Pathogenicity and histopathology of infectious spleen and kidney necrosis virus genotype II (ISKNV-II) recovering from mass mortality of farmed Asian seabass, Lates calcarifer, in Southern China

Megalocytivirus of family Iridoviridae was considered as one of the most alarming causative agents which caused fatally systemic infection and lead to mass mortalities to world-widely distributed freshwater and marine fish. In recent years, severe diseases with typical symptoms of body blackened, swims slowly to the water surface or surrounding the ponds were observed frequently in cultured Asian seabass, Lates calcarifer, in Zhuhai city of Guangdong province, where is the most important area for Asian seabass culture in mainland China. The causative agents were highly suspected as megalocytiviruses, however, the detailed and systemic studies about these diseases are still lacking. In this study, two cases of high morbidity and mortality of Asian seabass were investigated by systemic isolation and identification of pathogen. Virus isolation by MFF-1 cells and molecular detection confirmed that the causative agents were ISKNV genotype-II (ISKNV-II) of megalocytivirus. The mortalities from natural and artificial infected fish were 85.89% and 83.30%, respectively. A total of 21 single-nucleotide differences, including 20 same-sense mutations, were observed on the ISKNV-II compared with that of ISKNV-I. Histopathology examination showed that numerous abnormal enlarged cells, the most well-known ISKNV-specific clinicopathological features, were observed in spleen, kidney and liver. Results of immunohistochemistry (IHC) and immunofluorescence assay (IFA) examination confirmed that numerous abnormal enlarged cells with abundant viruses existed in these tissues. Moreover, large number of viral particles of diameter 150 nm were also observed both in infected tissues and MFF-1 cells by transmission electron microscope (TEM). Taken together, this is the first study with detailed and robust evidence confirming that ISKNV-II is the causative agent of the severe diseases in farmed Asian seabass in mainland China. This study contributes to the pathogenic and epidemiological studies of Asian seabass iridovirus disease in mainland China and other Asian countries and regions.