Abstract:Acute hepatopancreatic necrosis disease (AHPND) is an emerging shrimp disease causing great losses for the shrimp culture industry worldwide since 2010. In the present study, a bacterial strain 20160303005-1 was isolated from the hepatopancreas tissue of Litopenaeus vannamei with early mortality syndrome (EMS), and was identified as Vibrio parahaemolyticus based on its physiological and biochemical characteristics; and the analysis of both 16S rRNA and groEL gene sequence. The serotype of the bacterium is O1:KUT (K untypeable). It revealed positive amplification of the genes pirAVP and pirBVP which is related to cause AHPND in a virulence plasmid harboring this strain. However, the isolate examined showed negative amplification results for the virulent clinical V. parahaemolyticus strain markers—thermostable direct hemolysin gene tdh and TDH-related hemolysin gene trh. The immersion challenge test with L. vannamei was also employed to study pathogenicity and histopathology. The results showed that the isolate was highly virulent, with a median lethal dose (LD50) value of 7.96×103 CFU/ml. The empty gut in shrimp was observed at 6 h post-challenged. The hepatopancreas appeared pale and atrophic at 9 h. More than half of the shrimps died at 12 h, and up to 100% died at 24 h. Subsequent histological analyses showed that the hepatopancreas tubules collapsed with massive sloughing of hepatopancreas epithelial cells, which was the typical pathological characteristics of AHPND. Among 21 antibiotics tested, the isolate was resistant to amoxicillin, cefalotin, ticarcillin, cefuroxime, and cotrimoxazol; however, it was sensitive to gentamicin, ciprofloxacin, and other 14 antibiotics tested. These results provide basic data for epidemiology and drug control research on V. parahaemolyticus in aquaculture.
