Abstract

Background and Aim: Eimeria tenella is the most pathogenic species affecting chickens and a leading cause of economic loss due to coccidiosis. While live vaccines using virulent or attenuated strains are effective, they can still cause intestinal damage and reduce weight gain. Autophagy, a crucial host cell response during intracellular parasitic infections, shows variations in induction between virulent and precocious E. tenella strains that are not yet well understood. This study compares how host cell autophagy is triggered by the virulent E. tenella Shanxi strain (Tsx) and precocious Tsx (PTsx) strains, both in vitro and live animal experiments. 

Materials and Methods: Primary chick embryo cecal epithelial cells and specific pathogen-free chickens were infected with either low or high doses of Tsx or PTsx. Infection rates were determined through hematoxylin and eosin (H&E) staining. Autophagy levels were assessed by quantifying Beclin-1 mRNA expression via reverse transcription quantitative real-time polymerase chain reaction, evaluating LC3II puncta accumulation through immunofluorescence (IF), and calculating LC3II/I ratios using Western blot. In vitro experiments were carried out from 4 to 120 h post-infection, whereas in vivo evaluations took place on day 5 after inoculation. 

Results: In vitro, infection rates did not differ significantly between Tsx and PTsx groups during early stages (4–72 h), but Tsx showed significantly higher infection rates at 120 h. Both strains induced autophagy in a dose-dependent manner, as evidenced by increased Beclin-1 mRNA expression, LC3II puncta, and LC3II/I ratios compared with controls. These autophagy markers were consistently higher in Tsx-infected cells than in PTsx-infected cells at equivalent doses. In vivo findings mirrored in vitro trends, with stronger autophagy activation observed in Tsx-infected chickens, particularly at high doses. Autophagy activation was markedly amplified in vivo compared with in vitro, indicating the influence of the intestinal microenvironment. 

Conclusion: The highly virulent E. tenella strain Tsx causes intense and prolonged autophagy in host cells, while the less aggressive PTsx strain triggers a milder autophagic response. The level of autophagy activation is directly related to the parasite's virulence and infection dose. These results show that excessive autophagy plays a significant role in intestinal damage during E. tenella infection and highlight that reducing host autophagy activation is crucial for developing more effective live attenuated coccidiosis vaccines. 

Keywords: autophagy, coccidiosis, Eimeria tenella, host–parasite interaction, live attenuated vaccine, poultry disease, precocious strain, virulent strain.