Dynamic of Vibrio parahaemolyticus and PirA AND PirB toxins during the infectious process of Acute Hepatopancreatic Necrosis Disease (AHPND) in Litopenaeus vannamei  

In Mexico, during 2013 there were mortalities in L. vannamei shrimp farms associated with AHPND which caused a decrease in production of up to 70%. The causal agent was identified as toxigenic strains of Vibrio parahaemolyticus (Vp AHPND+), of different virulence, carrying a plasmid of 73 kb with genes coding for Pir type toxins (PirA and PirB).  In order to characterize the infectious process, we assessed the developmental capacity of AHPND+ strains and the expression of the PirA and PirB genes in stomach (ST) and hepatopancreas (HP) of shrimp and the aquarium water used during an experimental infection, using qPCR using AP3 primers and RT-PCR using primer designed by CIAD supported with in situ hybridization (ISH) and TEM (transmission electron microscopy).

Significant differences were found between post infection (p.i.) times in water and ST (P=0.000), but not in HP (P = 0.770), when bacterial density was maintained between 0.75-1.0 Log during the 24 hours of experimental infection indicating that the water and ST of shrimp are the environments for residence and multiplication during the infectious process. The expression of PirB gene in water was detected from 1 h p.i. to 24 h p.i., with significant difference (P=0.021) for each time p.i. (Fig 1). The maximum expression of PirB in both environments was quantified at 12 h p.i., the same time at which the first mortalities were recorded during the experimental infection.

The HIS and MET did not provide evidence that the bacteria colonized the ST and HP of shrimp, indicating that water and ST of shrimp are the preferred sites for multiplication of Vp AHPND+ during the infectious process, a period in which the expression of toxigenic gene PirB was in the same proportions in both places, behaving similarly to a constitutive gene.