Prevalence of infectious salmon anaemia virus (ISAV) in wild salmonids in western Norway

Abstract

Studies of infectious salmon anaemia virus (ISAV), an important pathogen of farmed salmon in Norway, Scotland, the Faeroe Islands, Ireland, Canada, the USA and Chile, suggest that natural reservoirs for this virus can be found on both sides of the North Atlantic. Based on existing information about ISAV it is believed to be maintained in wild populations of trout and salmon in Europe. It has further been suggested that ISAV is transmitted between wild hosts, mainly during their freshwater spawning phase in rivers, and that wild salmonids, mainly trout, are possible carriers of benign wild-type variants of ISAV. Change in virulence is probably a result of deletions of amino acid segments from the highly polymorphic region (HPR) of benign wild-type isolates after transmission to farmed salmon. Hence, it has been suggested that the frequency of new outbreaks of ISA in farmed salmon could partly reflect natural variation in the prevalence of ISAV in wild populations of salmonids. The aims of the present study were to screen for ISAV in wild salmonids during spawning in rivers and to determine the pathogenicity of resultant isolates from wild fish. Tissues from wild salmonids were screened by RT-PCR and real-time PCR. The prevalence of ISAV in wild trout Salmo trutta varied from 62 to 100% between tested rivers in 2001. The prevalence dropped in 2002, ranging from 13 to 36% in the same rivers and to only 6% in 2003. All ISAV were nonpathogenic when injected into disease-free Atlantic salmon, but were capable of propagation, as indicated by subsequent viral recovery. However, non-pathogenic ISAV has also been found in farmed salmon, where a prevalence as high as 60% has been registered, but with no mortalities occurring. Based on the results of the present and other studies, it must be concluded that vital information about the importance of wild and man-made reservoirs for the emergence of ISA in salmon farming is still lacking. This information can only be gained by further screening of possible reservoirs, combined with the development of a molecular tool for typing virulence and the geographical origin of the virus isolates. KEY WORDS: ISA virus · Wild salmonids · Natural reservoir Resale or republication not permitted without written consent of the publisherDis Aquat Org 66: [71][72][73][74][75][76][77][78][79] 2005 prior to the advent of intensive salmon aquaculture, suggests the long-term maintenance of ISAV in a wild reservoir. ISAV and raised levels of ISAV-specific antibodies have been found in wild salmonids with no clear relationship to aquaculture activity (Raynard et al. 2001a, K. Falk pers. comm.). Furthermore, studies based on sequencing of a so-called highly polymorphic region (HPR) of the haemagglutinin gene have suggested that all pathogenic ISAV isolates may be derived by differential deletions or by recombination of a potential ancestral sequence obtained from wild fish (Devold et al. 2001, Cunningham et al. 2002, Mjaaland et al. 2002, Nylund et al. 2003. Coupled to the fact that a r...

Transmission dynamics of an emerging infectious disease in wildlife through host reproductive cycles

“…Several observations in the infectious diseases of fish support this assumption. First, in anadromous salmonids, transmission of the infectious salmon anemia virus seems to occur frequently when mature salmon migrate upstream for spawning, and contact between infected mature salmon and susceptible young fish increases (Plarre et al, 2005). Second, infectious hematopoietic necrosis virus increases in sockeye salmon (Oncorhynchus nerka) during and after spawning when the host immunity may be weakened (Mulcahy et al, 1984).…”

Epidemiologic investigation of the re-emergence of infectious salmon anemia virus in Chile

“…Wild salmonids (Plarre et al 2005) and other wild fish populations (Nylund et al 2002) can play important roles as natural reservoirs or carriers of ISAV. In addition, vector-borne transmission of ISAV may occur by sea lice (Caligus sp., Lepeophtheirus sp.)…”

Viral encephalopathy and retinopathy (VER) in Atlantic salmon Salmo salar after intraperitoneal challenge with a nodavirus from Atlantic halibut Hippoglossus hippoglossus

“…As an internal control of RNA and cDNA yield, elongation factor α was used according to the description of Moore et al (2005). Heart ventricle tissue from 3 individuals (2 challenged and 1 control) from each sampling day were screened by Q-PCR, utilising specific probes and primers for detection of ISAV (Plarre et al 2005), SAV (Hodneland et al 2005) and IPNV (Watanabe et al unpubl.). These viruses are frequently detected in commercial salmon farms in Norway.…”