Horizontal and vertical transmission of covert mortality nodavirus in the marine fish

Covert mortality nodavirus (CMNV) is a vital emerging virus in aquaculture issued by World Organization for Animal Health recently. It can infect teleost fish, but the mode of transmission remains unclear. Therefore, this study investigated the potential of vertical and horizontal transmission of CMNV in marine fish using the marine model organism of marine medaka Oryzias melastigma. First, the clinical symptoms, viral load and histopathological changes of marine medaka artificially infected by intraperitoneal injection infection were consistent with those of fish species naturally infected with CMNV, indicating that marine medaka is an ideal model organism for CMNV infection. Subsequently, RT-nPCR analysis demonstrated a 10.5 % CMNV positivity rate in cohabitation-challenged marine medaka, with viral loads ranging from 3.28 × 10² to 3.21 × 10³ copies per gram of tissue. Histopathological analysis revealed severe lesions occurred in the brain, eye, kidney, and intestine of infected individuals, with positive hybridization signals of CMNV RNA probes were detected at necrotic sites in in situ hybridization (ISH) assay. These results provide strong evidence for the horizontal transmission of CMNV through waterborne in marine medaka. In addition, RT-nPCR and ISH analysis showed that ovarian tissue, testis tissue, fertilized eggs/embryos, and offspring fish from artificially infected parental marine medaka all tested positive for CMNV, demonstrating that vertical transmission can occur through either parent. The results of this study provide compelling evidence for both vertical and horizontal modes of transmission of CMNV in marine medaka.