Horizontal Plasmid Transfer Promotes the Dissemination of Asian Acute Hepatopancreatic Necrosis Disease and Provides a Novel Mechanism for Genetic Exchange and Environmental Adaptation.

Vibrio parahaemolyticus is an important foodborne pathogen and has recently gained particular notoriety because it causes acute hepatopancreatic necrosis disease (AHPND) in shrimp, which has caused significant economic loss in the shrimp industry. Here, we report a whole-genome analysis of 233 V. parahaemolyticus strains isolated from humans, diseased shrimp, and environmental samples collected between 2008 and 2017, providing unprecedented insight into the historical spread of AHPND. The results show that V. parahaemolyticus is genetically diverse and can be divided into 84 sequence types (STs). However, genomic analysis of three STs of V. parahaemolyticus identified seven transmission routes in Asia since 1996, which promoted the transfer of an AHPND-associated plasmid. Notably, the insertion sequence (ISVal1) from the plasmid subsequently mediated the genetic exchange among V. parahaemolyticus STs and resulted in the deletion of an 11-kb region regulating cell mobility and the production of capsular polysaccharides. Phenotype assays confirmed that this deletion enhanced biofilm formation, providing a novel mechanism for environmental adaptation. We conclude that the transmission mode of AHPND consists of two steps, the transmission of V. parahaemolyticus and the subsequent horizontal transfer of the AHPND-associated plasmid. This plasmid allows ISVal1 to mediate genetic exchange and improve pathogen fitness in shrimp ponds. Current shrimp farming practices promoted such genetic exchanges, which highlighted a risk of the emergence of new virulent populations, with potentially devastating consequences for both aquaculture and human health. This study addressed the basic questions regarding the transmission mechanism of AHPND and provided novel insights into shrimp and human disease management.IMPORTANCE Global outbreaks of shrimp acute hepatopancreatic necrosis disease (AHPND) caused by V. parahaemolyticus represent an urgent issue for the shrimp industry. This study revealed that the transmission mode of AHPND consists of two steps, the transregional dissemination of V. parahaemolyticus and the horizontal transfer of an AHPND-associated plasmid. Surprisingly, the introduction of the AHPND-associated plasmid also offers a novel mechanism of genetic exchange mediated by insertion sequences, and it improved the fitness of V. parahaemolyticus in a harsh environment. The results presented herein suggest that current shrimp farming practices promote genetic mixture between endemic and oceanic V. parahaemolyticus populations, which introduced the plasmid and accelerated bacterial adaptation by the acquisition of ecologically important functions. This entails a risk of the emergence of new virulent populations both for shrimp and humans. This study improves our understanding of the global dissemination of the AHPND-associated plasmid and highlights the urgent need to improve biosecurity for shrimp farming.