The screening procedure for IHNV is based on virus isolation in cell culture. Clinical disease occurs between 8вЊ and 15вЊ under natural conditions. The most prominent environmental factor affecting IHN is water temperature. bacterial coldwater disease), handling and other types of stress frequently cause subclinical infections to become overt. As with viral haemorrhagic septicaemia virus, good overall fish health condition seems to decrease the susceptibility to overt IHN, while co-infections with bacterial diseases (e.g. The age of the fish appears to be extremely important: the younger the fish, the more susceptible to disease.

Once IHNV is established in a farmed stock or in a watershed, the disease may become established among carrier fish.ДЄmong individuals of each fish species, there is a high degree of variation in susceptibility to IHNV. Egg-associated transmission is significantly reduced by the now common practice of surface disinfection of eggs with an iodophor solution, but is the only mechanism accounting for the appearance of IHN in new geographical locations among alevins originating from eggs that were incubated and hatched in virus-free water. Horizontal transmission is typically by direct exposure, but invertebrate vectors have been proposed to play a role in some cases. The transmission of IHNV between fish is primarily horizontal however, cases of vertical or вЂegg-associatedвЂ™ transmission have been recorded. Virus is shed via faeces, urine, sexual fluids and external mucus, whereas kidney, spleen, encephalon and the digestive tract are the sites in which virus is most abundant during the course of overt infection. The reservoirs of IHNV are clinically infected fish and covert carriers among cultured, feral or wild fish. Variations in the virulence of IHNV strains have been recorded during both natural cases of disease and in experimental infections ( LaPatra et al., 1993a). However, mouse monoclonal antibodies have revealed a number of neutralising epitopes on the glycoprotein ( Huang et al., 1994 Ristow and Arnzen De Avila, 1991 Winton et al., 1988 Xu et al.,1991), as well as the existence of a non-neutralising, group epitope borne by the nucleoprotein ( Ristow and Arnzen, 1989). On the basis of antigenic studies conducted with neutralising polyclonal rabbit antisera, IHNV isolates form a single serogroup ( Engelking et al., 1991a). Survivors of IHNV infection demonstrate a strong protective immunity with the synthesis of circulating antibodies to the virus ( LaPatra et al., 1993a) and, in certain individuals, a covert carrier state ( Drolet et al., 1995 Kim et al, 1999). Older fish are increasingly resistant to infection, but adult fish at spawning may shed virus in sexual products. High levels of virus are shed from infected juvenile fish. The causative agent, the rhabdovirus IHNV, multiplies in the endothelial cells of blood capillaries, haematopoietic tissues and nephron cells. Infection is often lethal due to osmotic imbalance leading to oedema and haemorrhage. Large mortalities have also been recorded among some wild stocks of Pacific salmon. The principal clinical and economic consequences of IHN occur at farms rearing rainbow trout in freshwater however, both Pacific and Atlantic salmon reared in freshwater or seawater can be severely affected. For more detailed reviews of the disease, see Bootland & Leong (1999), Pilcher and Fryer (1980), and Wolf (1988). The original geographical range of IHN was limited to the western parts of North America, but the disease has spread to continental Europe and Asia via the importation of infected fish and eggs. kisutch), and Atlantic salmon ( Salmo salar). Infectious haematopoietic necrosis (IHN) is an infectious disease of Pacific salmonid fish including rainbow or steelhead trout ( Oncorhynchus mykiss), Chinook ( O.

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