Viruses of the genus Megalocytivirus are globally emerging pathogens that are widespread in a diverse range of aquaculture and fisheries industries affecting both food and ornamental fishes. To help reduce taxonomic confusion, a binomial virus name of Megalocytivirus pagrus 1 was assigned to represent the three recognized genotypes of the former species Infectious spleen and kidney necrosis virus (ICTV, 2024). This includes infectious spleen and kidney necrosis virus (ISKNV; Genotype II), red sea bream iridovirus (RSIV; Genotype I) and turbot reddish body iridovirus (TRBIV; Genotype III). RSIV and ISKNV have been listed by the World Organization for Animal Health (WOAH) since their emergence. Recently revision from the WOAH Aquatic Standards Committee has specified the three genotypes of M. pagrus 1 as being notifiable. Of particular importance to Australia, barramundi (Asian seabass; Lates calcarifer) is a natural host for ISKNV (in marine and freshwater environments), RSIV and TRBIV with significant mortality events observed across South Asia. As an iconic Australian species, barramundi is important for aquaculture, recreational and cultural fishing practices. Further, several other endemic Australian freshwater finfish species, including some endangered species are also at risk from ISKNV.
Australia is considered free of megalocytiviruses and has strict import controls and biosecurity measures to support this status. In 2003, there was one mortality event associated with ISKNV at an aquaculture enterprise farming Murray cod (Maccullochella peelii) (Lancaster et al. 2003). Since, then there have been numerous detections of ISKNV associated with the trade of ornamental fish in apparently healthy and diseased fish (e.g. Rimmer et al., 2015, Becker et al., 2017). Incursions of M. pagrus 1 have been contained and eradicated through state-managed biosecurity responses and to date, there have been no detections in wild fishes in Australia. ISKNV DNA has been detected in frozen seafood products imported to Australia (Landos et al., 2021), but its viability in these products remains untested.
Despite being studied for nearly three decades, we actually have a poor understanding of the transmission risk posed by fish infected with ISKNV but are not showing any signs of disease (i.e. subclinical infection). The objective of this study was to assess the risk pathways for ISKNV incursion into Australia via live ornamental fish trade and frozen seafood products. By evaluating the minimum infective dose, we aimed to inform risk assessments and to quantify the potential for transmission from ISKNV-infected fish. Further, we evaluated the risks from frozen fish products contaminated with ISKNV. These findings will provide essential data for biosecurity assessments, helping to inform evidence-based policies to mitigate the transboundary spread of ISKNV and protect economically and ecologically important fish populations.
References
Becker, J. et al. (2017) FRDC Project 2014/001: Strategic approaches to identifying pathogens of quarantine concern associated with the importation of ornamental fish. Final Report. ICTV (2024) Virus Taxonomy: Megalocytivirus. Accessed online here. Lancaster, M.J. et al. (2003) Australian Veterinary Journal, 81(10), 633-634. Landos, M.A. et al. (2021) FRDC Project No 2019-126: Assessing the biosecurity risk of imported finfish in relation to exotic viruses. Final Report. Rimmer, A.E. et al. (2015) Preventive Veterinary Medicine, 122(1-2), 181-194.