The frequent occurrence of a low-virulent variant of the ISA virus (HPR0) in the hatcheries and statistic analyses of these finds support the hypothesis that spontaneous outbreaks of ISA can be caused by a mutation of the virus to a more pathogenic variant.
Infectious Salmon Anaemia (ISA) is a serious viral disease occurring in Atlantic salmon (Salmo salar L) and which results in substantial losses for the fish farming industry. The ISA virus has caused outbreaks of the disease in salmon-producing countries all over the world. Knowledge about how this virus is transmitted is essential if the industry and authorities are to be able to implement measures that can prevent and limit the spread of infection and outbreaks of the disease.
From 2003 to 2010, Trude Lyngstad has gathered and analysed information from a total of over three hundred Norwegian salmon locations. The locations studied were either already infected with ISA, or were neighbouring farms to infected plants, or aquafarms containing apparently healthy fish.
Samples of fish from these various locations were studied to find out whether they contained the ISA virus. All viruses discovered were genotyped in order to detect any relationship between them. These genetic data were then correlated with key information on the prevention of infection transmission between fish farms, such as the sea distance between salmon locations and infection pathways related to the running of the plants and fish sources.
Lyngstad used various different statistical methods for testing alternative transmission pathways for the ISA virus. For example, to what extent does the ISA virus transmit via horizontal pathways such as sea distance or production operations, or does it transmit via vertical pathways relating to the source of the fish?
The main conclusion of her study is that the ISA virus transmits infection through proximity to other salmon locations. This can explain about half of the outbreaks of the disease during the period 2003-2010.
However, local, horizontal dissemination of the ISA virus proved to be only part of the cause since many of the outbreaks during this period could not be explained by infection from neighbouring farms. A low-virulent variant of the virus (HPR0) was frequently detected in the farmed salmon studied. Earlier studies have suggested that this variant can mutate to a pathogenic variant that results in ISA, and this hypothesis was underpinned by Lyngstad's statistical analyses.
Taken as a whole, the results of this research represent an important contribution towards understanding how the ISA virus spreads between fish locations and how the low-virulent variant of the ISA virus plays a possible role in this dissemination.
The research project was financed by the Norwegian Research Council, the Norwegian Seafood Research Fund and the Norwegian Veterinary Institute and was carried out at the Epidemiology Department of the Veterinary Institute from 2007-2012.
Trude Marie Lyngstad defended her doctoral research on 19th June 2012 at the Norwegian School of Veterinary Science with a thesis entitled: Tracing transmission pathways for infectious salmon anaemia (ISA) virus.