Pathogens and histopathological characteristics of shrimp postlarvae bacterial vitrified syndrome (BVS) in the Litopenaeus vannamei

The shrimp postlarva vitrified syndrome broke out in spring of 2020 and spread explosively along the coastal areas from south to north of China. In order to find out the main pathogenic agents of shrimp postlarva vitrified syndrome, in this study, the pathogen was isolated and identified, and the histopathology was investigated. The postlarva symptoms included emaciation, dark cloud, decreased activity, anorexia, empty intestinal tract and stomach. The hepatopancreas showed atrophy, blurring of contour, paleness and even vitrified syndrome. Histopathological analysis showed that the epithelial cells of liver tubule were necrotic and exfoliated, liver tubule was filled with a large amount of debris tissue, leaving continuous glassy homogeneous areas. The intestinal tract was filled with tissue fragments, chorionic membrane fell off and even disappears. Ultrastructural pathological examination showed that the membranes of epithelial cells were ablated, organelles disintegrated, and nuclei were solidified. Subsequently, the cells disintegrated, fell off, and even the hepatic tubular tissue structure was ablated. The bacteria were found in the hepatopancreas, intestinal tract and gastric mucosa. The dominant strain was rod-shaped and curved, and no virions were found. Two dominant bacteria (Lv-A, Lv-B) were isolated from the diseased shrimp postlarvae. Artificial infection experiments illustrated that the strains of Lv-A and Lv-B were the causative pathogens with a median lethal dose of 1.62×10³ CFU/mL and 5.38×10³ CFU/mL, respectively. Based on molecular analyses (16S rDNA and gyrB), Lv-A and Lv-B were highly similar to Vibrio alginolyticus, V. neocaledonicus and Vibrio parahaemolyticus, preliminarily named shrimp postlarva bacterial vitrified syndrome (BVS). The chemotherapeutant sensitivity tests illustrated that Lv-A and Lv-B were sensitive to minocycline, doxycycline, nalidixic acid, etc. and resistant to neomycin, pipemidic, rifampicin, etc. This study provides theoretical basis and technical support for the effective prevention of BVS and the healthy development of shrimp industry.