Release and survival of infectious salmon anaemia (ISA) virus during decomposition of Atlantic salmon (Salmo salar L.)

Abstract

Infectious salmon anaemia (ISA) is a serious viral disease of farmed Atlantic salmon, Salmo salar L., caused by infectious salmon anaemia virus (Isavirus, Orthomyxoviridae). An ISA outbreak may vary when it comes to clinical manifestations and mortality levels. Some outbreaks have an aggressive development with an escalating mortality level (up to 90%). At these sites, efficient removal of dead fish can be a major challenge. Removal of dead fish is an important fish health preventive measure implemented at marine sites. In the present study we have examined the infection risk the dead fish may represent for the remaining fish at the site by examining both tissues from fish that died from ISA, and from the surrounding water (0–120h post mortem). The aim was to examine for how long infectious virus particles could be retrieved from dead salmon as this could indicate if such material may pose a risk for spreading of ISA virus. Although viral RNA was detectable in water from decomposing salmon by real-time RT-PCR, no infectious particles could be obtained from water either by inoculating cell cultures or by injecting water samples into fish. Nevertheless, the study showed that infectious virus particles could be retrieved from heart tissue of dead fish for more than 4–5days post mortem.Thus, daily removal of dead fish is an important preventative measure during ISA outbreaks. It is also of importance to remove moribund fish with ISA as these continue to produce and shed virus and may represent a greater risk with respect to the transmission of virus than the fish that are already dead.