Haemorrhagic septicaemia caused by Aeromonas hydrophila in striped catfish (Pangasianodon hypophthalmus) is one of the most important aquatic diseases in the Mekong Delta, Vietnam. However, antibiotic‐resistant A. hydrophila strains have become popular and resulted in inadequate control of the disease in striped catfish farms. This study investigates the protective efficacy of bacteriophage PVN02 against haemorrhagic septicaemia in striped catfish via oral administration. The phage‐containing pellets were prepared by spraying the phage solution on food pellets at 20 ml/kg. The rate of phage desorption from the food pellets into the water was very low; the phage titres in the water were approximately log 1.0 PFU/ml or undetectable. The in vivo experiment evaluating the protective efficacy of PVN02 against haemorrhagic septicaemia in striped catfish was conducted using 21 groups of 1,260 fish in 50‐L plastic tanks in triplicate. The catfish were fed twice daily with phage‐sprayed pellets. Different densities of bacterial suspensions were added into the tanks for 24 hr. Without the existence of the phage, the highest mortality rate was 68.3 ± 2.9% at the highest density of bacterial suspension. In contrast, the mortality rate at the highest density of bacterial suspension was significantly reduced to 8.33 ± 2.9% or 16.67 ± 2.9% at the phage dose of log 6.2 ± 0.09 or log 4.2 ± 0.09 PFU/g. This study provides a very practical manner of applying phage therapy to prevent disease in large‐scale striped catfish farms.

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Introduction: Striped catfish (Pangasianodon hypohthalmus) is a native freshwater fish species in the Mekong Delta, Vietnam, and significantly contributes to national aqua exports. Currently, however, the sustainable development of striped catfish farming is being affected by bacterial pathogen infections, of which hemorrhagic septicemia caused by Aeromonas hydrophila bacteria is one of the most common diseases. Methods: In this study, the stability of TG25P and CT45P phages to factors such as temperature, pH, and organic solvents was investigated, with the stability of TG25P being found to be higher than that of CT45P. Results: The activity of TG25P was retained to approximately 90% and 80% at 37oC and 50oC for 1 h, respectively. Its activity was maintained to greater than 80% at pH 5-9 for 24 h and approximately 90-100% in organic solvents, such as chloroform or diethyl ether, for 1 h. In addition, the stability and activity of TG25P for the control of A. hydrophila in striped catfish pond water was also evaluated for 48 h. Conclusion: TG25P was found to be highly applicable in the creation of low-cost phage-containing products for the prospective application of phage therapy in prevention and treatment of hemorrhagic septicemia in striped catfish.

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