Investigations on susceptibility of marbled rabbitfish Siganus rivulatus to various infectious marine bacteria -

Abstract

Growth of the aquaculture industry is accompanied with a rapid spread of fish disease. However, disease management is complicated by the fact that fish species vary in their susceptibility to pathogenic microorganisms, thus requiring that every species be studied independently. The present work was performed to investigate the susceptibility of marbled rabbitfish Siganus rivulatus to Aeromonas hydrophila, Mycobacterium marinum, Vibrio anguillarum, Streptococcus iniae, and Yersinia ruckeri. Fish were challenged with high concentrations of each bacterium and survival and hematological responses were evaluated. Furthermore, we investigated whether disease lesions found on wild fish contain any of the five bacterial strains. Fish were randomly distributed into 18 plastic tanks (12 fish per tank) containing 5 L of filtered seawater and supplied with rigorous aeration. Bacteria were added to each container at 107 CFU-ml and fish maintained for 60 mins. Fish were then removed, rinsed and placed in aquaria and observed for 10 days. Blood was then extracted from three fish of each aquarium and hematological parameters were assessed. Red blood cell and white blood cell counts as well as differential counts of lymphocytes and thrombocytes in bacterial treatments were similar to those of control fish. However, monocyte and neutrophil counts varied between the control and some treatments. The high number of neutrophils in fish challenged with M. marinum suggests that M. marinumis potentially pathogenic to S. rivulatus, possibly if the fish were stressed at the time of infection. Fish collected from Ein el Mreiseh with disease lesions were tested for the presence of the five bacterial species in their lesions. DNA was extracted from lesions, multiplied using PCR and standard primers for each species. Bands on gel electrophoresis were compared to DNA bands from pure bacterial cultures. No bands representing any of the five bacteria were found in the lesions. Although none of the five bacteria were found to be pathogenic to