May 13, 2010 (CIDRAP News) – Two new studies on viral pathogens in oysters and other mollusks shed new light on the need for better detection methods and the type of monitoring and treatments that are needed to ensure product safety.
Over the past few years the United States and Europe have reported several outbreaks of foodborne illness, especially norovirus cases, related to contaminated oysters and other mollusks.
The first study, by Spanish researchers, appeared yesterday in an early online edition of Emerging Infectious Diseases (EID). They analyzed 50 mollusk samples imported into Spain from Sep 2006 to Mar 2009 for the presence of three human enteric viruses, including two norovirus genotypes, hepatitis A, and astrovirus. Species included clams, oysters, cockles, and razor clams. The mollusks were imported from Morocco, Peru, Vietnam, and South Korea.
They used real-time reverse transcription polymerase chain reaction (RT-PCR) testing to detect norovirus and hepatitis A and standard RT-PCR to detect astrovirus.
They found that 20 (40%) of the 50 samples were contaminated by at least one virus, though they all had met current food safety standards. Norovirus genotype 1 was the most commonly detected virus, present in 24% of samples, followed by astrovirus (18%), norovirus genotype 2 (8%), and hepatitis A.
Six of the positive samples tested positive for more than one virus. The authors noted that coinfections with multiple virus strains could produce more severe symptoms and possibly lead to the emergence of new recombinant strains.
The researchers acknowledged the difficulty of detecting and monitoring viral contamination in shellfish samples, but they said new prevention strategies based on microbiological risk assessment could help ensure product safety and that it's essential to implement such steps and provide good lab training in developing countries that export the products.
In the second study, which appears in today's issue of Eurosurveillance, Irish researchers reported on a method they tested to reduce possible oyster contamination in harvesting areas linked to gastroenteritis outbreaks. They noted that methods for detecting norovirus in shellfish are relatively new and that processes to eliminate bacteria in oysters—putting them in clean seawater at ambient temperatures so they can purge their contaminants—do little to reduce virus levels in oysters.
They used oysters from an Irish harvesting area linked to norovirus outbreaks to test a modified "depuration" method that involved re-laying the oysters in a clean area for 17 days, and then subjecting them to elevated water temperatures (15C to 17C) for at least 4 days.
After treatment, the norovirus levels in the relaid oysters decreased from 2,900 to 492 genome copies, the group reported. Exposing them to the higher temperatures for 4 days reduced norovirus levels to 136 viral genome copies. The level fell below assay detection at 6 days.
The group concluded that growing evidence suggests that it is possible to gauge illness risk based on norovirus levels in oysters and that given the inadequacy of existing controls to prevent contamination, setting an appropriate virus standard would yield public health benefits. They also wrote that validated treatment processes can be used to produce a safe product, even when low levels of norovirus are detected in the treated oysters.
Early online EID report
May 13 Eurosurveillance report