INFLUENCE OF A NOVEL PROBIOTIC CANDIDATE ON IMMUNITY AND DISEASE RESISTANCE OF Pangasius hypophthalmus.
Edwardsiella ictaluri is an economically relevant pathogen for numerous important aquaculture species around the world. In the Asian catfish species Pangasius hypophthalmus, E. ictaluri causes a bacillary necrosis disease, contributing to large mortality rates of this foodfish. Traditionally, E. ictaluri infections are sometimes treated with antibiotics, but this method is currently not practical for the control of disease. In light of alternative methods for prophylaxis and control being necessitated, the use of probiotics for stimulating disease resistance has gained attention and been subject to scientific scrutiny in recent years. Thus, the present study aimed to elucidate the effect of a potential probiotic strain on immunity and resistance to bacillary necrosis in P. hypophthalmus.
This study first compares the effect of a control diet to a probiotic diet (Bacillus subtilis) on immunity of P. hypophthalmus. For this, 36 fish (mean weight of 100g) per diet treatment are currently stocked in two separate 800L recirculating aquaculture systems. Fish are being sampled at weeks 4 and 8 for several innate immunity parameters in the blood and lymphoid tissue. The examinations include blood leukocyte differentials, lysozyme activity, immunoglobulin titer, phagocytic activity, oxidative burst, and alternative complement activity.
The second portion of the study compares the effect of the control diet to the probiotic diet on the catfish species after challenge with E. ictaluri. A control group (n=40) and the probiotic group (n=40) are being challenged by immersion bath with E. ictaluri (1x107 CFU/mL) for 60 minutes. An extra 20 fish from the control diet, and 20 fish from the probiotic diet are being used as negative controls, with no bacterial immersion. Fish are being sampled at days 10 and 20 for liver, head kidney and blood.
Preliminary results (table 1) of a bacterial challenge assay demonstrated that probiotic fed fish reduced the appearance of clinical signs of bacillary necrosis in the catfish species compared to a control. Therefore, this study currently underway is further evaluating the benefits of direct-fed probiotic on disease resistance of fish.