1 Recent International Efforts to Improve Bivalve Molluscan Shellfish Safety
7
species. Therefore, to develop a tool that is applicable to other regions or other
bivalve species, or to answer risk management questions other than postharvest
refrigeration, it would be necessary to first modify the existing FAO/WHO risk
assessment model or develop a new model that considers and evaluates other
factors such as salinity, strain differences and bivalve species related factors. This
would require data from different regions and involving different bivalve species.
It was also noted that the Vibrio vulnificus calculator tool is less likely than is the
V. parahaemolyticus calculator to be applicable to a broader region than the United
States of America because of uncertainty about the dose–response relationship
(FAO/WHO 2012b).
Regarding monitoring the level of pathogenic V. parahaemolyticus in seafood
or water, the Expert meeting noted that monitoring on an ongoing basis could be
expensive but consideration could be given to undertaking a study over the course
of a year and using this as a means to establish a relationship between total and
pathogenic V. parahaemolyticus and V. vulnificus in the seafood and abiotic factors
such as water temperature and salinity. Once such a relationship is established
for the harvest area of interest and measuring abiotic factors may be a more cost
effective way to monitor. Further, the meeting noted that monitoring seawater
may be of limited value, because there is no linear relation between the levels of
V. parahaemolyticus and V. vulnificus in seawater and bivalves and the relation
may vary depending on region and species of bivalves. Since a range of methods
from conventional culture based to molecular methods are available, the meeting
found the identification of a single method for monitoring really challenging and
of limited value as molecular methods are evolving rapidly. The method selection
would depend on the specific purpose of the monitoring activity, the cost, the speed
with which results are required and the technical capacity of the laboratory. The
meeting recommended evaluating the performance criteria of methods, so that a
decision on a “fit for purpose” method could be facilitated.
The growth model for V. parahaemolyticus in the FAO/WHO risk assessment
is based on data for the American oyster (C. virginica). The Expert meeting
(FAO/WHO 2012b) noted that the model was also appropriate for estimating growth
in at least one other oyster species (Crassostrea gigas) but is not appropriate for
predicting growth in the Sydney rock oyster (Saccostrea glomerata). It was also
noted that the V. parahaemolyticus model currently used would over estimate
growth at higher temperatures (e.g., >25
ı C) in live oysters. The model was
based on studies that were primarily undertaken using natural populations of
V. parahaemolyticus as these are considered to be the most representative. While
data are limited and inconsistent with respect to the impact of strain on growth
rate, recent studies in live oysters are suggestive of differences between tdh/trh
positive (pathogenic) populations versus total or non-pathogenic populations of
V. parahaemolyticus. Since there is no data to evaluate the performance of the
growth models in any other oyster species or other filter feeding shellfish or other
seafoods, its use in other products cannot currently be supported, and if used should
be done with clear understanding of the associated uncertainty (FAO/WHO 2012b).
7
species. Therefore, to develop a tool that is applicable to other regions or other
bivalve species, or to answer risk management questions other than postharvest
refrigeration, it would be necessary to first modify the existing FAO/WHO risk
assessment model or develop a new model that considers and evaluates other
factors such as salinity, strain differences and bivalve species related factors. This
would require data from different regions and involving different bivalve species.
It was also noted that the Vibrio vulnificus calculator tool is less likely than is the
V. parahaemolyticus calculator to be applicable to a broader region than the United
States of America because of uncertainty about the dose–response relationship
(FAO/WHO 2012b).
Regarding monitoring the level of pathogenic V. parahaemolyticus in seafood
or water, the Expert meeting noted that monitoring on an ongoing basis could be
expensive but consideration could be given to undertaking a study over the course
of a year and using this as a means to establish a relationship between total and
pathogenic V. parahaemolyticus and V. vulnificus in the seafood and abiotic factors
such as water temperature and salinity. Once such a relationship is established
for the harvest area of interest and measuring abiotic factors may be a more cost
effective way to monitor. Further, the meeting noted that monitoring seawater
may be of limited value, because there is no linear relation between the levels of
V. parahaemolyticus and V. vulnificus in seawater and bivalves and the relation
may vary depending on region and species of bivalves. Since a range of methods
from conventional culture based to molecular methods are available, the meeting
found the identification of a single method for monitoring really challenging and
of limited value as molecular methods are evolving rapidly. The method selection
would depend on the specific purpose of the monitoring activity, the cost, the speed
with which results are required and the technical capacity of the laboratory. The
meeting recommended evaluating the performance criteria of methods, so that a
decision on a “fit for purpose” method could be facilitated.
The growth model for V. parahaemolyticus in the FAO/WHO risk assessment
is based on data for the American oyster (C. virginica). The Expert meeting
(FAO/WHO 2012b) noted that the model was also appropriate for estimating growth
in at least one other oyster species (Crassostrea gigas) but is not appropriate for
predicting growth in the Sydney rock oyster (Saccostrea glomerata). It was also
noted that the V. parahaemolyticus model currently used would over estimate
growth at higher temperatures (e.g., >25
ı C) in live oysters. The model was
based on studies that were primarily undertaken using natural populations of
V. parahaemolyticus as these are considered to be the most representative. While
data are limited and inconsistent with respect to the impact of strain on growth
rate, recent studies in live oysters are suggestive of differences between tdh/trh
positive (pathogenic) populations versus total or non-pathogenic populations of
V. parahaemolyticus. Since there is no data to evaluate the performance of the
growth models in any other oyster species or other filter feeding shellfish or other
seafoods, its use in other products cannot currently be supported, and if used should
be done with clear understanding of the associated uncertainty (FAO/WHO 2012b).