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United States Department of Agriculture

Agricultural Research Service


Location: Food Safety and Intervention Technologies

Title: Enteric virus and vibrio contamination of shellfish: intervention strategies

item Richards, Gary

Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 13, 2013
Publication Date: November 15, 2013
Citation: Richards, G.P. 2013. Enteric virus and vibrio contamination of shellfish: intervention strategies. Symposium Proceedings. In: Proceedings of the Korean Society of Food Science and Nutrition.Food Safety for the Food Industry, Gwangju, South Kore, November 13-15,2013.pp.57-58

Technical Abstract: INTRODUCTION. Molluscan shellfish include oysters, clams, mussels, and cockles, which can cause illnesses from a variety of human pathogens. Enteric viruses, like norovirus and hepatitis A virus, are generally transmitted to shellfish through fecal contamination of shellfish harvesting areas, although post-harvest processing contamination can also lead to illnesses. Noroviruses are the most common cause of foodborne illness throughout much of the world. In addition to enteric viruses, naturally occurring Vibrio vulnificus and Vibrio parahaemolyticus are bacteria that threaten the safety of molluscan shellfish. Vibrio vulnificus has a high mortality rate (around 50%) for the infected consumer, while V. parahaemolyticus seldom causes death but is a common source of infection in many countries. Virus- and Vibrio-contaminated products lead to: a) outbreaks of illness among shellfish consumers, b) loss of consumer confidence in the safety of shellfish, c) increased need for regulatory intervention, d) product recalls, and e) substantial financial loss to the industry, consumer, and regulatory agencies. VIRUS MONITORING OF SHELLFISH. The inactivation of enteric viruses within shellfish tissues is difficult to achieve. Currently, the best management practice to prevent norovirus infection is to make certain the shellfish are obtained from clean waters and are handled in a sanitary manner. Thorough cooking is effective for the inactivation of viral contaminants; however, many consumers like shellfish raw or only lightly cooked. Methods to extract and test for viruses in shellfish have been developed and should be implemented in regular monitoring programs. Molecular biological methods are available for the detection of norovirus, hepatitis A virus, and other enteric viruses in shellfish and should be used for general screening purposes and in outbreak investigations, but their limitations must also be recognized. Once contaminated, shellfish can maintain high virus levels, especially if they are stored in an environment that is cold, dark, and wet. Unlike many bacteria, human norovirus and hepatitis A virus do not replicate outside the human host, thus they do not increase in numbers during shellfish storage. VIBRIO-CONTAMINATED SHELLFISH. Vibrio parahaemolyticus is the major source of seafood-borne illness in the United States and in some parts of Asia. Some serotypes are more virulent than others. The pandemic O3:K6 serotype has been the focus of attention in recent years. We characterized the persistence of V. parahaemolyticus O3:K6 in United States coastal waters as well as factors that affect its colonization of the intestinal tract of an adult mouse model. Our findings and their significance will be discussed. Another Vibrio, namely Vibrio vulnificus, can cause serious illness and death in individuals who have compromised immune systems, especially in people with liver disease. Factors associated with V. vulnificus levels in seawater will be discussed. Both V. parahaemolyticus and V. vulnificus are naturally present in marine waters during summer months when seawater temperatures are high; therefore, monitoring for vibrios in seawater or shellfish should be performed, especially during the summer. Health authorities should also monitor for possible Vibrio outbreaks and trace back to their origin shellfish that were known to cause illness. During periods of outbreaks, products already in the market place should be recalled and harvesting should be suspended until levels of vibrios in the shellfish or seawater drop to acceptable levels. PROCESSING INTERVENTIONS FOR VIRUSES AND VIBRIOS. Vibrios are more easily controlled in shellfish than enteric viruses. Controls include: a) bacteriological monitoring for Vibrio levels in harvesting areas, b) restriction in harvesting based on the levels of vibrios present, c) using adequate ice during product shipment and storage to retard the outgrowth of vibrios, and d) post-harvest processing technologies. Unlike enteric viruses, vibrios can replicate within the shellfish both pre- and post-harvest; therefore, proper icing of shellfish can minimize the outgrowth of vibrios in harvested products. Thorough cooking is the most effective means to eliminate enteric viruses and vibrios from shellfish; however, the demand for raw product has led to the need for alternative processing techniques. Some processing techniques, like commercial depuration, have been used with varying levels of success. Shellfish depuration is not fully effective in eliminating viruses or vibrios from shellfish, but depuration may reduce levels slightly. Very low levels of norovirus can cause illness; therefore, depuration is never an adequate processing strategy for virus-contaminated products. High pressure processing is another technique that is effective in reducing vibrios and hepatitis A virus from shellfish; however, the levels of pressure required for the inactivation of human norovirus are higher than commercially feasible. Freezing is effective in reducing Vibrio levels in shellfish; however, freezing does not appear to inactivate human norovirus. Our laboratory found that repeated freezing and thawing of norovirus up to 14 times did not significantly reduce virus levels. Unfortunately, there is currently no satisfactory method to totally inactivate norovirus in raw shellfish. Enhanced monitoring is perhaps the best and most practical means to reduce norovirus outbreaks. Monitoring for signs of fecal pollution of the harvesting areas is essential for safe shellfish. Contamination events, like heavy rains and resulting sewage overflows, should trigger closure of shellfish beds. Dumping of boat wastes should not occur in areas where shellfish are raised and shellfish should never be handled by ill harvesters. Together, these common sense practices would reduce the incidence of enteric virus illness. As for emerging intervention strategies for vibrios, our laboratory is evaluating the use of natural predators to kill vibrios in seawater and shellfish. Natural predators, like bacteriophages and the predatory Bdellovibrio and like organisms (BALOs), may be used in conjunction with conventional processing technologies, like shellfish depuration, to reduce vibrios in shellfish. Among the BALO’s that significantly reduce vibrios in seawater are members of the saltwater genus Bacteriovorax. Bacteriovorax were shown to be present in seawater from the Atlantic and Pacific oceans throughout the year. Bacteriovorax spp. and bacteriophages that are currently being isolated against vibrios could serve as a simple processing strategy to reduce vibrios in shellfish. SUMMARY. Worldwide distribution of foods, including shellfish, has increased over the past few decades and will continue to do so in the future. This has created an opportunity for the rapid, global distribution of pathogens through the food supply. The success of the industry in international markets will be determined in part by their success in minimizing viral and Vibrio contamination of their products. Future success in reducing shellfish-associated illness will rely on enhanced shellfish monitoring for signs of contamination, implementation of good hygienic practices for handling and distributing shellfish, and the development of innovative new processing technologies.

Last Modified: 4/23/2014
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