Posted: February 28th, 2012 - 8:12pm
Source: FSAI News Vol 14 Issue 1

Norovirus is a major cause of acute gastroenteritis in Europe. The virus is readily transmitted from person to person so outbreaks can easily become large, as shown in the outbreak that occurred in a Dublin Hotel just before Christmas. In this instance, around 580 people reported illness after attending events in the hotel. Food and water can also be sources of norovirus infection when they are either contaminated with faecal matter or cross-contaminated by food handlers.
Oysters contaminated with norovirus pose a particular risk to human health as they can accumulate norovirus particles from contaminated water and are often consumed raw. Over a five week period in January and February 2010, more than 70 cases of gastroenteritis in Ireland and the United Kingdom (UK) were linked to the consumption of oysters originating from an Irish oyster harvesting area. The oysters connected to two of the illness incidents in Ireland were available for testing and contained 2,040 and 2,350 NoV genome copies per gramme, respectively.
A major issue for the Authorities and the industry in Ireland surrounds decisions regarding the safety of oysters when they are found to contain norovirus genome copies but are nevertheless compliant with all applicable control requirements. In this context the FSAI requested advice from the European Food Safety Authority (EFSA) on the public health risks associated with establishing standards for noroviruses in oysters. This was the first time that Ireland had activated the mechanism available to Member States which allows Competent Authorities in Member States to request opinions from EFSA. The task fell to the EFSA Panel on Biological Hazards to issue a scientific opinion on:
(i) the use of real-time reverse-transcription polymerase chain reaction (rRT-PCR) as a means of detection and quantification of Norovirus in oysters;
(ii) limits that do not pose an unacceptable risk to consumers for norovirus genogroups GI and GII in oysters as determined by real-time PCR, and
(iii) treatment regimes (post-harvest interventions) that can be relied upon to reduce norovirus counts in oysters.
EFSA concluded that norovirus is highly infectious and low doses can result in disease. However, it noted that the amount of virus detected in oysters linked to human illness could vary greatly. It also concluded that norovirus genetic material was frequently detected in European oysters even when they complied with existing control standards.
The EFSA panel supported the use of the rRT-PCR method for detection and quantification of norovirus and this method is currently being standardised in CEN. However, it noted that the number of genome copies detected by rRT-PCR may not relate to infectious norovirus particles, and as a consequence the method can only be used to provide an indirect measure of risk. The Panel cautioned risk managers that quantification of norovirus genome copies by rRT-PCR could therefore, overestimate the risk. EFSA was clear that the lower the limit the greater the level of consumer protection achieved. It could not, however, quantify the public health impact of establishment of different limits based on norovirus genome copy numbers determined by rRT-PCR.
EFSA also concluded that current methods for purifying oysters such as depuration are not completely effective at removing norovirus from oysters and consequently improvements in these procedures are necessary. Because of this, EFSA clearly reiterated the need to protect oysters from contamination with norovirus during production in the growing areas.
The opinion went on to recommend that risk managers should consider establishing an acceptable limit for norovirus in oysters that are intended to be harvested and placed on the market in the EU. The establishment of a microbiological criterion was found to be a suitable approach to control, both for regulators and for producers. Such controls could be enacted using an agreed EU microbiological criterion at one or more stages in the production and retail cycle. In addition, scientists recommend carrying out an EU-wide baseline survey on norovirus contamination in oysters in order to estimate overall consumer exposure. Such a survey would also provide information that could be used to evaluate the public health impact of control measures put in place over time.