Infection of covert mortality nodavirus in Japanese flounder reveals host jump of the emerging alphanodavirus

Interspecies transmission of viruses, where a pathogen crosses species barriers and jumps from its original host into a novel species, has been receiving increasing attention. Viral covert mortality disease, caused by covert mortality nodavirus (CMNV), is an emerging disease that has recently had a substantial impact on shrimp aquaculture in Southeast Asia and Latin America. While investigating the host range of CMNV, we found that this virus is also capable of infecting populations of the farmed Japanese flounder Paralichthys olivaceus, a vertebrate host. The infected fish were being raised in aquaculture facilities that were also producing marine shrimp. Through RT-nPCR, targeting the RNA-dependent RNA polymerase (RdRp) gene of CMNV, we found that 29 % of the fish sampled were positive. The amplicons were sequenced and aligned to the RdRp gene of shrimp CMNV and were found to have 98 % identity. Histopathological examination indicated that CMNV-positive fish showed vacuolation of nervous tissue in the eye and brain, as well as extensive necrosis of cardiac muscle. In situ hybridization showed positive reactions in tissues of the eye, brain, heart, liver, spleen and kidney of infected fish. Transmission electron microscopy showed the presence of CMNV-like particles in all of the above-mentioned tissues, except for brain. The novel finding of a shrimp alphanodavirus that can also infect farmed P. olivaceus indicates that this virus is capable of naturally crossing the species barrier and infecting another vertebrate. This finding will contribute to the development of efficient strategies for disease management in aquaculture.