dead fish material, i.e. the infection threat posed by a salmon that died
due to ISA.
Our findings show that ISA virions may be retrieved from heart tissue
after 5 days of decomposition in freshwater at 7 °C, but not in fish
decomposed at a higher temperature (18 °C). Initially, gill tissue was
sampled prior to decomposition for examination by real-time RT-PCR.
At the end of the experiment, however, itwas not possible to obtain further
gill samples due to the high degree of decomposition in this tissue.
It was only the heart tissue that retained its structural integrity, hence
this tissue was sampled instead. Although the levels of RNA at the
start and the end of the experiment are not directly comparable, as
these were from different tissues, they show that high levels of viral
RNA similar to the levels detected in gills could be detected at the end
of the experiment. The presence of ISAV particles in heart tissue was
determined by inoculation on to ASK cells, tested with IFAT and by
injection into fish, and further verified by sequencing.
No ISA virus was found in the dead fish allowed to decompose at
18 °C. The loss of infectivity in the 18 °C group is consistent with the
results found by Falk et al. (1997) in a laboratory study and with field
observations described in Poppe et al. (1999). It is also well known
that both enzymatic and microbiological activities are greatly
influenced by temperature, and faster degradation is therefore to be
expected.
Water samples examined by real-time RT-PCR showed that most
water samples were ISA virus RNA positive throughout the experiment
and that the RNA levels in these correlatedwith the disease status of the
fish (i.e., higher viral RNA levels in fish that died of ISA vs. moribund
fish). Real-time RT-PCR analysis of water samples also showed that
the viral RNA levels in water remained relatively constant during the
decomposition process, and in fact a tendency towards an increase in
viral RNA levels could be seen from 48 h onwards. This is most likely
due to breakdown of tissues that will free viral RNA. However, all
water samples examined from these fish during the decomposition
process were negative for infectious virus when inoculated on to cell
culture. It cannot be excluded that this could be due to the loss of viable
virus during filtration. To examine this further the same concentrated filtered
water samples were injected into naïve salmon smolts (n = 150).
Such a fish challenge test can be considered as the most sensitive infectivity
test applicable as it has been reported that 10 virus particles are
sufficient to establish infection via intraperitoneal infection (Gregory
et al., 2009). The fish challenge confirmed the findings in cell culture
since none of the fish became infected following injection of water
dead fish material, i.e. the infection threat posed by a salmon that died
due to ISA.
Our findings show that ISA virions may be retrieved from heart tissue
after 5 days of decomposition in freshwater at 7 °C, but not in fish
decomposed at a higher temperature (18 °C). Initially, gill tissue was
sampled prior to decomposition for examination by real-time RT-PCR.
At the end of the experiment, however, itwas not possible to obtain further
gill samples due to the high degree of decomposition in this tissue.
It was only the heart tissue that retained its structural integrity, hence
this tissue was sampled instead. Although the levels of RNA at the
start and the end of the experiment are not directly comparable, as
these were from different tissues, they show that high levels of viral
RNA similar to the levels detected in gills could be detected at the end
of the experiment. The presence of ISAV particles in heart tissue was
determined by inoculation on to ASK cells, tested with IFAT and by
injection into fish, and further verified by sequencing.
No ISA virus was found in the dead fish allowed to decompose at
18 °C. The loss of infectivity in the 18 °C group is consistent with the
results found by Falk et al. (1997) in a laboratory study and with field
observations described in Poppe et al. (1999). It is also well known
that both enzymatic and microbiological activities are greatly
influenced by temperature, and faster degradation is therefore to be
expected.
Water samples examined by real-time RT-PCR showed that most
water samples were ISA virus RNA positive throughout the experiment
and that the RNA levels in these correlatedwith the disease status of the
fish (i.e., higher viral RNA levels in fish that died of ISA vs. moribund
fish). Real-time RT-PCR analysis of water samples also showed that
the viral RNA levels in water remained relatively constant during the
decomposition process, and in fact a tendency towards an increase in
viral RNA levels could be seen from 48 h onwards. This is most likely
due to breakdown of tissues that will free viral RNA. However, all
water samples examined from these fish during the decomposition
process were negative for infectious virus when inoculated on to cell
culture. It cannot be excluded that this could be due to the loss of viable
virus during filtration. To examine this further the same concentrated filtered
water samples were injected into naïve salmon smolts (n = 150).
Such a fish challenge test can be considered as the most sensitive infectivity
test applicable as it has been reported that 10 virus particles are
sufficient to establish infection via intraperitoneal infection (Gregory
et al., 2009). The fish challenge confirmed the findings in cell culture
since none of the fish became infected following injection of water
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