الفهرس 
Materials and methodsOnly 14 pages are availabe for public view
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Abstract
Fishes are at the present superior to other vertebrates in number of individuals, and probably also in number of species. There are more than 25000 different species of fish that are known and many new forms are discovered every year (Greenwood, 1963). Most fish are ectothermic (”cold-blooded”), allowing their body temperatures to vary as ambient temperatures change. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments.
Fish is one of our most valuable sources of protein food. Worldwide, people obtain about 25% of their animal protein from fish by the increasing intensification of fish production. The successful breeding of commercially important animals depends upon the quality and quantity of their dietary protein. The recognition that fish contain proteins or superior nutritional properties places them in an especially important category of foods (Brogstrom, 1962). Like humans and other animals, fish suffer from diseases and parasites. Fish defences against disease are specific and non-specific. Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. If pathogens breach these defences, fish can develop inflammatory responses that increase the flow of blood to infected areas and deliver white blood cells that attempt to destroy the pathogens. Specific defences are specialised responses to particular pathogens recognised by the fish’s body that is adaptative immune responses. In recent years, vaccines have become widely used in aquaculture and ornamental fish, for example vaccines for furunculosis in farmed salmon and koi herpes virus in koi. Some commercially important fish diseases are VHS, ich and whirling disease. About 80% of fish diseases are parasitic especially in warm water fish (Eissa, 2002). No doubt that fishes harbor a wide range of ecto- and endoparasites infecting the alimentary canal, liver, kidney, reproductive organs, muscles as well as gills and skin (El-Naggar & Khidr, 1988 and Yang et al., 2005)Ecto-parasites are the most dangerous group that causes severe mortalities (Shalaby and Ibrahim, 1988). In Egypt there are long periods of optimum warm weather that enable external parasites for more production and cause bad effects on fish. The majority of the monogenitic trematodes of fishes are ectoparasites.
Monogeneans (flatworms) are among the most host-specific of parasites in general and may be the most host-specific of all fish parasites. Monogenetic trematodes of freshwater fishes could be considered as one of the most prevalent diseases affecting skin and gills, which included irritation, severe destruction of the gills, impaired breathing as well as severe losses too (Sineszko and Axelrod, 1980). They are the most abundant ectoprasitic flukes of fish, with greater diversity of species occurring in tropics than in the temperate regions of the world (Rohde, 1982). They spend their entire life cycle as parasites on gills and skin of fish, hold to the fish by the use of hooks and attachment organs at the posterior end. These parasites attack fish causing massive destruction of skin and gill epithelium. Even moderate infection of these organisms on small fish may prove a fatal disease since the infection may cause the fish to stop feeding (Meyer, 1966).Fishes act not only as final host but also as intermediate host for larval stages of many parasites like encysted metacercariae of different species of Trematodes (Clinostomum complantum, Clinostomum phalacrocoracis and Euclinostomum heterostomum), which affect fish causing retardation of growth especially for young fish and increasing the possibility of the secondary infections by decreasing the fish immunity (Elamei, 2001).
Infection of fish by encysted metacercariae of digentic trematode is due to the contamination of water resources as rivers, lakes and sea water with human, animals and bird settlements which harboring eggs voided in their feces,