Infectious spleen and kidney necrosis virus (ISKNV) is a globally important pathogen that infects more than 165 species of freshwater and marine fish. Infections may be subclinical or cause severe disease epidemics in wild fish, aquaculture, and ornamental fish typically with 20-60% and up to 100% mortality. The species ISKNV includes three closely related but distinct genotypes: red seabream iridovirus (RSIV), ISKNV-genotype, and turbot reddish body iridovirus (TRBIV). The recognised host and geographical range of each genotype has recently expanded reflecting an emerging disease situation.
The World Organization for Animal Health (WOAH) recognized the importance of ISKNV by listing red sea bream iridoviral disease (RSIVD) as notifiable under a definition including infection with RSIV and ISKNV-like viruses (WOAH, 2019). Presently, infection with TRBIV is not included in this definition, although this is being reviewed by the Aquatic Animal Standards Commission. TRBIV infection was originally considered to be restricted to flatfishes until recent reports of disease in food and ornamental fish, such as barramundi (Lates calcarifer) and rock bream (Opleghathus fasiatus). Currently, ISKNV, RSIV and TRBIV are reported to be endemic in SE Asia and cause recurrent moderate to severe epidemics in aquaculture with sporadic emergence of disease in additional locations.
This paper will review current literature and make recommendations to address barriers to minimizing the spread and impacts of ISKNV. Firstly, a uniform and simplified nomenclature that reflects functional differences in viruses is recommended. ISKNV disease control and revision of WOAH standards are currently hampered by confusing nomenclature. A plethora of virus names are in use due to the wide host range and terminology derived from the host fish and the virus family (Iridoviridae), without indicating the important genotype designation. At genotype level, confusion derives from the use of ISKNV as the name of the species and one of the genotypes. Further confusion arises when referring to the genotypes collectively by the genus name (Megalocytivirus) as this includes other important virus species. Improved and efficient methods for characterisation and classification of viruses are required because ISKNV is challenging to grow in cell culture. To support vaccination programs, improved genetic characterisation is required because of the close relationship of different genotypes and their association with similar pathology in an overlapping host range without showing evidence of cross protection. Finally, there is the need for further validation of fit-for-purpose diagnostic tests to facilitate international trade. These require defined purposes reflecting appropriate virus nomenclature and the different needs of disease diagnosis and high-throughput surveillance including detection of subclinical infection. Together, these recommendations will support regional surveillance and reporting to promote improved biosecurity related to ISKNV.