Relaying to Decrease the Concentration of Oyster-Associated Pathogens

Abstract

Oysters experimentally contaminated with indicator bacteria, Salmonella and poliovirus were used in relaying studies designed to measure microbial elimination under a variety of environmental conditions. Two factors, level of microorganism in the oyster and temperature of the water, were important in determining the length of time necessary to purge the contaminating organisms. Oysters under physiological stress cleansed at a slower rate than did healthy oysters. Based on the expected level of pathogen contamination in naturally polluted oysters, healthy relaid oysters were capable of cleansing in a 7-d period provided the temperature was above 10°C. These results were verified by following the elimination of indicator bacteria and poliovirus in commercially relaid oysters. Fecal indicator bacteria and enteric pathogenic bacteria were eliminated at similar rates but fecal coliform levels did not correlate with virus elimination. Relaying waters may contain some indicator bacteria and this study suggested that fecal coliforms may not be useful as end-point indicators for this method of oyster purification. Depuration (7,9) and relaying (3,25,28) are methods

Processing Strategies to Inactivate Enteric Viruses in Shellfish

“…It has been suggested that the phagocytic process in hemocytes, which involves lysosomal enzymes, toxic oxygen intermediates, and antimicrobial peptides, may be responsible for killing bacteria in bivalves (Canesi et al 2002), but this has not been demonstrated for human enteric viruses to date. Viruses are known to be retained by bivalves for significantly longer periods of time than bacterial indicators such as E. coli and fecal coliforms (Cook and Ellender 1986;Collins 1989, 1990). Recombinant NoV (genogroup I) was shown, using immunohistochemical techniques, to be localized within the phagocytes and lumen of the digestive diverticula of the Pacific oyster, Crassostrea gigas, after 12 and 24 h of bioaccumulation (Le Guyader et al 2006).…”

Bacteriophage as models for virus removal from Pacific oysters (Crassostrea gigas) during re-laying

“…The most important would appear to be re-laying in water less polluted than that in growing areas [6][7][8][9]. Re-laying can bring about a rapid reduction in the levels of bacterial contamination [3] but less is known about its impact on the number of viruses.…”

Use of Rotavirus Virus-Like Particles as Surrogates To Evaluate Virus Persistence in Shellfish

“…The shellfish were likely to have been contaminated following a flood in the spring and retained infectious virus for 1 to 2 months before the outbreak. Virus carriage for an extended period of time has been suspected previously to have an important impact on public health (8,16,25), after detection of infectious hepatitis A virus for at least 3 weeks and for 6 weeks by RT-PCR in an oyster laboratory experiment (17). After a norovirus outbreak following oyster consumption, the follow-up of the harvest area for approximately 2 months showed the persistence of the virus strain contaminating shellfish at levels up to 1,000 RT-PCR units per oyster prior to depuration of the shellfish (21).…”

“…Depuration is a dynamic process whereby shellfish are allowed to purge themselves of contaminants either in a natural setting or in land-based facilities, whereas relaying is the practice of transferring shellfish harvested from contaminated areas to clean shellfish-growing waters (31). Data on virus persistence are needed to address how long viruses may persist in contaminated oyster beds so that early reopening of beds that might pose a risk to the health of consumers can be prevented.Working with live bivalve mollusks under laboratory conditions may introduce artificial parameters that do not adequately account for environmental factors (i.e., nutriments, temperature, or suspended matter, which can modify filtration rates) important for interpretation of results (8,17,27,31,33,35). The use of large depuration tanks and professional equipment allows better reliability and reproducibility of different environmental parameters (aeration, UV, water quality, speed of water recycling, temperature, disinfection of wastewater).…”

“…Working with live bivalve mollusks under laboratory conditions may introduce artificial parameters that do not adequately account for environmental factors (i.e., nutriments, temperature, or suspended matter, which can modify filtration rates) important for interpretation of results (8,17,27,31,33,35). The use of large depuration tanks and professional equipment allows better reliability and reproducibility of different environmental parameters (aeration, UV, water quality, speed of water recycling, temperature, disinfection of wastewater).…”