Cases report of covert mortality nodavirus infection in indoor farming Penaeus vannamei

In late 2020, unusual patterns of shrimp mortality were reported in semi-intensive shrimp culture systems in Dongying city and Weifang city, China. In order to elucidate the cause of continual death that occurred in local shrimp (Penaeus vannamei) farms. Samples of shrimp and their frozen bait were collected and analyzed from 4 diseased farms in Dec 2020. Diseased shrimp showed abnormal behavior and symptoms including gathering at the pond bottom, whitish muscle, hepatopancreatic atrophy with color fading, etc. The molecular detecting results showed that 100 % of P. vannamei samples were tested to be positive of covert mortality nodavirus (CMNV). The histopathological lesion of tissues taken from moribund shrimp was similar to typical histopathology features of CMNV-infection. In situ hybridization (ISH) results indicated that massive CMNV-positive hybridization signal was observed in diseased shrimp tissues. Meanwhile, all of samples of frozen bait (i.e., Artemia sp. and Acetes sp.) were determined to be CMNV positive with much high viral load. The healthy shrimp individuals could be infected by the purified CMNV from Artemia sp.in the artificial challenged test. This study demonstrated that the frozen bait Artemia infected with CMNV was confirmed to be the pathogen carrier in the CMNV infection case, and represented a new potential pathogenic introduction threat to indoor shrimp farming. It was highly recommended to avoid using pathogen status unknown frozen bait in the shrimp aquaculture.