14 Bioaccumulation and Removal Dynamics of Murine Norovirus in Manila Clams. . .
173
Previous studies have detected viral particles in the digestive tract lumen, inside
gastrointestinal cells and in phagocytes (both in the epithelium and in connective
tissue) (Le Guyader et al. 2006b; Mcleod et al. 2009; Wang et al. 2008). Viruses
present in the digestive tract lumen could theoretically be removed relatively rapidly
via defecation when shellfish are placed in clean water. The specific attachment
and internalization of intact viral particles into gastrointestinal cells or captured by
phagocytes in the main ducts could be a viral mechanism to avoid being degraded
by the digestive system and may provide an explanation for the generally poor
efficiency in removing viruses from shellfish (Le Guyader et al. 2006b; Mcleod
et al. 2009). The relatively high levels of final viral loads in the depurated samples
observed in this work support this hypothesis.
In terms of risk to the consumer, interpreting the real significance of these results
is complicated. Although the final viral loads remain at relatively high levels in
all samples, molecular techniques like qRT-PCR detect RNA copies rather than
infective particles, and nucleic acids remain detectable for long periods. Assays are
currently in progress to assess the infectivity of the virus detected after mollusc
depuration.
Both the findings reported here, as in other studies mentioned above, suggest
that virus concentration in shellfish is not a passive process. It may depend on many
factors such as mucus production, glycogen content, water temperature, gonadal
development (Maalouf et al. 2010), and possibly on the presence, amount and/or
distribution of specific receptors in host tissues. In this way, not only a different virus
or virus strain could behave differently in shellfish, but also one virus could behave
differently in different bivalve species and/or in different stages of the mollusc life
cycle.
Conclusions
Depuration can reduce viral levels in shellfish, but not sufficiently to consider them
safe. The final viral load in samples remains in relatively high concentrations, and
virus, unlike bacteria, can be infectious at very low doses. Thus, it is necessary
to reduce virus to near negligible levels to improve the safety of shellfish but it
is also necessary to understand that the depuration process is a method capable
of reducing relatively low levels of contamination but not, at least for now, for
highly contaminated shellfish. This study will provide the baseline for future studies
focused on improving the efficacy of viral depuration of shellfish.
Acknowledgements This work was supported in part by Grant 10MMA200010PR from the
Conseller´ ıa de Econom´ ıa e Industria, Xunta de Galicia (Spain) and contract 2008/CP776 from
INTECMAR (Spain). The donation of viral stocks of vMC 0 and MNV-1 kindly provided by
Dr. Albert Bosch (University of Barcelona, Spain) and Dr. Herbert W. Virgin IV (University of
Washington, USA), respectively, is appreciated.
173
Previous studies have detected viral particles in the digestive tract lumen, inside
gastrointestinal cells and in phagocytes (both in the epithelium and in connective
tissue) (Le Guyader et al. 2006b; Mcleod et al. 2009; Wang et al. 2008). Viruses
present in the digestive tract lumen could theoretically be removed relatively rapidly
via defecation when shellfish are placed in clean water. The specific attachment
and internalization of intact viral particles into gastrointestinal cells or captured by
phagocytes in the main ducts could be a viral mechanism to avoid being degraded
by the digestive system and may provide an explanation for the generally poor
efficiency in removing viruses from shellfish (Le Guyader et al. 2006b; Mcleod
et al. 2009). The relatively high levels of final viral loads in the depurated samples
observed in this work support this hypothesis.
In terms of risk to the consumer, interpreting the real significance of these results
is complicated. Although the final viral loads remain at relatively high levels in
all samples, molecular techniques like qRT-PCR detect RNA copies rather than
infective particles, and nucleic acids remain detectable for long periods. Assays are
currently in progress to assess the infectivity of the virus detected after mollusc
depuration.
Both the findings reported here, as in other studies mentioned above, suggest
that virus concentration in shellfish is not a passive process. It may depend on many
factors such as mucus production, glycogen content, water temperature, gonadal
development (Maalouf et al. 2010), and possibly on the presence, amount and/or
distribution of specific receptors in host tissues. In this way, not only a different virus
or virus strain could behave differently in shellfish, but also one virus could behave
differently in different bivalve species and/or in different stages of the mollusc life
cycle.
Conclusions
Depuration can reduce viral levels in shellfish, but not sufficiently to consider them
safe. The final viral load in samples remains in relatively high concentrations, and
virus, unlike bacteria, can be infectious at very low doses. Thus, it is necessary
to reduce virus to near negligible levels to improve the safety of shellfish but it
is also necessary to understand that the depuration process is a method capable
of reducing relatively low levels of contamination but not, at least for now, for
highly contaminated shellfish. This study will provide the baseline for future studies
focused on improving the efficacy of viral depuration of shellfish.
Acknowledgements This work was supported in part by Grant 10MMA200010PR from the
Conseller´ ıa de Econom´ ıa e Industria, Xunta de Galicia (Spain) and contract 2008/CP776 from
INTECMAR (Spain). The donation of viral stocks of vMC 0 and MNV-1 kindly provided by
Dr. Albert Bosch (University of Barcelona, Spain) and Dr. Herbert W. Virgin IV (University of
Washington, USA), respectively, is appreciated.