Present control of IHNV relies upon hatchery practices. Eggs and fry should be from stocks with no previous history of the virus and reared in specific pathogen-free (SPF) water. Eggs from fish stocks with a history of IHNV outbreaks are treated with iodine-containing surfactant to inactivate virus (Amend and Pietsch, 1972). Iodophor (100 mg l-1) egg disinfection destroys at least 99.98% of IHNV on the surface of green and eyed eggs (Goldes and Mead, 1995). In most cases this treatment has been successful in preventing the transmission of IHNV; however, occasional outbreaks of disease still occur in the progeny after treatment with iodophor and rearing in SPF water (Myers et al., 1990).
Controlling the spread of IHNV by disinfection of water-supplies has had limited success. Ozonation, ultraviolet light, chlorination-dechlorination and the addition of elemental iodine or other germicides, such as ethanol, phenol, cresol soap solution or methanol, have all been effective under laboratory conditions (Winton, 1991; Inouye et al., 1992). In field trials, problems have arisen because of mechanical and electrical failures. Nevertheless, the approach has sufficient promise, and hatcheries with limited or no SPF waters are beginning to incorporate water disinfection equipment into their facilities (J.R. Winton, personal communication).
Twenty-four antiviral compounds were tested in vitro in CHSE-214 cells and 11 were toxic for the virus (Hasobe and Saneyoshi, 1985). Of these, five were tested in steelhead trout fry and there were more survivors in the treatment groups (14-34%) than in the control, untreated group (8%). The five compounds (6-thioinosine, 5-hydroxyuridine, 9-(5)-(2,3-dihydroxypropyl) adenine, virazole and chloroquine) were added to the water daily or on alternate days after infection. Hudson et al. (1988) examined the antiviral compounds amantadine, mitisazone, bis-benzimidazole and ribavirin. Amantadine was very effective against IHNV in rainbow trout cells in culture. The other compounds were also effective but had some associated cytotoxicity. It is likely that the costs of these antiviral compounds make their use prohibitive for aquaculture. An antiviral compound produced by Pseudomonas fluorescens has also been found to be effective against IHNV and O. masou virus (OMV), a fish herpes virus (Kamei et al., 1988a; Kimura et al., 1990). The compound, a cyclic peptide of 1126 Da, has been shown to be very effective in vitro, producing a 94% plaque reduction at concentrations of 12.5 mg ml-1. The compound did not inhibit the fish viral pathogen IPNV, a non-enveloped birnavirus, suggesting to the investigators that the inhibitor is specific for enveloped viruses.
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