Isolation and identification of a megalocytivirus strain (SKIV-TJ) from cultured spotted knifejaw (Oplegnathus punctatus) in China and its pathogenicity analysis.

Abstract

The spotted knifejaw (Oplegnathus punctatus) has recently emerged as a highly economically significant farmed fish in China. However, due to increasing environmental pollution and breeding density, a range of infectious diseases, including the iridovirus pathogen, have begun to spread widely. In this study, we isolated and identified a strain of Megalocytivirus, SKIV-TJ, from cultured spotted knifejaw in Tianjin, China. We observed significant cytopathic effects (CPE) in SKIV-TJ-infected spotted knifejaw brain (SKB) cells, and electron microscopy showed numerous virus particles in the cytoplasm of SKB cells 6 days post-infection. The annotated complete genome of SKIV-TJ (GenBank accession number ON075463) contained 112,489 bp and 132 open reading frames. Based on the multigene association evolutionary tree using 26 iridovirus core genes, SKIV-TJ was found to be most closely related to Rock bream iridovirus (RBIV). Cumulative mortality of spotted knifejaw infected with SKIV-TJ reached 100% by day 9. A transcriptomic analysis were conducted and a total of 5517 differentially expressed genes were identified, including 2757 upregulated genes and 2760 downregulated genes. The upregulated genes were associated with viral infection and immune signaling pathways. Our findings provide a valuable genetic resource and a deeper understanding of the immune response to SKIV infection in spotted knifejaw.