1 SURVEY OF WILD SALMONID FISH IN GREAT BRITAIN IN YEAR 2000 FOR INFECTIOUS SALMON ANAEMIA (ISA) R S Raynard, P F Dixon + , R Gardiner + , W R Gardiner, R Grant, A G Murray, C Longshaw + , A Gregory, S Quickfall + , A I M Macdonald, A Sheppard + , C O Cunningham, D M Stone + , N Bain, G Taylor + , B J Hill + and R M Stagg FRS Marine Laboratory, PO Box 101, Victoria Road, Aberdeen AB119DB + CEFAS Weymouth Laboratory, The Nothe, Barrack Road, Weymouth, Dorset, DT4 8UB SUMMARY A survey of wild populations of Atlantic salmon and brown trout for the presence of ISA virus was conducted in 2000. Rivers were sampled that were known to have wild salmon populations and included widespread geographic areas of Scotland, northern England, Wales and south west England. No isolations of ISA virus were made and no clinical signs of ISA disease were observed. Six fresh water samples and one sea water sample of salmonids were positive for ISAV by RT-PCR all of these positive results were obtained in Scotland. Similarities in nucleotide sequences of segment 8 of amplified RNA from wild samples and the ISA virus responsible for ISA disease in Scottish farmed salmon provides strong evidence of an ISAV infection of wild salmonids. The distribution of infected fish is consistent with a non-random distribution with 0.6% of fish in Scottish rivers being positive. No RT-PCR positive results were obtained from samples in England and Wales. INTRODUCTION Infectious salmon anaemia (ISA) is a viral disease of marine Atlantic salmon, Salmo salar, and is characterised by severe anaemia, leucopenia, ascites and haemorrhagic liver necrosis (Thorud and Djupvik, 1988; Evensen et al., 1991). ISA has, to date, been confined to salmon in marine farms or hatcheries using sea water without disinfection. In Norway, the aetiological agent was identified as an orthomyxovirus-like enveloped virus (Hovland et al., 1994; Nylund et al., 1996) with an RNA genome consisting of 8 segments (Mjaaland et al., 1997). Krossøy et al. (1999) proposed that ISAV be classified in the family orthomyxoviridae in a new genus, aquaorthomyxoviridae. This virus has been identified as the causative agent of ISA in Canada (Mullins et al., 1998; Bouchard et al., 1999; Lovely et al., 1999), and Scotland (Bricknell et al., 1998; Rodger et al., 1998; Turnbull et al., 1999; Stagg et al., 1999). Several of the segments of the ISAV genome have been characterised to date and significant differences in the nucleotide and amino acid sequences from North America, Norway and Scotland have been found (Blake et al., 1999; Cunningham and Snow, 2000; Inglis et al., 2000). The Scottish isolates are more closely related to Norwegian than the Canadian New Brunswick strain (Cunningham and Snow, 2000; Inglis et al., 2000). Identification of potential vectors and reservoirs of ISAV is important in understanding the epidemiology of ISA. In this respect, experimental studies by Nylund and Jakobsen (1995) and Nylund et al. (1995) show that Salmo trutta (brown trout and sea trout) have the potential to act as carriers of ISAV while not developing clinical disease. ISA virus was transmitted from