Abstract
The occurrence of infectious salmon anemia (ISA) outbreaks in marine farmed Atlantic salmon constitutes a recurring challenge in Norway. Here, we aim to identify risk factors associated with ISA outbreaks with an unknown source of infection (referred to as primary ISA outbreaks). Primary ISA outbreaks are here defined by an earlier published transmission model. We explored a wide range of possible risk factors with logistic regression analysis, trying to explain occurrence of primary ISA with available data from all Norwegian farm sites from 2004 to June 2017. Explanatory variables included site latitude and a range of production and disease data. The mean annual risk of having a primary outbreak of ISA in Norway was 0.7% during this study period. We identified the occurrence of infectious pancreatic necrosis (IPN), having a stocking period longer than 2 months, having the site located at high latitude and high fish density (biomass per cage volume) in the first six months after transfer to sea site as significant risk factors (p < 0.05). We have identified factors related to management routines, other disease problems, and latitude that may help to understand the hitherto unidentified drivers behind the emergence of primary ISA outbreaks. Based on our findings, we also provide management advice that may reduce the incidence of primary ISA outbreaks.
Highlights
Infectious salmon anemia (ISA) is an infectious disease that causes high mortality in farmed Atlantic salmon [1], with severe implications for production economics
A possible issue regarding the independence of data used may be that some of our presumed primary cases were infected by an unknown undetected neighboring farm infected with HPR-del ISA virus (ISAV), and did not represent a primary infectious salmon anemia (ISA) outbreak
We found that infectious pancreatic necrosis (IPN) increased the risk of experiencing a primary ISA outbreak more than two-fold (Table 2), concurrent with previously published results suggesting that fish with a history of IPN are more prone to diseases such as pancreas disease (PD) and heart and skeletal muscle inflammation (HSMI) [33]
Summary
Infectious salmon anemia (ISA) is an infectious disease that causes high mortality in farmed Atlantic salmon [1], with severe implications for production economics. Infectious salmon anemia is caused by infection with the ISA virus (ISAV), which is from the Orthomyxoviridae family. Since being first recorded in 1984 in Norway, ISA outbreaks have been reported in farmed Atlantic salmon in countries such as Scotland, the United Kingdom, Canada, USA, the Faroe Islands, and Chile [2]. Infectious salmon anemia was a serious disease problem in salmon aquaculture in Norway during the late 80s and early 90s, with at least 80 new outbreaks in 1990 at the peak of the epidemic. There was a substantial decrease in the number of outbreaks, ISA was not eradicated and remained a disease problem. In addition to the animal welfare concerns, each outbreak induces high rates of mortality, which are costly [1], as well as management restrictions that limit production in the surrounding salmon production sites [4]
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