Isolation of a Ranavirus-type grouper iridovirus in mainland China and comparison of its pathogenicity with that of a Megalocytivirus-type grouper iridovirus

• First isolation of a Ranavirus-type grouper iridovirus in mainland China
• Comparative pathogenicity of Ranavirus- and Megalocytivirus-type GIV was studied.
• Both Ranavirus- and Megalocytivirus-type GIV are virulent to infected tiger groupers.
• GIV-M showed a higher virulence than GIV-R, but the latter showed more rapid lethality.
• The differential histopathology caused by GIV-R and GIV-M was also studied.

Megalocytivirus- and Ranavirus-type grouper iridoviruses (GIV-M and GIV-R, respectively) are causative agents in cultured groupers. GIV-R is recoded in the grouper industry in Chinese Taiwan and Singapore but rarely found in other regions or countries, including mainland China. In this study, a novel GIV-R, designated as GIV-R-SY1301, was isolated and identified from diseased juvenile hybrid groupers (tiger grouper Epinephelus fuscoguttatus ♀ × giant grouper Epinephelus lanceolatus ♂) in Sanya, Hainan Island of China through PCR detection, cell culture, and transmission electron microscopy observation. Multiple alignment analysis based on the full-length major capsid protein (MCP) genes (1392 bp) showed that twenty-three and four nucleotide substitutions were observed among GIV-R-SY1301 and two early GIV-R isolates from Taiwan and Singapore, respectively. At the protein level, there are one or two amino acid substitutions in MCP among these isolates. Artificial challenge with GIV-R-SY1301 and a GIV-M isolate (OSGIV-HN11) indicated that both OSGIV-NH11 and GIV-R-SY1301 were highly virulent to infected tiger groupers. Cumulative mortalities of 97.5% (39/40) and 85% (34/40) were obtained in OSGIV-HN11- and GIV-R-SY1301-challenged fish, respectively. Histopathological analysis showed numerous abnormally enlarged cells in OSGIV-HN11-infected grouper spleen and kidney tissues but not in GIV-R-SY1301-infected grouper tissues. Abnormally enlarged cells in infected fish tissues could be a notable histopathological marker to distinguish GIV-M infection from GIV-R infection. Taken together, this study is the first to isolate and characterize GIV-R iridovirus in mainland China. GIV-R would be an alarming agent to grouper industry in mainland China, as well as GIV-M has been influencing.Statements of relevance1.First isolation and characterization of a Ranavirus-type grouper iridovirus in mainland China;2.Comparative pathogenicity of Ranavirus- and Megalocytivirus-type GIV was firstly studied; both Ranavirus- and Megalocytivirus-type GIV showed highly virulent to infected tiger groupers under artificial infection of the same viral dosages. By contrast, GIV-M showed a higher virulence than GIV-R, but the latter showed a more rapid lethality;3.The differential histopathology caused by GIV-R and GIV-M was also studied. Abnormally enlarged cells in infected fish tissues could be a notable histopathological marker to distinguish GIV-M infection from GIV-R infection. All these findings contributed important information for grouper aquaculture worldwide.