Food Protection Trends - July/August 2019 - 338

GeNeral iNtereSt paper

Managing Vibrio risk in oysters
angelo DePaola*
Angelo DePaola Consulting 12719 Dauphin Island Parkway, Coden, AL 36523, USA

SUMMARY
Vibriosis has increased more than any other illness caused
by a pathogen in the United States (U.S.) food supply since
the Centers for Disease Control and Prevention (CDC)
FoodNet program began in 1996. Foodborne Vibriosis is
almost exclusively associated with seafood, and most cases
are linked to raw oyster consumption. Vibrio parahaemolyticus
(Vp) and V. vulnificus (Vv), which are naturally occurring
bacteria inhabiting coastal areas around the world, are the
leading causes of seafood-associated illnesses and deaths,
respectively, in the U.S. Forces of nature and man, including
bacterial evolution, climate change, risky food-handling
practices, shifting geographical and seasonal production
and consumption patterns, expanding globalization,
improved recognition/diagnosis and fractured regulatory
oversight have spawned a perfect storm of ever-increasing
rates of vibriosis reported in the U.S. Control authorities in
states with the greatest morbidity and mortality in which
rapid cooling mandates have been implemented have seen
sustained Vv illness reduction burdens for five years and
appear to be driving down Vp illnesses as well in recent years.
To better inform risk management, national and international
risk assessments have generated forecasting and cold chain
confirmation tools calibrated to risk. Emerging post-harvest
processing and pre-harvest controls could further reduce
or practically eliminate the vibriosis risk associated with
consumption of raw oysters.
OVERVIEW
Foodborne vibriosis is almost exclusively associated
with seafood, and most illnesses are linked to raw oyster
consumption (30). Vibrio parahaemolyticus (Vp) and V.
vulnificus (Vv), naturally occurring bacteria inhabiting coastal
areas around the world (30, 46), are the leading causes of
seafood-associated illnesses and deaths, respectively, in the
U.S. (29). Vibriosis has increased more than illnesses caused
by any other pathogen in the United States (U.S.) food
supply since the Centers for Disease Control and Prevention
(CDC) FoodNet program began in 1996 (7).
Vp was first reported as a cause of disease in Japan
during an outbreak in the 1950s (24). Vp causes diarrhea
in healthy individuals and, occasionally, bloody diarrhea.
In rare cases, infections can become septic in those with
serious underlying chronic illnesses (50). Early on, it was
noted that most clinical Vp strains produced a thermostable

direct hemolysin (TDH) that was relatively rare (< 1%) in
environmental or seafood isolates (50). TDH production
was universally considered a pathogenicity marker and some
evidence indicated that it was a major virulence factor. TDH
is coded by the gene tdh, which is the targeted by a variety of
molecular assays to characterize pathogenicity of an isolate or
enrichment for presence or most probable number (MPN)
analysis (1). Subsequently a TDH-related hemolysin (TRH)
was identified and found to be coded by the trh gene. U.S.
outbreaks in the 1970s were caused by cross-contamination
of cooked shrimp and crabs by raw product (12). Reports
of sporadic cases linked to raw oyster consumption began in
the 1990s, and the first oyster-associated outbreak occurred
in the Pacific Northwest (NW) in 1997 and again in 1998
(4). However, the wakeup call occurred in 1998, when
consumption of raw Galveston Bay, TX oysters caused the
largest Vp outbreak in U.S. history (13). The culprit was the
O3:K6 (Sequence type 3) strain that had emerged in 1996
in Asia; the outbreak spread to North America, signaling the
first Vp Pandemic (35). Within a few years, the pandemic
had spread globally, with illnesses associated with food
reported in every continent except Antarctica and Australia
(34). Chile was the most severely affected country, with
thousands of cases reported in 2004, mostly associated with
consumption of raw mussels (25).
Vv was first reported in 1979 as a lactose-positive,
halophilic pathogenic bacterium occurring along the U.S.
Gulf Coast (2). Two syndromes were described, one
associated with wound exposure to seawater and the other
a primary septicemia from consumption of raw oysters
harvested from the Gulf Coast during warm months. The
primary septicemia associated with Vv has the highest case
fatality rate (35-50%) of any foodborne disease but is limited
to individuals with underlying chronic illnesses, especially
liver disorders (23, 29). Vv ingestion does not cause serious
illness in healthy individuals and is not associated with
outbreaks. Other pathogenic Vibrio spp. such as V. chlolerae
occur in oysters but are far less consequential for morbidity
and mortality in the U.S. than Vp and Vv (29). Although
this article focuses on Vp and Vv, most of the information
is applicable to these other pathogenic vibrios as well.
Sanitary controls that form the backbone of shellfish safety
globally do not protect the public against pathogenic vibrios.
Instead, control of oyster-associated vibriosis in the U.S.
is based on risk assessments conducted by the U.S. Food

*Author for correspondence: Phone: +1 251.455.3035; Email: andydepaola@gmail.com

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