Food Safety Magazine, February/March 2013

SEAFOODfoods is essential to reducing norovirus illnesses in the generalpopulation. If gloves are used to handle foods, they should bemaintained in a sanitary manner. Noroviruses are generally resistantto alcohol-based products, including hand sanitizers, sothorough washing is very important.Environmental Contamination of ShellfishUnlike most food products, in which handling is often thesource of contamination, bivalve molluscan shellfish (oysters,clams, mussels and cockles) are most commonly contaminatedby fecally polluted water in the harvest area. These shellfishfeed by filtering out particles in the water through their gillsProcessing InterventionsWashing. Since honest efforts can go only so far in providingsafe foods, postharvest processing interventions are often necessaryto enhance food safety. The outer surfaces of foods such asfruits and vegetables, leafy greens and berries should be washedwith potable water to remove surface contamination to thegreatest extent possible. Foods with porous or crinkly surfaces,like strawberries and curly leaf lettuce or spinach, are more difficultto wash and may require more effort to remove surfacecontamination. Several rinses of produce with copious amountsof water would be expected to lessen the number of virusespresent. Even then, viruses may remain within pores, cracksor surface folds. Peeling vegetables will also remove surfacecontamination. Melons should be thoroughly washed beforecutting to prevent the transfer of surface contamination to theinside of the fruit via the knife.The outer shell of molluscan shellfish should be washedclean to remove mud and potential surface viruses, particularlyif the shellfish are to be shucked and eaten raw. In a food processingenvironment, food contact surfaces should be washed“...bivalve molluscan shellfish (oysters, clams, mussels and cockles) aremost commonly contaminated by fecally polluted water in the harvestarea.”and diverting the particles to their mouths and digestive tracts.Bivalves can bioconcentrate viruses within their edible tissuesto many times the levels in the surrounding water. This makesraw shellfish susceptible to high levels of norovirus contaminationwhen the water is polluted. To minimize illnesses, regulatorsin the U.S. monitor shellfish growing waters for signs offecal pollution using fecal coliform (bacterial) standards. Thesebacteria are not the best indicators for virus pollution, becausebacteria and viruses have different tolerances to ever-changingenvironmental conditions. Nevertheless, the detection of bacterialcontamination of fecal origin signifies the likelihood thatnorovirus and other enteric viruses are also present. The converseis not true; that is, water that tests clean for fecal coliformbacteria may still contain viruses that are more persistent thanbacteria. Shellfish from such waters could still pose a threat tothe consumer. Other factors that contribute to shellfish contaminationinclude storm water runoff, particularly in areaswith septic tanks, faulty sewage treatment plants and the illicitdumping of boat wastes into harvesting areas. Dumping of boatwastes and vomiting overboard have been associated with specificoutbreaks. 3, 4 Harvesting only from areas approved by stateregulators will reduce the threat of illness but cannot guaranteesafety. Other factors, like storing shellfish in a sanitary manner,chilling them with “clean” ice and handling the shell stock andshucked products under hygienic conditions, are necessary precautionsto enhance shellfish safety.with dilute bleach solution to inactivate (kill) potential noroviruses,although the effective concentrations and exposure timesfor bleach and other disinfectants against human noroviruseshave not been defined.Heating. Pasteurization and/or cooking are effective meansto inactivate enteric viruses, but their effectiveness depends onthe type of cooking (frying, steaming, baking, boiling) and theduration. 5 Internal temperatures must be sufficient to inactivateviruses, although the times and temperatures required to inactivatehuman noroviruses have not been clearly defined. Cookingis effective and useful as a disinfection process for manyfoods, including vegetables. A short blanching may be sufficientto eliminate surface contaminants on fresh vegetables. Forshellfish, thorough cooking can leave them overly chewy, soalternative processing strategies are needed.Depuration. A popular method to cleanse shellfish of bacterialand other contaminants is known as controlled purificationor depuration. Molluscan shellfish are placed in tanks of cleanseawater and allowed to purge the contaminants from their systems,generally over a 3-day period. 6 Depuration has been practicedfor over 100 years and is effective in reducing bacterialcontamination and sand, but is less efficient in reducing entericviruses from shellfish. Enteric viruses apparently become sequesteredin motile, phagocytic hemocytes (blood cells) of theshellfish. Motile, phagocytic hemocytes can travel back andforth from the digestive tract, through the epithelial cells surroundingthe tract and into the shellfish’s connective tissues. 7Although these hemocytes have acidic interiors, enteric virusescan be highly acid tolerant. Laboratory studies have shown thathepatitis A virus was very acid tolerant and persisted for 21 daysin oyster hemocytes, whereas other viruses were less tolerant. 7No studies of acid persistence of human norovirus have beenperformed, but judging from the number of norovirus illnessescaused by shellfish, it is suspected that noroviruses may alsopersist in hemocytes for extended periods. Preliminary testing58 F o o d S a f e t y M a g a z i n e