Vibrio in seafood - FAO.org

• Effect of post harvest handling and processing• Ability of the organism to multiply to an infective doseAs a result, the exposure assessment was divided into separate modules, which corresponded todifferent stages leading potentially to consumer exposure: the harvest and post harvest, retail andconsumption modules.The Harvest Module estimates the prevalence of pathogenic V. parahaemolyticus at time ofharvest. The Post Harvest Module determines the role of post harvest processing and handling on thenumbers of pathogenic V. parahaemolyticus at consumption.As this section is currently based on the model developed in the United States FDA-VPRA(Anonymous, 2001) a similar approach and structure is being used. It also involves the collection of datafrom other countries, if possible on a regional and seasonal basis and then incorporating such data into themodel. In the development of FDA-VPRA model, because of harvesting and temperature differences, theUnited States harvest areas were divided into five regions, and each region was divided into four seasons.Differences existing in oyster harvesting practices and climates in the United States were sufficientlysignificant to identify five separate geographic regions (Northeast Atlantic, Mid-Atlantic, PacificNorthwest, Louisiana Gulf Coast and the remainder of the Gulf Coast) for each season, for consideration inmodelling each of the modules. Factors influencing the risk of illness posed by V. parahaemolyticus wereidentified and incorporated into each module as appropriate. Integration of the various parameterscomprising these modules into a quantitative risk assessment model will provide a more comprehensiveunderstanding of the relative importance and interactions among the factors influencing risk.2.2.5.2 AssumptionsWhile providing a framework for understanding the relationship of risk to various parameters, thedevelopment of the risk assessment model necessarily requires certain assumptions to fill the data gaps. Inthe development of the United States Vibrio risk assessment the assumptions incorporated in the modelwere reviewed by the National Advisory Committee on Microbiological Criteria for Foods (NACMCF,1998) at a public meeting in September 1999. In the current risk assessment the work undertaken to dateincluding the assumptions made have been reviewed by a group of experts at a joint FAO/WHO expertconsultation on risk assessment of microbiological hazards in foods that was convened in July 2001(FAO/WHO, 2001). Such a review step ensures that the assumptions are the best that can be made basedon current knowledge and also facilitates transparency of the risk assessment process.Based on the information currently available, for the Harvest Module, it was assumed that thepresence of the thermostable direct hemolysin (TDH) gene be used as the basis for pathogenicity. It is notcurrently known what average numbers of TDH-positive strains exist in shellfish, nationally or regionally(see also section 3.1.3 on TDH). The estimates made in the V. parahaemolyticus risk assessment, based onthe observed frequency of TDH-positive isolates, were the best possible from the data currently available.However, since it is currently not known how this frequency may vary from one year to the next, a twofoldup or down triangle distribution was assumed. Also, within a given year, there is uncertainty aboutthe variance of the percentage pathogenic V. parahaemolyticus in one composite of oysters to the next.For example, for the United States coastal area, with the exception of the Pacific Coast (where the rangewas 2% to 4%), the percentage pathogenic V. parahaemolyticus in a given year ranged from 0.1% to 0.3%.However, these estimates are based on older data and may not be predictive of future years, given that thefrequency of percentage pathogenic V. parahaemolyticus may be changing as new outbreak strains emergeor re-emerge, such as the emergence of O3:K6 or recurrence of known outbreak strains such as O4:K12. Ithas also been noted that the proportion of pathogenic strains occurring can vary from region to region, forexample these strains tend to occur with greater frequency in Asia than in the United States (FAO/WHO,13