Detection of noroviruses in shellfish in the Netherlands

Shellfish from oyster farms in the Netherlands and imported from other European countries were examined for viral contamination. A method that allows sequence matching between noroviruses from human cases and shellfish was used. The samples of shellfish (n = 42) were analyzed using a semi-nested RT-PCR that had been optimized for detection of norovirus in shellfish (SR primer sets). In addition, a different genome region was targeted using a second primer set which is routinely used for diagnosis of norovirus infection in humans (JV12Y/JV13I). To improve the detection limit for this RT-PCR a semi-nested test format was developed (NV primer sets). One of 21 oyster samples (4.8%) from Dutch farms was norovirus positive, whereas norovirus was detected in 1 out of 8 oyster samples (12.5%) and 5 out of 13 mussel samples (38.5%) collected directly after importation in the Netherlands. RNA from samples associated with an outbreak of gastro-enteritis in the Netherlands in 2001 was re-analyzed using the NV primer sets. At least one identical sequence (142/142 nt) was found in three fecal and in two oyster samples related to this outbreak. Further surveillance of norovirus by detection and typing of viruses from patients with gastroenteritis and shellfish is warranted to clarify the causes of future outbreaks.