Bacteriological and histopathological analysis of Penaeus vannamei experimentally infected with Vibrio parahaemolyticus-AHPND strains

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Abstract
Vibrio parahaemolyticusstrains have been identified as the causative agent of acute hepatopancreas necrosis disease (VpAHPND+) in shrimp. In the present work, a highly virulent (M0904) and less virulent (M0607) strain ofVpAHPND+ were used in experimental infections ofPenaeus vannamei. Throughout the experiment, histopathological analyses were conducted on shrimp tissues, and samples from the bottom (B) and water column (WC) of each aquarium were taken to estimate bacterial growth. The first deaths of shrimp challenged withVpM0904 occurred 4 h post-infection (hpi), reached 50% mortality at 17 hpi, and showed more acute pathological progression associated with AHPND despite both strains having similar bacterial densities from 0-4 hpi. The first deaths of shrimp inoculated withVpM0607 occurred at 13 hpi, a difference of 9 h. These shrimps displayed reduction of reserve vacuoles in R and B cells of the hepatopancreas, and reached 50% mortality at 27 hpi. As bacterial growth in the WC and B in the first hours was similar for both strains, bacterial density was not a significant factor inVpvirulence. Since pirAB toxin is the causative agent of AHPND, it is possible that there is a threshold level at which the toxin damages hepatopancreatic tissues, so production or secretion of the toxin can be dependent of theVpstrain. We propose new AHPND stage: remission with 3 main routes for the disease outcome after the remission stage (re-infection, chronic effect and recuperation). Hepatopancreatic embryonic cells can be used as a biological indicator of this stage.

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