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Abstract

Piscine iridoviruses infect a wide variety of fish and are classified into three genera: Lymphocystivirus, Ranavirus and Megalocytivirus. Lymphocystiviruses cause non-fatal, dermal infections, while ranaviruses and megalocytiviruses produce devastating, systemic infections with mortality reaching up to 100%. Although both ranaviruses and megalocytiviruses cause fatal systemic infections, they induce different pathology. In Taiwan, both ranaviruses and megalocytiviruses have caused serious epidemics in several mariculture fish species, including groupers. In this study, we infected the orange-spotted grouper (Epinephelus coioides) with either a megalocytivirus (TGIV, grouper iridovirus of Taiwan) or a ranavirus (GIV, grouper iridovirus), and then the two iridoviruses were investigated and compared in respect of their target organs, virulence, and effects on the expression of several immune-related genes in the spleen and head kidney. By measuring cumulative mortality rate, GIV was shown to have higher virulence than TGIV. By PCR, we found that TGIV mainly infected the spleen, head kidney, kidney, heart and gill, while GIV mainly infected the spleen and intestine. The assayed immune genes were hemoglobin subunit-β-2 (Hb), CC chemokine 19, Toll-like receptor 9 isoform A and B (TLR9-A and B) and Mx (myxovirus resistance). By real-time RT-PCR, we found that of the assayed genes, the expression of CC chemokine 19 was strongly induced in spleen by both viruses, whereas the expression of Mx were strongly induced by both viruses in both organs and the expression of Hb gene was induced only by TGIV in head kidney.

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