Outbreaks and risks of infectious spleen and kidney necrosis virus disease in freshwater ornamental fishes

We examined the distribution of iridoviruses in 10 freshwater ornamental fish species hatched in Korea and imported from other Asian countries using both 1-step and 2-step polymerase chain reation (PCR). None of the 10 fish species analyzed were free of iridovirus as shown by 2-step PCR positive results, and 3 species yielded 1-step PCR positive results with associated mortality. Cloned PCR amplicons of the adenosine triphosphatase (ATPase) and major capsid protein (MCP) genes in genomic DNA of iridovirus showed the same nucleotide sequences as that of infectious spleen and kidney necrosis virus (ISKNV) isolated from the mandarinfish Siniperca chuatsi. These results indicate the presence of ISKNV disease in various ornamental fish as new host species and that the disease is widespread throughout different Asian countries including Korea, Singapore and China. Such infections were either clinical with associated mortality (and 1-step PCR positive) or asymptomatic in fish that were externally healthy (and only positive in 2-step PCR). Molecular analyses of the K2 region performed on iridovirus samples isolated from freshwater ornamental fishes revealed deletion/insertion of repetitive sequences of various lengths (42 to 339 bp), depending on the ISKNV isolates, without substitutions. Experimental infection of pearl gourami Trichogaster leeri and silver gourami T microlepis with a tissue homogenate of pearl gourami infected by ISKNV induced 70 and 20% cumulative mortalities in the pearl and silver gourami, respectively.