Parasitic ciliates of Egypt: a model of cutaneous infection and immunity in some freshwater fish

Abstract

Remarkable deficiency in fish production, due to many factors including water pollution and parasitism, was evident The common parasitic ciliate Jchthyophthirius multifiliis is one of the most important protozoan pathogens of freshwater fish throughout the world. Examination of freshwater fish representing four different species of Tilapia from the River Nile revealed higher percent of infection in Oreochromis niloticus, followed by Oreochromis aureus, Sarotherodon galilaeus and Tilapia zilli. Upon the initial natural infections, typical white spots characteristic of "Itch" appeared on Oreochromis niloticus and Oreochromis aureus, while Sarotherodon galilaeus and Tilapia zilli showed only non-specific signs of mild skin irritation. Histological sections of the skin of the four examined Itch-infected Tilapia revealed large trophonts of the I. multifiliis that were prominently lodged in the epidermal layers. Most trophonts observed were adjacent to the basement membrane of the epithelial layer. Liver hepatocytes of Itch-infected Oreochromis niloticus fish appeared swollen with moderate vacuolation. Also, the presence of melano-macrophage centers (MMCs) was significantly higher in the Itch-infected group than in the control group. Spleen of parasite-infected fish had an overall pale appearance; with the white pulp expanded and red pulp decreased. Specifically, the spleen of Itch-infected Tilapia zilli fish showed the great number of erythrocytes in different developmental stage, which suggests that the spleen is an important erythrocytopoietic organ. The electron microscopical study confirmed the histological organization of the MMCs described by light microscopy. The main cellular elements of the MMCs in Itch-infected Tilapia were large phagocytic cells filled by engulfed material in variable degree of digestion. A significant difference was observed between the non-infected controls of Tilapia species in the levels of globulin alpha and beta. "Itch" infection in the four tested species of Tilapia induced significant elevations in the level of globulin alpha and beta. At the electron microscope, apoptotic cells collected from the four Tilapia species during parasitic infection revealed an aggregation on the apical and basal part of the ciliated and non-ciliated endothelial cells lining the gill epithelium. Blood lymphocytes collected from infected Oreochromis niloticus and Oreochromis aureus display a stronger phagocytic activity and killing assay than Sarotherodon galilaeus and Tilapia zilli. Itch-infected Sarotherodon galilaeus showed low detectable levels of IL-1 and the levels of IL-1 were significantly elevated in Itch-infected Tilapia zilli. Itch-infected Oreochromis aureus and Tilapia zilli still had significantly low levels of IL-2 than Oreochromis niloticus and Oreochromis aureus. Tilapia zilli had no detectable IL-8 levels in non- . infected and Itch-infected animals during the course of experiment. The higher level of TNF-a was detected in Sarotherodon galilaeus, while the lower level was detected in Tilapia zilli.