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Abstract
The results of bacteriological examination of 300 clinically diseased O. niloticus fish demonstrated that, 60 cases harboured bacteria of the family Enterobacteriaceae with an overall incidence of (20%) using API 20E system and the most predominant species were E. tarda (18.33%) and Y. ruckeri with an incidence of (13.33%) from the total number of isolates.
The tests used for detection of the virulence factors of Y. ruckeri and E. tarda were haemolysin, haemagglutinating activities and siderophore production.
Alpha-haemolysin was recorded in 62.5% of Y. ruckeri isolates against 5% (v/v) bovine, sheep and tilapia erythrocytes and Beta-haemolysin was recorded in 63.6% of E. tarda isolates.
The ability to agglutinate RBCs of Guinea pig was demonstrated in 75% and 72% of Y. ruckeri and E. tarda isolates respectively. In addition, 33.3% of haemagglutinating Y. ruckeri isolates and 50% of haemagglutinating E. tarda isolates were mannose resistant.
On the other hand, 25% of Y. ruckeri isolates were hydroxamate siderphore producer, while four out of 11 examined E. tarda isolates showed siderophore production.
Experimental infection was successful through I/P injection of Y. ruckeri and E. tarda in O. niloticus and the recorded mortality rates were 66.6% and 80% respectively at the end of experimental period.
ECPs of Y. ruckeri and E. tarda were highly virulent when injected intraperitoneally in O. niloticus produced mortality rates of 73.3% and 93.3% respectively.
The effect of 5 mg of levamisole / kg fish as a non-specific immunostimulant against Y. ruckeri and E. tarda was investigated and the results demonstrated reduction in mortality to be 25% in case of Y. ruckeri and 35% for its ECPs. Meanwhile, the mortality rate was 30% in case of E. tarda and 40% in case of its ECPs.