Infectious salmon anaemia virus in wild fish from Scotland

Following the outbreak of infectious salmon anaemia (ISA) at salmon farms i n Scotland, UK, a survey was established to determine the extent of infecti on in wild fish. All fish tested were free from the clinical symptoms of IS A, Isolations of ISAV were made from 5 sea trout within areas where ISA aff ected salmon farms were located, Evidence for ISAV in other sea trout was p rovided by ISA RT-PCR diagnostic tests. Results from ISA RT-PCR tests revea l evidence for ISAV being present in salmon parr, adult salmon and juvenile brown trout in rivers distant from salmon farms and indicate that, at the time of the survey (1998-1999), ISAV may have been widely distributed. Nucl eotide sequence analysis of segments 2 and 8 showed that for most sequences from wild fish there was 100 % homology with ISAV isolated from clinically affected farmed fish although evidence is presented which indicates variab ility in ISAV sequences from wild fish. Modelling the RT-PCR findings indic ates that ISAV among salmonid fish was spatially non-random. Brown trout, s ea trout and salmon (adult and parr) show a pattern of occasionally large n umbers of positive samples against a background of very low numbers.