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July 19, 2005
Quantitative Risk Assessment on the Public Health Impact of
Pathogenic Vibrio parahaemolyticus in Raw Oysters
Table of Contents
Appendix 11: Data Gaps and Future Research Needs
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.
Consumption
- 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.
Table of Contents