Increased susceptibility of white spot syndrome virus-exposed Penaeus vannamei to Vibrio parahaemolyticus causing acute hepatopancreatic necrosis disease

Abstract In this study, co-infection of white spot syndrome virus (WSSV) and AHPND-Vibrio parahaemolyticus (VpAHPND), was studied in juvenile Penaeus vannamei under the laboratory condition. Experimental shrimp were divided into 4 groups (group 1: WSSV only; group 2: VpAHPND only; group 3: WSSV + VpAHPND; group 4: not challenged), and for the co-infection, shrimp were challenged with dual pathogens (initially exposed to WSSV and subsequently exposed to VpAHPND). During the experiments, we observed an increased mortality rate in the co-infection group (group 3, N = 7/20 shrimp at day 2 post-infection [p.i.], and N = 15/20 shrimp at day 3 p.i.) than other groups (N = 1/20 shrimp at day 3 p.i., and N = 10/20 shrimp at day 4 p.i. in group 2; no mortalities in group 1 and 4). Also, we found obvious differences in WSSV infection among groups; strong WSSV-positives were detected by immunohistochemistry examination (brown coloration) and qPCR assay (the copy numbers between 2.0 × 103–3.7 × 104) in the co-infection group on the termination day, but these were not detected in the other groups. In addition, there were prominent differences in the histopathology of the hepatopancreas of surviving shrimp among groups; shrimp of the co-infection group showed typical histopathology signs of AHPND in the hepatopancreas, with sloughing of hepatopancreatic tubule epithelial cells (G4) on the termination day, but the hepatopancreas in shrimp of the other groups appeared normal. This study demonstrated that shrimp initially exposed to WSSV allows VpAHPND to cause faster and higher mortality than shrimp infected with VpAHPND alone, and also leads to severe WSSV infection, indicating that WSSV is a risk factor for VpAHPND in ponds. Therefore, shrimp farmers should pay attention to the management of co-infection in the pond.