The presence of parasites, especially nematodes, in the intestines of Cyprinus carpio fish causes structural deformities of the intestine (Necrosis), accompanied by the occurrence of cases of rupture in the villi, as well as infiltration of inflammatory cells in the villi and degeneration of cells in the mucosal layer. The results of the current study, through the histological section of the intestines of infected and uninfected Carasobarbus luteus, showed the pathological effects of nematodes on the intestines of Leuciscus vorax and Planiliza abu, accompanied by necrosis of the villi, as well as necrosis and damage to the muscular layer and epithelial tissue, fibrin deposition in the mucosal layer, peeling of the submucosal layer, necrosis and rupture of the intestines of Carasobarbus luteus. In addition, the infiltration of inflammatory cells into the villi causes a clear decrease in the number of villi, the tearing of the superficial epithelial cells in the mucosal layer, as well as necrosis and rupture in the muscular layer, which causes a change in the function of the intestine and a decrease in its efficiency, represented by poor nutrition, weight loss, and cases of lethargy and inactivity. The attachment of the threadworm to the epithelial layer leads to the withdrawal of part of the epithelial cells and the dissolution, erosion and death of the cells.