Vertical transmission infectious haematopoetic necrosis virus in the reproductive products

Vertical virus transmission is the transmission of virus from one generation to its offspring, regardless of whether the virus is located within or external to the egg (Fenner and White, 1976; Pilcher and Fryer, 1980a,b). An alternate definition is the transmission of a virus within the egg (Nicholson, 1982) and this narrower definition has caused confusion.

The most frequent evidence for vertical transmission is the association between the shipment of infected eggs into new geographical areas and resultant outbreaks of IHN (Plumb, 1972; Holway and Smith, 1973; Sano et al., 1977; Niu and Zhao, 1988). Further evidence of vertical transmission is that IHN has occurred in progeny from eggs disinfected with iodophor and raised in virus-free water (Wingfield and Chan, 1970; Ratliff, 1982; Mulcahy and Bauersfeld, 1983; Mulcahy and Pascho, 1985; Meyers et al., 1990; Roberts, 1993). However, vertical transmission may be an infrequent event. There are several reports where IHNV-infected parents did not produce IHNV-infected progeny when the eggs were raised in virus-free water (Amend, 1975; LaPatra, 1990; Engelking et al., 1991; LaPatra et al., 1991b; Yamazaki and Motonishi, 1992; Traxler et al., 1997). This has raised doubts as to whether vertical transmission really occurs. However, negative results do not eliminate the possibility of vertical transmission of IHNV. Mulcahy and Pascho (1985) indicated it was difficult to demonstrate vertical transmission and they were only successful three times in isolating IHNV from live and dead eggs and fry of infected adult sockeye salmon. They found that only a small portion of sockeye eggs and fry contained IHNV and that not all parents transmitted the virus to their progeny and not every egg from infected females contained the virus. These observations would explain why studies to demonstrate vertical transmission have failed when only a small number of eggs were used, and why IHN epizootics appear intermittently within hatcheries.

It is accepted that salmonid reproductive products can harbour IHNV, but it is controversial whether the virus is on the egg surface or within the egg. Observations that disinfection of eggs does not always prevent IHNV infection of progeny suggests that the virus is within the egg (Amend, 1975). However, the virus may be external. Another fish virus, IPN virus (IPNV), adheres strongly to the chorion of water-hardened eggs, but not to unfertilized eggs

(Ahne and Negele, 1985). Infectious pancreatic necrosis virus was isolated from eggshells after fry hatched and the fry may become infected by eating the eggshells. The chorion of unfertilized eggs is smooth, but hardened eggs have a rough, lobed and porous texture, which would provide anchorage for the virus and may prevent disinfectants from reaching the virus. A similar situation may be occurring with IHNV. Examination of eggshells for IHNV has not yet been done.

Fish species may be an important determinant for vertical transmission. When eggs of masu salmon and chum salmon were exposed to IHNV and then fertilized, the eggs and resulting fry were not infected (Yoshimizu et al., 1989). The stage of egg development influenced the susceptibility to IHNV replication. Exposure of unfertilized eggs to virus did not result in viral replication, but when eyed eggs were injected with IHNV, the virus replicated and the resulting fry suffered IHN-induced mortality. Egg-yolk components inhibited viral replication. An increase in IHNV susceptibility as the egg matured was correlated with a decrease in yolk components. Yoshimizu et al. (1989) concluded that direct vertical transmission of IHNV within the egg is doubtful. The anti-IHNV action of the yolk-sac components may be species-specific. Rainbow trout may have similar viral-inhibiting components in the yolk-sac, since fry developed IHN when eyed eggs were immersed in IHNV (Amend, 1975). However, sockeye salmon do not appear to have viral-inhibiting egg components, since Burke and Mulcahy (1983) found that IHNV infectivity was stable in egg homogenates and Mulcahy and Pascho (1985) isolated IHNV from eyed eggs reared in virus-free water. Inhibition of IHNV by egg components of other species does not appear to have been tested.

Infectious haematopoietic necrosis virus is frequently isolated from milt (Wingfield and Chan, 1970; Meyers et al, 1990; Yamazaki and Motonishi, 1992) and male salmonid fish may have a role in vertical transmission. Sockeye salmon and steelhead trout have a lower prevalence of infection and viral titres in milt than in the kidney and spleen (Mulcahy et al, 1987). The prevalence of infection in milt is lower than that in ovarian fluid, but the proportion of males with high milt viral titres may be equivalent to that of females with high virus concentrations in the ovarian fluid (Meyers et al, 1990). Virus strongly and quickly adsorbs to the surface membrane of steelhead trout and chinook salmon sperm (Mulcahy and Pascho, 1984). Sperm which has adsorbed IHNV from male fish or from infected ovarian fluid could deliver the virus directly into the egg during fertilization (Mulcahy and Pascho, 1984; Meyers et al, 1990). However, the role of sperm in IHNV transmission is still unknown. Contamination of masu salmon or chum salmon milt with IHNV did not result in infection of eggs or fry (Yoshimizu et al, 1989) and there is no direct evidence that sperm can transmit IHNV into eggs.

Additional factors such as hatchery practices, genetics and environmental conditions may influence vertical transmission. Large-scale hatchery operations raising over 20 million eggs annually may have an increased probability of vertical transmission. However, hatcheries rearing less than 7 million eggs may have no IHNV losses, despite females having a 100% prevalence of infection and high viral titres (Meyers et al, 1990). Infectious haematopoietic necrosis epizootics can be prevented by not mixing eggs from several females and by incubating the eggs in virus-free water. Mixing of eggs from many females may contaminate the eggs from females that had a low viral titre with a lethal dose of virus from a few females with high virus titres (Mulcahy et al., 1983b). The roles of fish genetics and environmental factors in vertical transmission are unclear.

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  • Mat
    Is ihnv transferred vertically?
    12 days ago

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