Pathological Injury and Gastrointestinal Microbiota Changes in Lambs Infected with Eimeria ovinoidalis Isolate from Gansu, China.

Coccidiosis poses a serious threat to the sheep industry, with Eimeria ovinoidalis recognized as the primary causative agent. However, quantitative analyses of the pathological damage induced by Eimeria ovinoidalis infection across the entire intestinal tract of sheep, together with investigations into the structural characteristics of the gastrointestinal microbiota, remain scarce. In this study, Eimeria ovinoidalis was isolated and purified from the feces of naturally infected sheep in Gansu Province, China. Purified sporulated oocysts were orally administered to lambs to establish an experimental infection model. Clinical signs were monitored throughout the experimental period, and following successful model establishment, oocyst burdens were quantified in each intestinal segment. Histopathological changes and alterations in the gastrointestinal microbiota were systematically evaluated using hematoxylin-eosin (H&E) staining, Sirius Red staining, and 16S rRNA gene high-throughput sequencing. The infected lambs developed typical clinical manifestations, including diarrhea and emaciation, and exhibited a significant reduction in body weight at 14 days post-infection (dpi) (P < 0.05). The highest oocyst burdens were detected in the ileum, cecum, colon, and rectum. Histopathological examination revealed that infection with the Gansu isolate primarily affected intestinal segments extending from the ileum to the colon and was characterized by mucosal epithelial sloughing, lamina propria edema and hemorrhage, crypt destruction, and massive inflammatory cell infiltration. Following infection, eosinophil density increased markedly throughout the intestinal tract (P < 0.01), with the greatest relative increase observed in the colon (1113.13%) and the highest absolute density detected in the ileum (12.01 ± 4.46 cells/10⁴ μm²). Analysis of the 16S rRNA gene sequencing data demonstrated that microbial richness and diversity were significantly reduced in the cecum, colon, and several other intestinal segments of infected lambs (P < 0.05). At the phylum level, the relative abundance of Firmicutes decreased, whereas that of Campylobacterota increased in the hindgut; in contrast, the abundance of Bacteroidota increased in the stomach. At the genus level, pathogenic bacteria, including Campylobacter, were enriched, whereas beneficial bacterial genera were depleted in the hindgut. These findings indicate that infection with the Gansu isolate of Eimeria ovinoidalis induces severe intestinal tissue damage and gut microbial dysbiosis in sheep. These findings provide an important theoretical foundation for elucidating the pathogenic mechanisms of ovine coccidiosis and for developing more effective prevention and control strategies.