Co-infection of infectious spleen and kidney necrosis virus and Francisella sp. in farmed pearl gentian grouper (♀Epinephelus fuscoguttatus ×♂E. lanceolatus) in China - A case report

From October–December 2018, a mass mortality among cultured pearl gentian grouper (♀Epinephelus fuscoguttatus × ♂E. lanceolatus) occurred on a private grouper farm in Zhanjiang City, Guangdong Province, China. Approximately 120 dead fish from each pond were recorded daily, and the cumulative mortality was up to 52% within 56 days. Diseased fish showed clinical signs such as lethargy, anorexia, slow swimming or staying near the pond edge, and dark body surface and petechiae of the gills. The notable internal gross pathological changes of diseased fish were splenomegaly, renomegaly, hepatic congestion, and the absence of apparent white nodules. Histopathological examinations revealed that hypertrophic cells were dispersed in various tissues and many spherical bacteria-like organisms were found in macrophages. PCR detection and sequence analysis confirmed the simultaneous presence of infectious spleen and kidney necrosis virus (designated as ISKNV-ZJ) and Francisella sp. (designated as zj09) in the diseased fish. Phylogenetic analysis revealed that virus strain ISKNV-ZJ was closely related with ISKNV and Murray cod iridovirus (MCIV), while bacterial strain zj09 was closely related with strains F. halioticida. This is the first report of natural co-infection of ISKNV and Francisella sp. in pearl gentian grouper. •A bacterial strain zj09 was identified as Francisella from diseased grouper.•ISKNV genotype of Megalocytivirus was detected from all diseased grouper.•Co-infection of ISKNV and Francisella sp. was confirmed among the diseased grouper.•Hypertrophic cells were dispersed in various tissues.•Many spherical bacteria-like organisms were found in macrophages.