Sources of IHNV other than salmonid fish have not been identified, but potential sources include freshwater and marine invertebrates, sediment and other fish species. Depending on temperature, microflora and electrolyte concentration, IHNV survives in fresh water for from 3 days to several months (Wedemeyer et al., 1978; Toranzo and Hetrick, 1982; Yoshimizu et al, 1986; Kamei et al, 1988b). Infectious haematopoietic necrosis virus survives longer in salt water than in fresh water (Winton et al, 1991), and, although it is unlikely that fish become infected through the ocean water, because of the high dilution factor, this remains a possibility. Ingestion of marine organisms harbouring IHNV is another possible mechanism of infection, but IHNV has not been isolated from marine organisms (Traxler et al, 1997).
Salmon leeches (P. salmositica) can harbour IHNV; however, there is little relationship between leech viral titres and those of the host sockeye salmon (Mulcahy, 1986; Yamamoto et al, 1989; Mulcahy et al., 1990). Although leeches can have high IHNV titres, it is not clear whether IHNV is actually replicating in the leech or whether the leech is concentrating and storing the virus from fish blood. Infectious haematopoietic necrosis virus has also been isolated from copepods (Salmincola sp.) and the common mayfly (Callibaetis sp.). Copepods always had lower viral titres than those of fish gill tissue (Mulcahy et al, 1990). Transmission from invertebrates to salmonid fish has not been demonstrated, but the role of invertebrate reservoirs cannot be ruled out.
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