Vibrio spp. are common pathogens in fish and shellfish aquaculture with global estimates of economic loss due to vibriosis reported as high as US $3 billion per year. Acute hepatopancreatic necrosis disease (AHPND) is one of the most devastating vibrio infections affecting shrimp aquaculture. AHPND is caused by a toxin-producing strain of Vibrio parahaemolyticus that harbors the pVA1 plasmid encoding toxins PirAVp and PirBVp. Non-AHPND strains of V. parahaemolyticus are found to be involved in white gut/white fecal syndromes in Litopenaeus vannamei, along with other known/unknown causative agents. Other significant pathogenic Vibrio spp. in shrimp culture systems include V. harveyi., V. alginolyticus, V. vulnificus and V. mimicus. This paper details the research on addressing vibriosis in general and on specific evaluation of potential solutions for AHPND and associated syndromes.
In vitro antimicrobial assays were conducted to screen selected phytobiotic compounds for their activity against AHPND and non-AHPND strains of V. parahaemolyticus and against V. harveyi. Studies were conducted using response surface methodology with standard as well as field isolated strains, and prototype formulations were prepared using optimized blends of shortlisted compounds. An in vivo experimental challenge with V. parahaemolyticus (XN89AHPND) after a 42-day feeding period was conducted in white leg shrimp, L. vannamei, to evaluate the effect of supplementation of a prototype formulation on the growth performance and survival under normal and AHPND challenge conditions. The trial design comprised of four groups including the product prototype (0.5 % inclusion in feed), uninfected control, infected control and an antibiotic control. There was no significant difference in growth observed among the treatments after 6-weeks of feeding period (p>0.05). Subsequent to the feeding period, the shrimp was challenged with a virulent isolate of V. parahaemolyticus (XN89) at 105 CFU/mL (LC-50). Survival rates after challenge were significantly different (P<0.05) among the treatments, with a mortality rate of 83.34% in the infected control group. Supplementation of the product prototype improved the survival rates (60%) to be comparable to that of the antibiotic control, 120 h post-challenge. Bacteriological analysis of the gut and hepatopancreas of dead and moribund shrimp in all the treatment groups showed the presence of positive V. parahaemolyticus colonies. Histopathological analysis showed hepatopancreatic tissues with normal tubule epithelia in the uninfected group and thin collapsed hepatopancreatic tubule epithelia in the antibiotic, prototype and infected control groups indicative of AHPND.
The selected product formulation based on phytobiotic compounds was tested in conditions of suspected vibriosis and associated syndromes in commercial shrimp culture systems. Sixteen studies were conducted in shrimp farms at various locations with characteristic symptoms of white gut and/or white fecal syndrome. Trials demonstrated significant improvement in recovery from the vibrio infections based on survival (p<0.05) and clinical symptoms. The results indicate the potential of phytobiotic compounds in addressing AHPND, white gut and other vibrio associated syndromes in white leg shrimp.