Pathogenicity experimentA total of one hundred and sixty Nile tilapia (O. niloticus) weighed 70 g were placed to eight 60-l aquarium tanks with freshwater at 15°C. Fish were fed on commercial diet ad libitum and maintained under 12 h light/ 12 h dark photoperiod. Water quality during all experiments was: dissolved oxygen 6.8–7.5 mg/l, temperature 15°C, pH 6.65–6.87, unionized ammonia <0.001 mg/l and nitrite <0.10 mg/l. Fish were acclimated for 2 weeks. Fish experimental protocol and handling was performed with regardto the ethical committee of veterinary medicine faculty and local Mansoura university rules. The fish were placed 20 fish /aquarium in duplicate aquaria per each group (40 fish/group). The experimental groups were: control negative group without zoospores exposure; infected group exposed to 2 × 〖"10" 〗^"4" zoospores of S. ferax per liter of water (spore/L) and 〖"KMnO" 〗_"4" group is the infected group treated after one week with 2.5 ppm of 〖"KMnO" 〗_"4" ; by adding 150 mL of the stock solution to each treated tank, FCA group is the infected group treated after one week with intraperitoneal injection of 0.1 ml of FCA and both groups lasted for 2 weeks after, and control positive group exposed to 2 × 〖"10" 〗^"4" spore/L for the 3 weeks experimental period. Fish in all groups were descaled on different regions on the body using sharp scalpel then, 2 × 〖"10" 〗^"4" spore/L were added to all groups except the control negative one. All aquariawere covered to minimize contamination. Water changed once a week during the treatment period with replacing the concentration of the 〖"KMnO" 〗_"4" to be at the same exposure level for group 2. The aquaria were checked daily after the challenge for two weeks, and dead and moribund fish were removed for examination. Skin scrapings, gill and fin biopsies of removed fish were examined. S. ferax infection
was confirmed via identification of broad aseptate hyphae, sporangia and encysted zoospores with light microscopy.
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