Abstract:Spotted knifejaw (Oplegnathus punctatus) is a new species of mariculture fish with high economic value in China. In August 2017, an outbreak of unknown etiology occurred in spotted knifejaw juveniles cultured in the offshore cages of a marine fish farm. Investigation showed that the water temperature was 26℃~28℃ and that the infected fish were mainly juveniles at the age of 4~5 months. The total length of the diseased fish was (16.3±1.6) cm and their body weight was (156.9±37.0) g. The cumulative mortality of 800000 juveniles was more than 90% within 2 weeks of disease onset. Clinical examination showed that the diseased juveniles had no obvious apparent lesions but had poor vitality and rapid breathing. Necropsy findings included a swollen and brittle spleen, kidney erosion, and hemorrhagic spots in the liver. No parasites were detected, and no pathogenic bacteria were isolated from the liver, spleen, and kidney of the diseased fish. Histopathological sections showed a large number of enlarged cells (about 20 μm in diameter) in the spleen and kidney tissues of diseased fish. In addition, a large number of hexagonal virus particles with a diameter of about 145 nm were observed in the electron microscope sections of the spleen and the body kidney. The spleen homogenate of naturally infected fish was filtered and injected intraperitoneally into healthy fish at a water temperature of 26℃. The fish in the infected group began to die after 10 days post infection (dpi), and the cumulative mortality reached 95% within 14 dpi. Artificially infected fish showed symptoms similar to those in naturally infected fish, and a large number of enlarged cells and similar virus particles could be observed in the spleen and kidney tissue sections. Severe iridovirus infection was detected in the spleen and kidney tissues of naturally and artificially infected fish using specific PCR primers. The major capsid protein gene (MCP) of the iridovirus, at 1362 bp in length was cloned and sequenced, and the phylogenetic tree of the family Iridoviridae was constructed. The results showed that the virus belonged to the red sea bream iridovirus (RSIV) of genus Megalocytivirus in the family Iridoviridae. Based on the results of epidemiological investigation, clinical symptom observation, etiological detection, histopathological observation, artificial infection experiments, molecular biology diagnosis, and viral phylogenetic analysis, it was confirmed that the disease causing the mass death of spotted knifejaw was Megalocytivirus disease, and that the pathogen was a strain of RSIV. This study demonstrates for the first time that RSIV can cause large-scale death in the spotted knifejaw. Furthermore, the research results provide an important reference to diagnose and control viral diseases in the spotted knifejaw.
