الفهرس 
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Abstract
a) The knowledge of fish fanning dates back to several thousand years
according to old chinese writings and Egyptian bus-reliefs. This explains
the importance of fish as a food source of high protein content. The
protein of fish origin is of high biological value for human being as it is
very essential for growth and maintance of vital functions. Moreover,
certain localities of the world depend mainly on the fish as a source of
animal protein, so it would be wise economically to cultivate such
important source of protein to meet the dietary need of the expanding
population.
Nile tilapia (Oreochromis niloticus) plays a good role in the
intensive, semi-intensive and extensive artifical breeding culture
programmes in different locations in Egypt (Badran and Eissa, 1991).
The fish culturing, especially the intensive culture, acts as predisposing
for appearance and spreading of many fish diseases. Fish diseases due to
bacterial infection are considered one of the major problems in
aquaculture particularly in the intensive culture (Austin and Austin,
1987). Most of the bacterial causative agents associated with such
diseases are naturally saprophytic organisms, widely distributed in the
aquatic environment (Frerichs and Hendrie 1985). Only a
comparatively few species are classified as true obligate pathogens. Both
classes of organisms, however, may be present on the external body
surface or within the tissue of apparently healthy fish. Stress factors such
as poor water quality, temperature changes, over-crowding and parasitism allow these bacteria to become pathogenic to fish (Badran and Eissa, 1991) .. 1