Histopathological Changes Associated with Coccidia Infection in Pearlscale goldfish (Carassius auratus)

Abstract

This study investigated the pathological characteristics of coccidial infection in Pearlscale goldfish exhibiting clinical signs of systemic disease. Affected fish displayed severe clinical manifestations including widespread cutaneous ulceration, hemorrhage, abdominal distension, and ascites. Molecular analysis using coccidiosis-specific primers (18E-Coc2r and Coc1f-18R) confirmed the presence of coccidian DNA in multiple organs, including the intestine, spleen, and kidney, indicating that the pathogen had disseminated systemically. Histopathological examination revealed increased mela- no-macrophage centers (MMCs) in the spleen and kidney, glomerulonephritis characterized by in- flammatory cell infiltration, macrophage accumulation in gill tissue, and the presence of coccidial cysts containing merozoite within the intestinal submucosa. These findings demonstrate that the cocci- dial infection was not confined to the gastrointestinal tract but had disseminated systemically, causing multi-organ damage and chronic immune activation. The identification of merozoite-containing cysts in the intestinal submucosa provides definitive evidence of active parasitic replication, as merozoite represent the asexual reproductive stage characterized by highly active intracellular proliferation. This case underscores the importance of integrated molecular and histopathological diagnostics for accurate identification of protozoan infections in ornamental fish and highlights the need for preventive manage- ment strategies, including water quality control and maintenance of appropriate stocking densities, to minimize disease occurrence in freshwater ornamental fish aquaculture systems.