الفهرس 
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Abstract
Background: Aquaculture is an important source of protein, especially Nile tilapia fish. Fungal diseases, especially saprolegniasis, are one of the most important pathological problems that cause the death of a large number of fish. Aim of this study: Studying the prevalence of Saprolegniasis in Nile tilapia fish in different fish farms at different localities and using traditional diagnostic methods and proving the importance of using the molecular diagnosis of Saprolegnia fungi and the effect of infection with Saprolgenia diclina on the immune response in tilapia fish in addition to studying the histopathological changes caused by the infection and microscopic structural changes in the tissue using optical and electronic microscopy. Method: The study contains methods of isolating and classifying of Saprolegnia fungi through sexual and asexual reproduction, as well as molecular diagnostics, as well as preparing samples to study histopathological and microscopic structural changes in tissues using optical microscopy and electron microscopy, and conducting a laboratory experiment to find out the extent of the isolated virulence used and its effect on vital indicators specific to immunity, oxidative stress and Gene expression. Results: Presenting clinical symptoms and anatomical description of diseased fish samples and displaying images of pathological changes on different tissues in diseased fish using optical microscopy and electron microscopy and clarifying the prevalence of Saprolegniasis in the areas under study. The results of molecular diagnostics, as well as the immune response of Nile tilapia fish, infected with Saprolgenia diclina, the rate of change in vital signs of immunity and oxidative stress in addition to pathological changes in tissues. Conclusion: A summary of the study’s findings on the importance and accuracy of molecular diagnostics in the diagnosis of fungal fish diseases and that there is a relationship between water quality and the spread of disease Saprolegnia in fish. Therefore, water quality must be examined periodically to ensure the production of the finest types of fish, and that Saprolegnia is responsible for the deaths of a large number Of fish.. Recommendations: There is an urgent need to control pollution to which farmers are exposed to reduce the rate of infection and that traditional diagnostic methods should be side by side with modern diagnostic methods as well as the impact of infection on the immune response in fish.