Effects of immunostimulants on the immune response and resistance of striped catfish (Pangasianodon hypophtalmus) to bacterial disease : in vitro and in vivo approaches.

Abstract

The striped catfish (Pangasianodon hypophthalmus) farming system in the Mekong Delta of Viet Nam has been vigorously developed in recent years. It has resulted from expansion of farming areas, application of new farming techniques and an increase of intensification level. As in other developing striped catfish culture venture, health problems including disease outbreaks have been encountered. The use of chemicals and antibiotics in striped catfish farming has increased for prevention and treatment of diseases. These compounds have attracted criticism because of a potential hazard for fish health, environment risk and human consumers. The present study aimed to evaluate the efficiency of different immunostimulating compounds in striped catfish farming in order to reduce the use and impact of antibiotics and chemicals by alternative method of immunostimulation. A combination of in vitro and in vivo approaches was used to investigate the effects of Lipopolysaccharide (LPS) on immune responses of striped catfish. Some potential results of this research were highlighted. The innate immune system of striped catfish was significantly stimulated by LPS injection or feeding through the increase of respiratory burst, lysozyme and complement activities. Total of immunoglobulins was also enhanced in LPS treatment. A challenge test with a common bacterial pathogen (Edwardsiella ictaluri) for striped catfish was performed and documented that LPS could significantly reduce the cumulative mortality of fish, especially in low dose such as 3 mg LPS/kg fish for injection and 0.01% of LPS for oral. Proteomic analysis performed in both approaches suggested that LPS could stimulate the over-expression of several immune proteins such as complement component, lysozyme precursor, transferrin, immunoglobulin,… In addition, the results from in vitro also showed that the response of striped catfish to LPS involved an increase of many protein expression related in cytoskeleton, stress response, cell signaling, carbohydrate metabolic process as well as muscle contraction. All these protein responses result in an increase of the ability of striped catfish peripheral blood mononuclear cells (PBMC) to produce and secrete pro-inflammatory mediator, which is fundamental to fight against a pathogen invasion. Comparing the efficiency of different immunostimulants on immune responses and bacterial resistance of striped catfish was also investigated. The results suggested that all tested immunnostimulants (β-glucan, bovine lactoferrin, chitosan, levamisole and vitamin C) could significantly enhance the immune response in striped catfish and protect fish from bacteria damage. Among them levamisole at the dose 5 mg/kg fish performed as the best immunostimulant. From above results, levamisole and LPS were selected as immunostimulant in a comparative experiment aiming to evaluate whether these compounds could efficiently replace an antibiotic treatment in infected striped catfish by E. ictaluri. Both LPS and levamisole stimulated the immune system of striped catfish and reduce the cumulative mortality after challenge with bacteria. The fish mortality in treatments LPS and levamisole without anitibiotics did not significantly differ with the one of control (without immunostimulants) treated with antibiotics. These results are good evidences to support that LPS and levamisole can replace antibiotic treatment to protect striped catfish against Edwardsiella ictaluri. However, validation in field trials needs to be performed before advocating the use of LPS and levamisole in intensive striped catfish culture practices. Overall, the use of immunostimulants in striped catfish aquaculture appears as an excellent method to enhance the immune system and increase the bacterial resistance of this fish species, supporting the replacement of antibiotics by alternative method such as the immunomodulation.(DOCSC03) -- FUNDP, 201