Virus-contaminated oysters: a three-month monitoring of oysters imported to Switzerland

Abstract

Molluscan shellfish are known to be carriers of viral and bacterial pathogens. The consumption of raw oysters has been repeatedly linked to outbreaks of viral gastroenteritis and hepatitis A. Switzerland imports over 300 tons of oysters per year, 95% of which originate in France. To assess the level of viral contamination, a 3-month monitoring study was conducted. Therefore, the sensitivities of several previously described methods for virus concentration were compared, and one protocol was finally chosen by using dissected digestive tissues. Eighty-seven samples consisting of five oysters each were analyzed for Norwalk-like viruses (NLVs), enteroviruses, and hepatitis A viruses from November 2001 to February 2002. The oysters were exported by 31 French, three Dutch, and two Irish suppliers. Eight oyster samples from six French suppliers were positive for NLVs, and four samples from four French suppliers were positive for enteroviruses; two of the latter samples were positive for both viral agents. No hepatitis A viruses were detected. The sequences of NLV and enterovirus amplicons showed a great variety of strains, especially for the NLVs (strains similar to Bristol, Hawaii, Mexico, and Melksham agent). The data obtained indicated that imported oysters might be a source of NLV infection in Switzerland. However, further studies are needed to determine the quantitative significance of the risk factor within the overall epidemiology of NLVs.

FIG. 1.

RT-PCR analysis of NLVs in 10 oyster samples in week 8 by using genogroup-specific primers and agarose gel electrophoresis. The arrow indicates the position of the 203-bp amplicon. Lanes 3, 7, and 8, NLV genogroup II sequence positive; lane P, positive control; lanes N, negative controls; lane M, marker.

FIG. 2.

Phylogenetic analysis performed with a 132-bp region of the RNA polymerase of NLV genogroup II detected with genogroup-specific primers (19), in which seven oyster samples were used. Two of the eight French exporting regions considered were positive for NLV genogroup II, and 6 of 31 French suppliers provided positive samples. Samples obtained from regions 1 and 2 are indicated by rounded rectangular and rectangular boxes, respectively. The previously published sequences of Lordsdale virus (GenBank accession number X86557), Camberwell virus (AF145896), Bristol virus (X76716), Mexico virus (U22498), Melksham virus (X81879), and Hawaii virus (U07611) were also included.

FIG. 3.

Phylogenetic analysis performed with a 132-bp region of the RNA polymerase of NLV genogroup II detected with generic primers. Weekly analyses of oyster samples were performed, and each sample consisted of five oysters from a supplier. One of the eight French exporting regions (samples from 6 of the 31 French suppliers investigated), in addition to the two regions detected by using genogroup-specific primers (Fig. 2), was positive for NLV genogroup II. Samples obtained from regions 2 and 3 are indicated by rectangular and hexagonal boxes, respectively. The previously published sequences of Lordsdale virus (GenBank accession number X86557), Camberwell virus (AF145896), Bristol virus (X76716), Melksham virus (X81879), and Hawaii virus (U07611) were also included.