الفهرس 
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Abstract
This study was carried on 162 red swamp crayfish Procombrous clarkii, 100 0. niloticus, 100 Nile catfish C. gariepinus, 10 common carp C. carplo and 24 striped gray mullet M cephalus. Gross examination of Procambrous clarkil, revealed some clinical signs, in few cases appeared as soft, brittle and yellowish colored carapace with fuzzy like growth on the articular memrane and eyes. Also destruction of the articular membrane was observed. These lesions were closy similar to that described in case of crayfish plague. However, Microscopical examination of wet-mount preparation from these lesions revealed presence of heteroplaria species. Clinical signs of Aphanomycosis in examined fish (C. gariepinus &. M cephalus) appeared as excess mucous secretion with dark grey coloration of the skin, detached scales, descrate superficial ulcers, abdominal swelling in some cases and inflamed destructed fin. Although some fishes did not show any clinical signs, they covered Aphanomyces species isolates. Wet mount preparation taken directly from the suspected lesions revealed presence of sparsely branched long to very long non septated hyphae with tapered end. The percentage of isolation of Aphanomyces sp among examined red swamp crayfish was 43.8%, while the percentage of isolation among the examined fish was 9.8% represents 12%, 2%, 10% and 33.33% in C. gariepinus, 0. niloticus, C. carpio and M. cephalus, respectively. On the other hand, the prevalence of infection among obtained C. gariepinus was 8% and 16.67% in M cephalus. while 0. niloticus and C. ca;pio showed no clinical signs or any sort of infection indicating that the both fishes were resistance to Aphanomyces infection. Isolation of the fungus was carried out on glucose peptone yeast extract (OPY) broth and GPY agar supplemented with antibacterial (chloramphnicol, streptopencid, penicillin 0 sodium) and antimycotic (Amphotricin B, Fluconazole, Metalexyle). On sporulating media (SM), the Aphanomyces sp appeared as long hyphae containing one row of rectangular spores (primary spores) linked together with cytoplasmic thread. The spores released at the tip of hyphae forming balls of primary spores. Few primary spores failed to release forming retained refractile encysted mature spores within the sporangium. No sexual organs (antheridia, Oogonia) observed on the sporulating media even after long incubation period. Results of experimental infection for Cl. gariepinus showed mortality rate 40-60%, while the rate reached 80-100% in fish treated with a synthetic cortizon. Experimental infection in 0. niloticus in both coticostroid treated and untreated revealed negative results. Reisolation from experimentally infected C. gariepinus with the isolated Aphanomyces sp.revealed positive results. The histopathological examination of skin of C. garipenius experimentally infected with Aphanomyces species revealed multiple granulomas formed from aggregation of inflammatory cells mainly epitheliod cells. Congestion, hemorrhages, severe myonecrosis and aggregation of multinucleated giant cells were detected. Moreover, PAS fungal hyphae surrounded with inflammatory reaction was also seen.