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Vibrio parahaemolyticus Risk Assessment - Appendix 11: Data Gaps and Future Research Needs

July 19, 2005

Table of Contents

The Vibrio parahaemolyticus risk assessment has provided a framework to significantly advance our ability to describe our current state of knowledge about this important foodborne pathogen, while simultaneously providing a framework for integrating and evaluating the impact of new scientific knowledge on enhancing public health. However, as demonstrated in the risk assessment, deficiencies of the current research with respect to risk assessment were identified. There are several uncertainties associated with the model due to insufficient or absent data. This has brought several future research needs or further data gathering to the forefront as discussed below, which would reduce the uncertainties and improve the risk assessment.

Incidence/frequency of pathogenic V. parahaemolyticus in water and shellfish

  • More studies are needed to determine the relative abundance of pathogenic V. parahaemolyticus in the different regions, particularly the mid-Atlantic and Northeast Atlantic regions. A more accurate estimate of the incidence of pathogenic V. parahaemolyticus in these two latter regions would improve the risk assessment.
  • Additional research is needed to determine the possibility of changes in the relative abundance of pathogenic V. parahaemolyticus during different seasons of the year in the different geographical regions, as well as the identification of associated environmental factors (e.g. temperature or salinity effects). Data on densities of total and pathogenic V. parahaemolyticus under a variety of conditions would considerably strengthen the VPRA. Further studies investigating (i.e., to either substantiate or refute) previous finding of higher ratios of pathogenic V. parahaemolyticus at lower water temperatures (DePaola et al., 2003a) would be particularly informative. Similar data on levels of pathogenic V. parahaemolyticus at the point of sale or consumption could provide more valid exposure estimates.
  • There is a need for research on the dynamics and causes of temporal "spikes" in pathogenic levels and whether or not the interim monitoring plan, as devised, can identify these spikes as they occur (i.e., is it effective?)
  • Information is also needed on the role of oyster physiology and immune status on levels of pathogenic V. parahaemolyticus in the oyster. There is a need to determine if there is any correlation between the number of pathogenic V. parahaemolyticus and the percentage of oysters diseased.
  • It would be appropriate to further investigate V. parahaemolyticus O3:K6, and its incidence, because it has been shown to be more resistant to mitigation strategies and appears to require fewer microorganisms to cause illness than other pathogenic V. parahaemolyticus.

Impact of overnight submersion of intertidally harvested oysters

  • Research is needed to determine whether the predicted level of 90% reduction in illness can be achieved when oysters are stacked in baskets and allowed to remain submerged in the water overnight.

Growth rate of V. parahaemolyticus

  • Further knowledge of the growth rate of V. parahaemolyticus within oysters at temperatures other than 26 °C would help decrease the uncertainty with respect to the difference between growth in the oyster vs. bacterial broth culture; including the issue of potential differences in the growth rate of pathogenic strains versus total V. parahaemolyticus populations.

Impact of hydrographic flushing

  • Additional quantitative studies are needed on the rates of hydrographic flushing (water turnover) in shellfish harvest areas based on levels of freshwater flows, tidal changes, winds, depth of harvesting area to show how these factors may influence pathogenic V. parahaemolyticus levels.

Impact of post-harvest handling and processing

  • Additional data on the genetic diversity that we are likely to encounter will enable better evaluation of the phenotypic characteristics that affect ability to tolerate mitigations, growth rates, acid tolerance, etc.
  • Studies are needed to obtain more accurate estimates of the distribution of cooling rates of commercial oyster shellstock in an industry setting.
  • Quantitative studies are needed to determine the effect of refrigerated wet storage with UV treatment (depuration under refrigerated conditions) as a means of further reducing V. parahaemolyticus post harvest.
  • A multi-season, nationwide retail study would be required to determine the pathogenic V. parahaemolyticus density in market oysters.


  • A survey of the oyster retail market in the different regions would provide a better indication of the actual proportion of the oyster harvest that goes to the raw oyster market.
  • Better consumption information would be helpful in determining the actual amount of oysters consumed per serving as well as per annum in the different regions.

Improved dose-response data

  • More intensive investigations of shellfish foodborne disease outbreaks in such a way as to examine the relationships between the dose of contaminated food items ingested and the attack rate and severity of the resulting illness controlling for host factors.
  • More research is needed to determine whether different pathogenic strains differ in virulence and in the levels of pathogen required to cause illness.
  • More research on the potential virulence factors other than TDH (e.g., urease, TRH, enterotoxins, invasive ability) is needed to determine if the ability to cause disease is increased or decreased by the presence of additional virulence factors. Vibrio parahaemolyticus strains that do not produce TDH, TRH, or urease have been found to induce fluid accumulation in suckling mice and diarrhea in a ferret model after oral inoculation in a dose-dependent manner (Kothary et al., 2000). Correlation between clinical and environmental incidence of these strains is yet to be determined.
  • Additional research is needed to determine the difference in virulence between the strains that have the above virulence properties, as well as between strains that are tdh+/trh- and tdh+/trh+. Research on the genetic diversity among pathogenic strains needs to be explored to determine if the degree of pathogenicity among pathogenic strains is associated with additional genetic markers and the temporal and environmental dynamics related to the emergence of individual strains within the harvest areas. The current risk assessment assumes all tdh+ strains to be equally virulent but more recent reports indicate that strains with tdh+/trh+ have a different promoter sequence for the tdh gene and produce much less TDH than tdh+/trh- strains (Nishibuchi, 2004). This an important finding since ~95% of the tdh+ strains from Gulf and Atlantic oysters (and 100% from Pacific oysters) are tdh+/trh+. Nishibuchi's findings are further supported by CDC data that show that most US clinical isolates are tdh+/trh- even when O3:K6 (tdh+/trh-) are excluded.

Improved state surveillance systems

  • More data from State surveillance systems would provide a better knowledge of the actual illnesses occurring due to consumption of raw oysters containing pathogenic V. parahaemolyticus. This would also help to better characterize the immune and general health status of individuals that become ill, as well as if there are other contributing factors such as taking stomach acid suppressors.
  • There is a need to look at the seasonality of CDC illness data, especially for the Gulf. The illness peak in late spring is probably real as the reporting system should not vary seasonally. It may be that tdh levels peak then.

Impact of consumer handling of raw oysters

  • More information is needed on post retail consumer handling of raw oysters, such as storage conditions (time and temperature), kitchen practices (possibility of cross-contamination), etc. This would provide some indication as to whether the consumer has a role in increasing or decreasing levels of V. parahaemolyticus in raw oysters at time of consumption.

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