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.

The study was conducted to evaluate the susceptibility of 58 Vibrio parahaemolyticus bacterial isolates to 16 antibiotics. These bacterial isolates were recovered from diseased shrimp which displayed typical pathology of AHPND such as hepatopancreatic atrophy, empty gut, and hepatopancreatic changes including hemocytic infiltration and bacterial infection. Results of antibiotic susceptibility testing by the disk diffusion method showed that single resistance to tested antibiotics was relatively rare. The bacterial strains were resistant to amoxicillin (100%), cephalexin (100%), Sulfadiazine Sodium (94.7%), and Erythromycin (87.7%). Sensitivity of tested strains was recorded with doxycycline (84.2%) and oxytetacylin (49%). The minimum inhibitory concentrations (MICs) were determined for the sensitive isolates using a broth macro dilution method. The majority of tested isolates had an MIC value of 2μg/mL with doxycycline. The current study suggests caution in the use of antibiotics for the prevention and treatment of AHPND in shrimp farming.

A field study on the water quality parameters and prevalence of diseases was carried out in four locations in the Mekong Delta, Vietnam. A total number of 960 samples (816 with disease signs and 144 without disease sign) of farmed shrimp were collected over 8 sampling months (2 production cycles). Samples were collected every two weeks and when abnormal behavior from cultured shrimp was observed. Five major groups of gross clinical signs were recorded among diseased shrimp samples. Diseases that appeared through the sampling months were: (1) atrophy and pale-colored hepatopancreas (HP) with the empty or little food in the mid gut; (2) empty/little/discontinued food in the midgut and (3) slow growth, HP atrophy and gut with discontinued food. White feces disease was not found in April but appeared for the rest of sampling months with the highest prevalence in July (39,2%) and the lowest prevalence in September (4,9%). White spot disease appeared from October to December (at the end of when the monsoon season and low temperature).

A total of 291 white leg shrimp samples were collected from 70 cultured ponds in Soc Trang, Bac Lieu and Ca Mau provinces in the Mekong Delta and subjected to endoparasitic detection in the digestive tract. Collected shrimps displayed unhealthy behaviors such as stop or less feeding and lethagic swimming. Pathological signs in the gastrointestinal tract include (1) empty midgut and stomach together with pale and atrophy hepatopancreas; (2) empty, little or discontinued food in the midgut; (3) slow growth and variation in sizes; and (4) white feces. The results from fresh and Giemsa stained smears methods revealed that 96.5% of sampled shrimps were infected by gregarine parasite at different developmental stages. A prevalence infection of 24.7% was recorded with Vermiform present in the hepatopancreas by fresh smear and histology. Histopathological analysis noted that 7.9% of collected shrimp samples had Enterocytozoon hepatopenaei (EHP) spores in hepatopancreas and midgut and confirmed by PCR analysis.

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