bacterial shedding, fecal score, average daily gain, feed conversion ratio, serum antibody level and histopathological changes were observed and recorded.

In Experiment 1, typical clinical symptoms and pathological manifestations of PPE were successfully reproduced in all infection groups inoculated with tissue homogenates of different LI strains. LI was detectable in feces from day 4 post-inoculation, and bacterial shedding persisted until day 28 post-inoculation. Diarrhea occurred in the infection groups from day 4 post-inoculation, with the most severe symptoms observed from day 19 to 21 post-inoculation. All infection groups showed lower average daily gain and higher feed conversion ratio compared with the negative control group. Serum antibody monitoring showed that seroconversion occurred from day 14 after the establishment of infection and lasted until day 28 post-inoculation. Pathological examination revealed thickened intestinal mucosal folds in the infection groups; hematoxylin-eosin (HE) staining showed intestinal crypt hyperplasia and decreased goblet cells, and LI was detected in the cytoplasm of intestinal crypt epithelial cells by immunohistochemical staining (IHC). Overall, JX703 tissue homogenate induced the most prominent symptoms with the strongest virulence. In Experiment 2, the results of gradient dose test showed that inoculation with LI-HuB23 in 10⁶ LI per piglet was sufficient to establish effective infection, accompanied by fecal bacterial shedding, diarrhea, and corresponding pathological changes. The severity of symptoms increased with the elevation of infection dose, and the 10¹⁰ LI per pig group exhibited the most typical patterns in terms of peak fecal scores, pathological scores, and reductions in average daily gain.

A PPE infection model was successfully established using Chinese LI isolates and field-derived infected tissue homogenates in this study. The selected evaluation indexes had an important reference value for related research, including fecal bacterial shedding, fecal score, average daily gain, feed conversion ratio, serum antibody level and histopathological changes. The infection model established with an inoculation dose of 10¹⁰ LI per pig presented more typical clinical and pathological characteristics of PPE.