Histopathologic and phylogenetic assessment of abomasal coccidiosis in small ruminants

Eimeria gilruthi, an underreported coccidian parasite, primarily infects the abomasum of small ruminants. It has an unclear life cycle, and its phylogenetic relationship remains poorly understood. Most E. gilruthi infections are incidental, with rare descriptions of those associated with clinical disease. This study aims to characterize the lesions, immune sresponse, and molecular identity of E. gilruthi in naturally infected small ruminants. Histopathology, special stains, and immunohistochemistry for immune cells were used to assess tissue responses in the abomasum of 4 goats and one sheep. Molecular characterization was performed targeting the 18S and ITS1 regions of its rDNA and cox3 regions of its mDNA, followed by phylogenetic analysis. All animals harbored megaloschizonts within hypertrophic host cells, accompanied by lymphohistiocytic and eosinophilic abomasitis in the deep mucosa without evidence of sexual stages or oocyst formation. Immunohistochemistry revealed an inflammatory infiltrate composed of CD3⁺ T and CD20⁺ B lymphocytes and IBA-1⁺ macrophages, especially around immature schizonts, with fewer eosinophils. Molecular analysis revealed genetic divergence among isolates. While Eimeria from goats clustered with E. gilruthi, E. christenseni, and E. alpacae, Eimeria from sheep grouped with E. bovis, E. zuernii, and E. ellipsoidalis, suggesting potential host-specific lineages or different species. These findings enhance current understanding of E. gilruthi immune response and emphasize the need for further research into its taxonomy, life cycle, and host specificity to clarify its impact on small ruminant health.