Fish genotype significantly influences susceptibility of juvenile rainbow trout, Oncorhynchus mykiss (Walbaum), to waterborne infection with infectious salmon anaemia virus.

The aetiological agent of infectious salmon anaemia (ISA) is an orthomyxo-like virus with a negativestranded RNA genome consisting of eight segments (Dannevig, Falk & Namork 1995; Falk, Namork, Rimstad, Mjaaland & Dannevig 1997; Mjaaland, Rimstad, Falk & Dannevig 1997). The ISA virus (ISAV) constitutes the only member of the Isavirus genus (Kawaoka, Cox, Haller, Hongo, Kaverin, Klenk, Lamb, McCauley, Palese, Rimstad & Webster 2005). ISA was first observed in Norway in Atlantic salmon, Salmo salar L. (Thorud & Djupvik 1988). The disease has caused considerable economic loss to the Norwegian salmon farming industry and has also been reported from Scotland (Rowley, Campbell, Curran, Turnbull & Bryson 1999) and from the eastern coast of North America (Bouchard, Keleher, Opitz, Blake, Edwards & Nicholson 1999; Lovely, Dannevig, Falk, Hutchin, MacKinnon, Melville, Rimstad & Griffiths 1999). Clinical signs of ISA include anaemia and leucopenia, exophthalmos, haemorrhages, ascites, petechiae in viscera, and congestion of the liver, spleen and gut (Evensen, Thorud & Olsen 1991). The cumulative mortality in infected salmon farms may reach 100% over several months. Natural outbreaks of ISA have been observed only in Atlantic salmon, but the virus was detected from apparently normal individuals in other species including coho salmon, Oncorhynchus kisutch (Walbaum), (Kibenge, Garate, Johnson, Arriagada, Kibenge & Wadowska 2001) and brown trout, Salmo trutta L. (Raynard, Murray & Gregory 2001). ISAV was also isolated from coho salmon showing jaundice in a Chilean fish farm (Kibenge et al. 2001), and ISAV was subsequently considered as one of the aetiological agents (Smith, Larenas, Contreras, Cassigoli, Venegas, Rojas, Guajardo, Perez & Diaz 2006). No mortality was recorded in experimentally infected adult Arctic char, Salvelinus alpinus (L.), rainbow trout, Oncorhynchus mykiss (Walbaum), brown trout, chum, Oncorhynchus keta (Walbaum), chinook, Oncorhynchus tshawytscha (Walbaum), and coho salmon, and steelhead trout, O. mykiss (Snow, Raynard & Bruno 2001; Rolland & Winton 2003). When injected with ascitic fluid from infected salmon, sea trout, S. trutta, did not develop clinical signs of ISA apart from an abnormal haematocrit (Nylund & Jakobsen 1995). However, ISAV could replicate in this species, and blood samples were successfully used to infect Atlantic salmon. Furthermore, it was recently reported that the injection of ISAV NBIS A01 or 810/9/99 induced low mortalities in juvenile rainbow trout (Kibenge, Kibenge, Groman & McGeachy 2006). These observations suggested that ISAV is able to replicate Journal of Fish Diseases 2007, 30, 631–636