Infectious haematopoietic necrosis virus primarily causes disease in the genus Oncorhynchus and was first identified in the Pacific North-West of the USA. During the 1950s, IHNV caused severe losses of sockeye salmon (O. nerka) at hatcheries in Washington (Rucker et al., 1953) and Oregon (Wingfield et al., 1969). The virus might have been introduced into Oregon from Washington in unpasteurized sockeye salmon viscera that were fed to young fish, a practice that was soon stopped (Amend and Wood, 1972). A similar disease was next reported in hatchery-reared chinook salmon (Oncorhynchus tshawytscha) in California (Ross et al., 1960; Wingfield and Chan, 1970). In 1967, IHN occurred in young sockeye salmon in British Columbia and was reported for the first time in rainbow trout (Amend et al., 1969).
With the realization that IHNV could devastate hatchery populations of young sockeye salmon, chinook salmon and rainbow trout, several surveys were carried out to determine the distribution and prevalence of IHNV in the Pacific North-West. Prior to the 1970s, IHNV was not a problem in Washington stocks of resident chinook or coho salmon (Oncorhynchus kisutch), but two stocks of sockeye salmon were infected (Amend and Wood, 1972). During the 1970s, IHN epizootics increased in Oregon stocks of rainbow trout, steelhead trout (anadromous O. mykiss), chinook salmon and kokanee salmon (land-locked O. nerka) (Groberg et al., 1980; Mulcahy et al., 1980). The source of the virus was not clear but it was suggested that the import of infected eggs from Washington and wild adult kokanee salmon or rainbow trout harbouring the virus might have introduced the virus into the area. In 1977, IHNV began causing severe losses of rainbow trout in Idaho (Busch, 1983). During the early 1980s, IHNV was isolated from additional salmonid species and fish losses due to IHN increased dramatically. By 1982, IHNV had been isolated in cutthroat trout (Oncorhynchus clarki) and there were very high IHN losses in rainbow and steelhead trout in the Columbia River basin (Groberg, 1983; Groberg and Fryer, 1983). The first reported natural IHN epizootic in Atlantic salmon fry occurred in 1984 in Washington (Mulcahy and Wood, 1986) and, in 1986, IHNV was isolated for the first time from adult chum salmon (Oncorhynchus keta) with no clinical disease (Hopper, 1987). In Idaho, IHNV became enzootic in rainbow trout (Busch, 1983) and was isolated from adult steelhead trout and kokanee salmon (Groberg, 1983). Currently, IHNV can be isolated from salmonid fish in Oregon, Washington, Idaho, California, Alaska and British Columbia and is considered endemic in the Pacific North-West (Pilcher and Fryer, 1980a,b; Wolf, 1988).
In Alaska, IHNV began causing epizootics in sockeye salmon in 1973 and is now enzootic (Grischkowsky and Amend, 1976). Infectious haematopoietic necrosis was a major cause of failure in the culture of sockeye salmon until 1981, when a statewide policy devised by the Fisheries Rehabilitation, Enhancement and Development Division (FRED) proved successful in minimizing losses (Meyers et al., 1990). However, outbreaks of IHN are still occurring in enhanced populations of sockeye salmon smolts in at least one lake system (Follett and
Burton, 1995). The virus has also been isolated in Alaska from diseased chinook salmon and chum salmon (Follett et al., 1987).
Several other states have had sporadic IHN outbreaks in rainbow trout, and this was usually associated with the import of infected eggs or fry. The first epizootics outside the Pacific North-West occurred in 1969 in Minnesota (Plumb, 1972) and South Dakota (Wolf et al., 1973). Disease outbreaks have also occurred in West Virginia (Wolf et al., 1973), Montana (Holway and Smith, 1973), New York (Carlisle et al., 1979), Colorado (Janeke, 1984) and Utah (Amos, 1985).
Infectious haematopoietic necrosis virus has spread by the movement of infected eggs and/or fry outside North America (Hill, 1992). In Japan, IHNV was apparently introduced in 1968 with eggs imported from Alaska (Sano et al., 1977) and yearly IHN outbreaks occur in nearly all areas that rear salmonid fish (Fukuda et al., 1992). Rainbow trout, yamame (Oncorhynchus masou), and amago trout (O. masou macrostomus) are most severely affected but sporadic outbreaks have been reported in sockeye and kokanee salmon, masu salmon (O. masou masou), chum salmon, cherry salmon (Oncorhynchus rhodurus), Atlantic salmon, brown trout (Salmo trutta), brook trout (Salvelinus fontinalis) and Japanese char (Salvelinus leucomaenis) (Kimura and Awakura, 1977; Sano et al., 1977; Yamazaki and Motonishi, 1992). In 1985, IHNV was spread to northeast China by importation of infected eggs from Japan (Niu and Zhao, 1988; Zhao and Niu, 1994). Importation of infected rainbow trout eggs into Italy resulted in the first diagnosis of IHNV in 1987 (Bovo et al., 1987) and IHNV has spread to at least six regions within the country (Bovo et al., 1991). Other countries in which IHNV has been identified include Taiwan (Chen et al, 1985; Wang et al., 1996), France (Laurencin, 1987; Hattenberger-Baudouy et al., 1989, 1995a,b), Belgium (Hill, 1992) and Korea (John et al., 1992; Park et al., 1993).
It is likely that the geographical range of IHNV will continue to increase. The migration of infected anadromous fish species may spread IHNV (Hill 1992) and the movement of infected eggs and fish within and between countries will probably continue to occur, despite attempts in several countries to avoid this type of transfer by strict fish health certification requirements.
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