Food Protection Trends - July/August 2019 - 340

and was associated with using ice from a shellfish depuration
plant to cool cooked seafood; the attack rate approached
100% (32). This outbreak was identified retrospectively
months later, as Europe does not have a systematic vibriosis
surveillance system. Vp attack rates associated with
consumption of raw shellfish tend to be much lower, and
sporadic illnesses such as occur in the U.S. are much less
likely to be detected without systematic surveillance (32).
Harvest practices
Vibrios are normal microflora of oysters and proliferate in
their tissues at temperature-dependent rates before and after
harvest (8, 9, 23, 27, 50). When oysters are taken out of the
water or emerge intertidally, their shells close and they cannot
purge vibrios. As the vibrios proliferate in the oyster tissues,
the risk of illness in those who consume the oysters also increases. Historic harvest practices worldwide expose oysters to
warm ambient air, allowing vibrios to proliferate exponentially.
Unlike many other animal food sources, oysters are not
slaughtered during harvest and can live for days out of water.
Thus, spoilage and shelf life were of little economic concern
in the case of live oysters than for less risky but perishable
seafood products that have long been subjected to rapid
cooling. Market surveys of live U.S. oysters in 1998-1999
and 2007 frequently found Vp and Vv levels greater than
100,000 MPN/g in samples from the Gulf and Mid-Atlantic
during summer months (11, 18), levels 100-fold greater than
at harvest and approaching the maximum levels than can be
caused by temperature abuse of live oysters. The U.S. industry
and regulators were long reluctant to mandate the stringent
time/temperature controls needed to prevent growth of Vp
and Vv. Many doubted the relationship between exposure
and risk, and most harvesters operated out of small boats that
were considered impractical for on-board cooling. Typically,
industry had up to ten or even more hours to place oysters
under refrigeration, even for the warmest areas and seasons.
Stacking burlap sacks of oysters onto pallets and loading
them into refrigerators at that time was standard practice.
The VPRA and VVRA assumed that ten hours was needed for
the internal temperature of the sacks to be below 10°C, the
minimum Vp growth temperature (23, 50).
Shifting seasonal and geographical production and
consumption patterns
Late fall and winter, especially around holidays such as
Thanksgiving, are historically the times of highest oyster
demand as well as the periods when vibriosis risk is lowest.
Glycogen levels in oyster tissues peak in the winter after the
spawning season, increasing yield and organoleptic "fatter"
appeal. With the recent advent of triploid oysters that do not
spawn, aquaculture production of these oysters has increased
to meet the growing demand for half-shell oyster consumption
during the peak summer vacation season in coastal areas, when
vibriosis risk is greatest. The VPRA has estimated that half of

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Food Protection Trends July/August

all oysters were consumed raw, based on data from the 1990s
(50). Industry and state regulators assume that most farmed
oysters are now consumed raw. Perhaps more consequential
to increasing vibriosis numbers is the geographical shift
of production to the Pacific NW in the late 1990s and the
Northeast (NE) Atlantic over the past decade. The increased
production in these regions primarily reflect farmed oysters
destined for raw consumption. While Vibrio abundance
in oysters is an order of magnitude lower in these cooler
regions than in the warmer Gulf waters, epidemiological data
indicates that the Vp risk per serving of oysters from higher
latitude areas may be an order of magnitude higher than for
Gulf oysters during the Vp risk season (3, 4, 5, 6, 11, 18, 19,
50). Fortunately, the Vibrio season in the Pacific NW and NE
Atlantic is short, beginning in mid-June and typically being
over sometime in September. The Vv risk remains negligible
in these cooler regions, as cooler waters suppress its growth far
more than growth of Vp (11, 18, 31, 46).
Human host susceptibility
The VVRA used CDC data that indicated that ~7% of
the U.S. population had underlying health conditions that
predisposed them to primary Vv septicemia, based on data
from the 1990s (23). As people age they are more likely to
have chronic illnesses and to use medications that increase
susceptibility to infectious diseases, including vibriosis.
Changes in rates of alcoholism, cancer, diabetes and
infectious diseases such as HIV, and hepatitis B and C could
influence the pool of high-risk individuals.
Improved recognition, reporting and diagnostics
Greater recognition of vibriosis in the health care
community due to high profile outbreaks and improved
diagnostics, especially real time PCR, has likely reduced underreporting (1, 45). However, CDC revised its underreporting
rate for Vp in 2011 from 20:1, which was used in the VPRA,
to >150:1 (38, 48). The VPRA had assumed 2800 Vp oysterassociated cases annually in 2005, but annual U.S. foodborne
Vp illnesses are now estimated to be around 50,000 (48,
50). The estimated underreporting rate for vibriosis, with
the exception of that caused by Vv, is much higher than for
other major foodborne pathogens (48). CDC has recently
implemented culture-independent diagnostic testing
protocols in many states, which will likely detect vibriosis
evidence that may have been undetected by historical culturedependent testing protocols (3). Authorities agree that this
will complicate comparisons with historical culture-dependent
data even if the new approach more accurately reflects actual
risk. Reported illness could increase while risk is declining, as
the result of improved management practices.
Fractured regulatory oversight
The Interstate Shellfish Sanitation Conference (ISSC),
which makes shellfish safety policy for the National Shellfish

Food Protection Trends - July/August 2019

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