The fate of calicivirus in oysters in a 10-day depuration was assessed. The norovirus gene was persistently detected from artificially contaminated oysters during the depuration, whereas feline calicivirus in oysters was promptly eliminated. The prolonged observation of norovirus in oysters implies the existence of a selective retention mechanism for norovirus within oysters.Noroviruses have been one of the major causative agents of acute gastroenteritis and are often related to food poisoning from eating oysters (12,15). Cases of food poisoning caused by norovirus-contaminated raw oysters have been increasing, and marine industries related to oysters have been significantly damaged by the contamination of cultivated oysters with noroviruses. Many researchers are devoted to developing procedures for efficient removal or inactivation of pathogenic viruses in contaminated oysters (5,14), and viral surrogates have been employed to evaluate the removal or inactivation efficiency of noroviruses in oysters because there are no suitable cell culture systems to cultivate noroviruses (25). Feline calicivirus (FCV) is one of the most frequently used surrogates (1,6,11,26). Norovirus and FCV belong to the same family, Caliciviridae, which are small, nonenveloped, icosahedral viruses with a linear, positive-sense, single-stranded RNA genome (7). Since norovirus and FCV have almost the same configuration, these two viruses are expected to have similar fates in oysters. However, the similarity or discrepancy of behaviors between norovirus and FCV in oysters has not been significantly evaluated thus far.In this study, the persistency of norovirus genogroup II (GII) in oysters was compared with that of the FCV f4 strain. Oysters (Crassostrea gigas) were artificially contaminated with norovirus GII and FCV f4 for 72 h; the contaminated oysters were moved to a water body without viruses for a 10-day depuration. The gene concentrations of test viruses in oysters were quantified with the real-time quantitative reverse transcription-PCR (qRT-PCR) during the contamination and depuration periods. FCV strain f4 used in this study was given by Y. Tohya, University of Tokyo, and was multiplied in CRFK cells cultivated with minimum essential medium including 10% bovine serum according to Tohya et al. (23). Norovirus GII used in this study was purified from a stool specimen of a patient with acute infectious gastroenteritis in Miyagi Prefecture, Japan. About 20 g of stool was suspended in 200 ml of distilled water and centrifuged at 10,000 ϫ g for 30 min. The supernatant, including norovirus particles, was collected and stored at Ϫ20°C. The genotype of norovirus GII in the specimen was found to be norovirus GII genotype 6 (GII.6), which was revealed by RT-PCR and sequencing of the norovirus GII gene as described previously (24).A contamination test of oysters with norovirus GII and FCV f4 was conducted at Miyagi Prefecture Fisheries Research and Development Center. Three water baths were prepared, each containing 160 liters of sand-filtere...
