الفهرس 
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Abstract
A total number of 215 cultured O.niloticus(70±5g) were collected alive from fish culture in Shack-ShokatFayoum Governorate during summer, autumn, winter and spring seasons for determination of two zoosporic fungi prevalence.
The prevalence of Saprolegniosis was 5.45%, 0%, 6.66% and 2.85% among cultured O.niloticus, during summer, autumn, winter and spring respectively with overall prevalence of 4.65% certainly among cultured Niletilapia.
Saprolegniosis naturally infected O. niloticusshowed variety of clinical signs including fluffy whitish to grayish cotton-like fungal growth on the skin, gills, and / or eyes of the infected fish, which can be seen only when the infected fish present in the water. This fungal growth however, changed to glistening slimy matted mass on the affected parts of the body when the fish out concerning of the water. Advanced cases showed destruction of the fins, hemorrhages and ulceration leaving row musculatures. On microscopical examination of mount preparation taken from different lesions, manyaseptate hyphae and zoosporangia were observed in various lesions. In most cases, no gross signs were observed in the internal organs.
Samples taken from the skin, gills and / or eye of the apparently healthy and diseased fish were cultivated on GY agar medium resulted in isolation of 10 cotton-like whitish colonies. Identification of saprolegniosiscausative agents was depending on their morphological, biochemical characteristics, growth at different temperature degrees, growth at different pH and different concentrations of NaCl (salinity). The isolated A.prolefirodes strains grow at 25 and 30˚C, while no growth was recorded at 37˚C while the isolated S.diclina strains grow at 25˚C, while no growth was recorded at 37˚C and faint growth at 30˚C.Concerning the effect of pH on the vegetative growth of tested fungi, A. proliferoides and S. diclina optimal pH was ranged between 7.5 - 8.5. On the other hand; A. prolefiroids BNS-M005 was able to tolerate up to 5 pptNaCl, however, it rarely grew at 10ppt NaCl andno growth of fungus was observed on (GY) agar containing 15-30 pptNaCl. On the other hand, isolates of S.diclina BNS-M003 was able to tolerate up to 25 pptNaCl, but no growth of fungus was observed on (GY) agar containing 30 pptNaCl.
The isolated A. proleferoides had stronger ability to produce lipase than lecithenase and protease, while the tested isolates of S. diclina could produce protease more than lecithenase and lipase.All tested species of SaprolegniaandAchlya could not utilize xylose however; they showed high capabilities to utilize fructose and galactose.Isolation of saprolegniosis causative agent (S.diclina& A. prolefiroides), from diseased O. niloticuswas done using GYA agar medium.The postmortem lesion was slimy grayish brown patches distributed on the affected areas together with haemorrhges, erosions and ulceration of the skin. Destruction of the gills was also reported.
Identification of the pure isolates of two zoosporic fungi (S.diclinaand A.proliferoides), were carried out on the basis of conventional biochemical tests andbiochemical pathogenicity.
The morphological characters of S.diclinaand A. proliferoides colonies on GY agar their variation in shape and characterization by puffy, whitish, moist colonies which reach full plate after 5 days, at 25°C. Rigid hyphae penetrated into the agar (i.e. separable agar) in A.proliferoides, on the other hand in S.diclina, the fungus was cotton-like whitish colony on GYA, reached full plate after 5 days, at 25°C.
The pathogenicity of (A.prolefiroidesBSN- M005 andS.diclina BSN-M003) was carried by bath immersion of groups of fish on water with high fungal suspension 2 x 10⁵ and others on water with low zoosporicsuspension 2× 10²spore/mL with skin scarification before immersion in two groups by “ami-momi” treatment for establishment of the disease. Experimental infection of fish groups immersed in high (2X10⁵) and low (2X10²) concentrations of (S.diclinaBNS-M003and A.prolefiroidesBNS-M005) was associated with mortality rates of 100 and 60 %, respectively.