A confirmed case of infectious myonecrosis virus (IMNV) infection in cultured Penaeus vannamei in China

Infectious myonecrosis virus (IMNV) is a highly contagious shrimp virus that is epidemic in few countries. The risk of global transmission of IMNV is increasing due to the frequent transboundary trade of farmed shrimp. Here, we describe a case of IMNV infection in cultured Penaeus vannamei (P. vannamei) in Liaoning province of China. Abnormal deaths in cultured juvenile shrimp from a local farm started in July 2020. The diseased shrimp were tested to be strongly positive for IMNV by nested reverse transcription (RT) PCR and real-time RT-PCR assay. The genome of a novel IMNV variant (designated CN-20-1) was cloned from the diseased shrimp and shared 97%98% sequence identity with the reference strains. Phylogenetic analysis showed that the novel variant was more closely related to strains isolated from Indonesia than from Brazil. The characteristic histopathological changes of IMNV infection, including myonecrosis of skeletal muscle fibers and appearance of lymphoid organ spheroids (LOS), were observed in the diseased shrimp tissues. IMNV infection in the diseased shrimp was also confirmed by electron microscopy and in situ hybridization (ISH). In the artificially challenge experiment, purified IMNV at an injected dose of 2.0 x 10<^>7 copies could cause 100% mortality of healthy shrimp within 9 days post-infection, and the infected shrimp exhibited typical clinical signs and histopathological changes of IMNV infection. Strong hybridization positive signals of the IMNV probe were detected in the tissues of artificially challenged shrimps in the ISH assay. The IMNV variant (CN-20-1) was suspected to have been introduced into the farm through the importation of IMNV-contaminated P. vannamei broodstock from Thailand in the investigation of the origin of IMNV. Subsequent active surveillance in 2020-2021 showed that the IMNV infections did not spread to adjacent areas but occurred only at the original site. However, as the risk of transboundary transmission of IMNV is high, continuous monitoring of IMNV and implementing strict measures to prevent its spread through trade are really urgent to control its threat to global shrimp farming.