A novel method for estimating the time-dependent decay of infectivity of infectious salmon anemia virus (ISAV) in raw seawater

Abstract

Understanding the key factors that influence virus transmission between fish is essential for effective risk assessment and disease management in aquaculture. In particular, accurate estimates of how long a virus can remain infectious in specific aquatic environments are important for predicting the spread of viruses between fish farms. Infectious salmon anemia virus (ISAV), the causative agent of infectious salmon anemia (ISA) has severe economic consequences for Norwegian salmon farmers if an outbreak occurs. Previous lab-based studies have reported that ISAV can remain infectious in water for periods ranging from hours to months, depending on the experimental approach, water conditions, and detection methods used. Here, we have developed a novel laboratory-based method to more accurately measure the decay of ISAV infectivity in raw seawater (RSW), minimizing viral manipulation steps that could potentially reduce infectivity. Identical aliquots of ISAV were incubated in RSW at 10 degrees C for different time-periods. These pre-incubated ISAV samples were then used to bathchallenge salmon fry and infectivity was subsequently measured by determining the prevalence of ISAV-positive fish at 132 degree-days post-challenge (ddpc) using qPCR. Using this strategy, we calculated the decay rate constant of ISAV infectivity in RSW at 10 degrees C to be 0.451 day-1, which corresponds to an infectivity half-life of 36.9 h. This method offers a novel approach to more accurately estimate virus decay rates in aquatic environments, providing crucial data to improve ISAV risk assessment and disease control strategies.