The epizootiology of IHNV is not completely understood and the source of virus infecting salmonids is still unknown. However, survival of IHNV in a fish population depends on a close association of the virus with the life cycle of the fish host (Fig. 2.5). Similarly to other salmonid viruses, IHNV typically causes an acute disease in young salmon and trout. Mortalities of fry and fingerlings can be as high as 90%, but occasional epizootics have been reported in smolts and older fish (Yasutake, 1978; Busch, 1983; Burke and Grischkowsky, 1984; Roberts, 1986; Traxler, 1986). The source of virus infecting young salmonids may be from vertical transmission or by direct water-borne exposure to IHNV from an unknown reservoir or host. During an epizootic in young fish, the virus is transmitted horizontally from fish to fish and infectious virus is readily isolated up to approximately 50 days after viral exposure (Amend, 1975; Busch, 1984; Bootland et al., 1995; Drolet et al., 1995). Thereafter, infectious virus is usually not isolated again until the fish near or reach sexual maturity (Amend, 1975; Busch, 1984; Meyers et al, 1990; Hattenberger-Baudouy et al, 1995b). Two mechanisms may explain why infectious virus is isolated at only two stages of the salmonid life cycle. First, IHNV may be entering a latent state in fish surviving an IHN epizootic, with virus reactivation occurring only as fish mature sexually (Amend, 1975). Alternatively, the fish may completely clear the virus but become reinfected prior to or during their spawning migration (Elston et al., 1989; Traxler et al, 1997). Although non-piscine reservoirs have not been identified, there is evidence to support the suggestion that both mechanisms exist.
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