White Feces Syndrome (WFS), a phenomenon where white or pale feces manifests in cultured shrimp, has emerged as a problem in shrimp production across the globe. Typically seen in populations infected with Ecytonucleospora hepatopenaei (EHP) and various Vibrio spp., this occurrence can precede large economic losses. We describe here a simple experimental challenge method to reproduce WFS in vivo under laboratory conditions using juvenile SPF P. vannamei. Two Vibrio parahaemolyticus strains were tested: (i) a WFS-associated isolate lacking the pirA and pirB toxin-encoding genes present in acute hepatopancreatic necrosis disease (AHPND) and (ii) a translucent post-larval disease (TPD) strain carrying vhvp genes.
Seven challenge groups were included in this bioassay: (1) negative control (SPF) shrimp, (2) shrimp challenged with WFS Vibrio alone, (3) shrimp challenged with a sublethal dose of TPD alone, (4) shrimp challenged with both WFS Vibrio and a sublethal dose of TPD, (5) EHP-infected shrimp alone, (6) EHP-infected shrimp challenged with WFS Vibrio, and (7) EHP-infected shrimp challenged with a sublethal dose of TPD. There were two 90 L tanks per challenge group and 10 shrimp stocked into each tank.
In challenge group (6) and (7), white fecal strings started appearing six days post-challenge and persisted until study end (day 21) or, for group 7, until early termination (day 11) due to mortality. In these groups (6 and 7), higher mortality was observed compared to shrimp infected with EHP alone or challenged with Vibrio spp. alone, suggesting an EHP infection enhances susceptibility to even a sub-lethal dose of Vibrio bacteria.
To summarize, the data gathered from this study demonstrates a reproducible challenge model with predictable results within an estimated 21 days of Vibrio immersion. This method can be implemented to help develop genetic lines of shrimp that are tolerant/resistant to EHP/AHPND/TPD, as well as screen candidate therapeutics that are effective in mitigating WFS.