Food Protection Trends - July/August 2019 - 344

the Puget and Long Island Sounds have become available on
the internet (43). Doubling times during Vp risk season in
the Puget Sound can range to more than a day to less than an
hour between the early morning and mid-afternoon in the
same day. There are also efforts to improve the accuracy of
Vibrio models with region specific data on the relationship
of Vibrio levels with other water quality data beyond
temperature and salinity (14).
Post-harvest processing (PHP) and cooking
Vibrios are much more susceptible than other foodborne
pathogens to most disinfection measures. Cooking virtually
eliminates the risk and is the most effective measure,
especially for high-risk consumers with chronic underlying
illnesses. PHP can be as effective as cooking in preventing
illness caused by vibrios while preserving most of the
organoleptic qualities of raw oysters (17, 51). Mild heat
treatment was commercialized in the 1990s to eliminate the
Vv risk of Gulf oysters, creating a new category of oyster
usage, "PHP" that could be used on the label to indicate
added safety. While PHP was capable of reducing Vibrio
populations by orders of magnitude, it was incapable of
achieving total Vibrio inactivation and zero risk. ISSC
adopted the label "processed to reduce Vp and/or Vv to
nondetectable levels." "Nondetectable" was originally defined
as < 3 MPN/g but was changed to < 30 MPN/g based on
Vibrio risk assessments (23). This policy opened the door to
an extraordinary period of oyster PHP innovations that are
unparalleled for any other food commodity. Freezing coupled
with extended frozen storage soon emerged as the most
popular PHP, as this is a simple and inexpensive method
that also extends shelf life. More innovative processing using
high hydrostatic pressure to inactivate vibrios also makes
shucking oysters easier by separating the adductor muscle
from shell attachment. Irradiation kills vibrios but unlike
other PHPs, is not lethal to oysters. Validation is required
for PHPs with an added safety label, but many processors
are freezing oysters without making any labeling claims, thus
avoiding the expense of validation and verification. Industry
experts estimate that about 15% of the half-shell oyster
market represent PHP products. Perhaps no other product
in the food supply has been subjected to the range and extent
of PHP to successfully reduce pathogen risk as oysters have
been. A U.S. survey of PHP oysters demonstrated that Vp and
Vv levels were typically orders of magnitude lower than 30
MPN/g, and I am not aware of any definitive evidence that
vibriosis has been linked to PHP oysters (17).
Pre-harvest controls
Pre-harvest controls were not modeled in early Vibrio
risk assessments, as they were considered ineffective or
impractical for controlling naturally occurring pathogens
in oysters. The potential effectiveness of this approach to
reducing Vv levels was first demonstrated in the 1990s by

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Food Protection Trends July/August

relaying oysters to high salinity (>32 ppt) Gulf of Mexico
waters (39). Vv levels were consistently reduced to < 3
MPN/g within 17 days, but this approach has not been
commercialized. Pre-harvest controls were first implemented
in response to the 2004 Alaska outbreak linked to a warming
anomaly. In 2005, AK control authorities mandated lowering
gear containing oysters below the thermocline for a month
before entry into commerce when water temperatures
reached 15°C. Vp levels decreased by approximately one
log unit in oysters held below the thermocline (< 10°C),
compared with those in gear at the surface, and fewer
illnesses were reported than in 2004 under similar climate
conditions. Some harvesters in British Columbia, Canada
have adopted this approach to reduce Vp levels to < 100/g,
the Canadian Vp standard. Another approach, relocation of
oysters out of site, was implemented in Katama Bay, MA in
2015 after several years of reactionary closures for exceeding
NSSP illness allowances. Oysters moved into cooler Atlantic
Ocean waters outside of the bay prior to being sold have not
been linked to any illnesses, and Katama was not closed in
the three years afterwards. There is also growing evidence
that on-shore depuration of oysters in refrigerated seawater
(12.5°C) can reduce Vp levels by several log units within
a week. These findings have sparked industry interest, but
this approach has not yet been commercialized (49). Onshore depuration expands treatment options far beyond
refrigeration. Innovative approaches abound; physical
forces (UV, electrical, magnetic), chemical (antibiotics,
polyphenol) and biological agents (phage, bacteria) and
many more have been proposed to aid Vibrio depuration.
Global tracking of emergence and movement of Vp
outbreak strain
The key assumption of the VPRA that all pathogenic
strains with tdh are equally infectious has long since
been proven erroneous by orders of magnitude (33, 50).
Nearly 95% of predicted illnesses were attributed to the
Gulf, where tdh+ strains are more than ten-fold more
abundant than in other U.S. regions. While few reported
illnesses are associated annually with the Gulf harvest (3),
the introduction of the pandemic O3:K6 Vp strain into
Galveston Bay, TX in 1998 caused attack rates comparable
to that of norovirus, with hundreds of reported illnesses and
likely thousands of unreported illnesses (13). Improving
systems for recognizing and tracking the emergence and
global spread of highly infectious outbreak strains is critical to
mitigating the Vp risk. FDA initiated this effort in 2008 with
the creation of a publicly accessible Multi-locus Sequence
Typing (MLST) site at Oxford (UK) University (26). The
site is transitioning toward Whole Genome Sequencing
(WGS), which provides greater resolution for distinguishing
strains and tracking evolution. State health departments
are sharing illness isolates with investigators doing cuttingedge research expanding knowledge on routes of spread

Food Protection Trends - July/August 2019

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