Salmonid alphavirus (SAV) and pancreas disease (PD) in Atlantic salmon, Salmo salar L., in freshwater and seawater sites in Norway from 2006 to 2008

Abstract

A cohort study was initiated in the spring of 2006 to investigate epidemiological aspects and pathogenesis of salmonid alphavirus (SAV) subtype 3 infections and pancreas disease (PD). The aims were to assess involvement of the freshwater production phase, the extent and frequency of subclinical infections and to follow PD-affected populations throughout the entire seawater production cycle, as well as investigate possible risk factors for PD outbreaks. Fish groups from 46 different Atlantic salmon freshwater sites in six counties were sampled once prior to seawater transfer and followed onto their seawater sites. A total of 51 Atlantic salmon seawater sites were included, and fish groups were sampled three times during the seawater production phase. SAV subtype 3 was not identified by real-time RT-PCR from samples collected in the freshwater phase, nor were any SAV-neutralizing antibodies or histopathological changes consistent with PD. In the seawater phase, SAV was detected in samples from 23 of 36 (63.9%) studied sites located within the endemic region. No SAV subtype 3 was detected in samples from seawater sites located outside the endemic region. The cumulative incidence of PD during the production cycle amongst sites with SAV detected was 87% (20 of 23 sites). Average fish weight at time of PD diagnosis ranged from 461 to 5978 g, because of a wide variation in the timing of disease occurrence throughout the production cycle. Mortality levels following a PD diagnosis varied greatly between populations. The mean percentage mortality was 6.9% (+/-7.06) (range 0.7-26.9), while the mean duration of increased mortality following PD diagnosis was 2.8 months (+/-1.11) (range 1-6).

Identification of a wild reservoir of salmonid alphavirus in common dab Limanda limanda, with emphasis on virus culture and sequencing

“…These data might also be related to the kinetics of infection with SAV isolates of differing subtypes, as persistence of viral RNA in the tissues of farmed Atlantic salmon from which the virus was not isolated has been described (Christie et al 2007, Graham et al 2010, Jansen et al 2010. It has also been noted that initial isolation of SAV can be complex, and a degree of cell-culture selection or adaptation may be necessary in some cases .…”

“…Considerable existing data suggest that horizontal transmission represents an important route of spread of PD and that infection pressure has a strong effect on the probability of recording an outbreak in a single cohort (McLoughlin et al 1996, Kristoffersen et al 2009). Jansen et al (2010) reported that if infection occurs in a population, disease is highly likely to develop, assuming fish spend an adequate time in that environment. Through experimental studies, SAV has been shown to survive for 5.7 d at 10°C in sterile saltwater with organic loading ).…”

“…Through experimental studies, SAV has been shown to survive for 5.7 d at 10°C in sterile saltwater with organic loading ). Moreover, viral RNA has been shown to persist in tissues for extended periods (Andersen et al 2007, Christie et al 2007, Graham et al 2010, Jansen et al 2010), potentially posing an infection risk to healthy fish.…”

Low virulent infectious salmon anaemia virus (ISAV-HPR0) is prevalent and geographically structured in Norwegian salmon farming

“…1). The screening data comprised samples from 5 collaborating projects with varying sampling protocols: a cohort study of pancreas disease (Jansen et al 2010), a case-control study of cardiomyopathy syndrome (B. Fineid pers. comm.…”

“…We thank K. Haugbro and E. Trettenes for technical work in the laboratory, and E. Brun for useful discussions. The cohort study of pancreas disease (Jansen et al 2010), the case-control study of cardiomyopathy syndrome (B. …”