Human norovirus disease burden of consuming Crassostrea gigas oysters: A case-study from Japan

Shellfish borne viral gastroenteritis outbreaks are most commonly attributed to human norovirus. Minimizing the norovirus bioaccumulation in oysters and improved norovirus removal/inactivation techniques are paramount in reducing the shellfish-borne norovirus outbreaks. Genotype-dependent bioaccumulation of human norovirus in oysters can affect the norovirus removal/inactivation efficiency and ultimately the quantity of human norovirus ingested by the consumer. To evaluate this phenomenon, Crassostrea gigas oysters collected from a shellfisheries area in Eastern Japan were artificially contaminated for 72 h in a sea water bath with only human norovirus GII.2 strain, GII.17 strain and a bath containing a mixture of GII.2 + GII.17. After 72-h contamination, only GII.17 was bioaccumulated in the oysters. Then depuration of oysters was conducted for 48 h, as customary in commercial shellfisheries areas in Japan. However, depuration didn't contribute to significantly reduce the norovirus bioaccumulated in the oyster digestive tract. Based on the norovirus concentration in the depurated oyster digestive tract, human norovirus disease burden by consuming raw oysters was calculated and it was above the 10⁻⁴ DALYpppy, which is the acceptable disease burden. Since all the norovirus particles are not infectious, infectivity ratio; the ratio between infectious particles and total virus particles was introduced and the disease burden was recalculated. Even after incorporating infectivity ratio, disease burden value was higher than 10⁻⁴ DALYpppy. These results emphasize that depuration doesn't contribute to significantly reduce the human norovirus accumulated in the oyster digestive tract and therefore the disease burden. Minimizing the exposure of shellfisheries areas to human norovirus can be a suitable strategy to reduce the human norovirus disease burden of consuming raw oysters.