Two new studies on viral pathogens in oysters and mollusks shed new light on the need for better detection methods and treatments that are needed to ensure shellfish safety.
Over the past few years both the United States and Europe have reported several foodborne illness outbreaks related to contaminated oysters and other mollusks.
The first study, by Spanish researchers, appeared in Emerging Infectious Diseases (EID). In it, scientists 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.
According to the Center for Infectious Disease Research and Policy, real-time reverse transcription polymerase chain reaction (RT-PCR) testing was used to detect norovirus and hepatitis A and standard RT-PCR was used to detect astrovirus.
Researchers found that 40 percent of the 50 samples were contaminated by at least one virus, though they all had met current food safety standards. Present in 24 percent of samples was norovirus genotype 1, followed by astrovirus–found in 18 percent of samples, norovirus genotype 2–found in 8 percent, and hepatitis A.
Six of the positive samples tested positive for more than one virus. The authors noted that infections with multiple virus strains could produce more severe symptoms and possibly lead to the emergence of new recombinant strains.
The researchers recognized the difficulty of detecting and monitoring viral contamination in shellfish samples, however, they believe the 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 Eurosurveillance, Irish researchers reported on a method they tested to reduce possible oyster contamination in harvesting areas linked to gastroenteritis outbreaks. The research team 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.
Oysters from an Irish harvesting area linked to norovirus outbreaks were used to test a modified “depuration” method involving the re-laying of oysters in clean areas for 17 days, then subjecting them to elevated water temperatures (15C to 17C) for at least 4 days.
The researchers reported that after treatment, norovirus levels in the re-laid oysters decreased from 2,900 to 492 genome copies. 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.