Description of a natural infection with decapod iridescent virus 1 in farmed swimming crab

As an emerging viral pathogen, Decapod iridescent virus 1 (DIV1) is a severe threat to the farmed and wild crustaceans, and it can spread between shrimp and crabs. Although swimming crab (Portunus trituberculatus) was identified as one of the DIV1 susceptible species by experimental infection mimicking the natural pathway, no case of natural infection was reported to date. Samples of Po. trituberculatus suffered from severe mortality were collected from a farm in Shandong Province of China. Diseased Po. trituberculatus exhibited slow movement and loss of appetite, while dying crabs showed empty stomachs and guts. Real-time PCR revealed that Po. trituberculatus samples were DIV1 positive and had a very high DIV1 load (up to 5.68 × 1010 copies/μg DNA) in gill tissue. Histopathological examination revealed eosinophilic inclusions and pyknosis in the hemocytes of the Po. trituberculatus gill tissue. Transmission electron microscopy of ultrathin sections showed that there were typical DIV1 particles in the gill tissue and hemocytes, which confirmed the disease of the farmed Po. trituberculatus was caused by the infection with Decapod iridescent virus 1 (iDIV1). Further bioassay by oral administration with tissues of diseased crabs and hemolymph coagulation examination showed that the diseased Po. trituberculatus had a significantly longer coagulation time than in healthy individuals. This study reported a severe case of natural infection with DIV1 in cultured swimming crab, further confirming the risk of DIV1 to the swimming crab farming industry and providing a new approach for on-site diagnosis of iDIV1 or other potential pathogens in Po. trituberculatus.