Pathological Changes in Gills of Tigris Asp Leuciscus vorax (Heckel) Due to Parasitizing with Paradiplozoon sp. (Monogenoidea, Oligonchoinea, Diplozoidae) from Al-Gharraf River Southern Iraq

Tigris asp (Leuciscus vorax Heckel), locally known as "Shilik," is a native freshwater fish and an important table species for the Iraqi people. Recently, the population of this species has declined due to poor water quality, reduced water discharge, the presence of invasive species, anthropogenic impacts, and pollution. This study investigates the pathological effects of twin-worm (Paradiplozoon sp.) infestations on the gills of Leuciscus vorax (Tigris asp) from the Al-Gharraf River in Dhi-Qar Province, Iraq. Fish were collected between October and December 2023. Histopathological analysis revealed significant lesions, including necrosis of secondary lamellar epithelial cells, severe fibrosis in the primary lamellae, and vascular changes such as aneurysms. These lesions disrupted oxygen uptake, causing respiratory distress. Inflammatory cell infiltration, predominantly lymphocytes, was also observed, indicating an immune response to the infestation. The histopathological results showed fusion of gill filaments due to epithelial hyperplasia, which reduced the gill surface area and impaired respiration. Fibrotic changes and abnormal vascular formations further obstructed the respiratory surface, worsening fish health. The study recommends integrating advanced diagnostic methods, such as PCR, immunohistochemistry, and ultrasonography—for early detection and improved understanding of the parasite’s lifecycle and host-parasite interactions. These tools, combined with histopathological analysis, can enhance disease detection and help manage parasitic infestations, ensuring fish health and sustainability. This report represents the first study in Iraq on the pathological effects of Diplozoidea members on the gills of freshwater fish. It is also one of the few global reports documenting the impact of this relatively large parasite group on such vital fish tissue.