Aw Schucks: When the world is NOT your oyster!

This is the eleventh of 16 student posts, guest-authored by Ilze Berzins. 

When one hears the words “food-borne illness”, what comes to mind?  For me, I think of a medium rare, pink, juicy hamburger, or something like potato salad that may be made with mayonnaise containing raw eggs, or maybe a fresh green garden salad sprinkled with sprouts.   I am sure we have all heard about outbreaks or recalls surrounding these familiar dishes.  And the usual suspects contaminating these food stuffs are often bacteria with familiar names such as E.coli or Salmonella.   The fear of getting “food poisoning” from these products may encourage us to buy fresh seafood from the grocery store, or order the shrimp or salmon when out at a restaurant instead of steak, thinking seafood might be a safer bet.  And of course, aren’t health benefits from seafood supposed to be an added bonus?  But not so fast!    Believe it or not, one can also become ill when eating, or even just handling, seafood.  Gives the phrase “fresh catch of the day” a whole new meaning!

Seafood is a broad term…and no…it is not the game you played as a child that while eating you asked people if they wanted seafood and you opened your mouth (get it?  SEE food?).   Seafood refers to all species of fish and shellfish, coming from both fresh and saltwater sources.  Salmon, snapper, trout, tuna, perch, tilapia are just a few of the fish that come to mind.  But what is shellfish?  Shellfish consists of a mixed group of mollusks and crustaceans that have a shell or shell-like body covering known as an exoskeleton.  For the mollusks, this would include oysters, clams, mussels, abalone, and scallops.  And although they don’t have an obvious shell (it is very small or absent), squid and octopus are also in this group.  Crustaceans that are edible shellfish include lobsters, shrimp, crabs, and crayfish.  Yum.

What can make you sick from eating seafood?  There are several main categories including infectious pathogens (parasites, viruses, and bacteria – oh, my!), contaminants (heavy metals such as mercury in the form of methymercury), and drug and chemical residues.  These can be synthetic compounds but can also come from natural bio-toxins.  One such compound is found in the tissues of small aquatic organisms in overgrowth situations or “blooms” such as in red tide events.

Eating raw or undercooked seafood definitely increases the risk of a food-borne illness, but even just handling or processing seafood can cause problems for humans (7, 10).  Topically acquired or contact zoonoses can be acquired through stings, bites, or spine/pincer puncture wounds.  Zoonoses are illnesses transmitted from animals to humans.  Some groups at risk include commercial fishermen, fish farmers (aquaculture), hobbyists, aquarists working in public display facilities, fish food processors, chefs, and even you when you are preparing dinner!  Bacteria are the primary causative agents for zoonotic infections through a contact route. Viruses, fungi, and parasites contact infections have rarely been reported (7, 8).   However, I am not quite sure how to classify a recent case report on a “parasite-like” infection by the squid Todarodes pacificus (5).   A 63-yr-old Korean woman was reported to have experienced severe pain in her oral cavity immediately after eating a portion of parboiled squid along with its internal organs. She did not swallow the portion, but spat it out immediately. She complained of a pricking and foreign-body sensation in the oral cavity.   Twelve small, white spindle-shaped, bug-like organisms stuck in the mucous membrane of the tongue, cheek, and gingival were completely removed. On the basis of their morphology and the presence of sperm, the foreign bodies were identified as squid spermatophores!!  Not sure if I would call that “parasite-like”?!   I digress!  Let’s focus!

There are lots of species of seafood and many reports of associated food-borne illnesses (1,2).  Where should we start?  In this blog, I will focus on a group within the mollusks, the bivalves.  Bivalves are mollusks with two shells including oysters, clams, and mussels.   I want to focus on this group because given the propensity of humans to eat raw oysters, eating them raw elevates the risk of acquiring a viral or bacterial food-borne illness.   In subsequent blogs, I will review other members of the mollusks, then on to crustaceans, and of course, fish, so stay tuned!

Safety issues for bivalves center around two categories; first, the quality of water in which these animals are grown in, and second, the conditions under which they are harvested, processed, and distributed (1,2,8).  In 1925, the Bureau of Chemistry (now the United States Food and Drug Administration) met to establish guidelines for the oyster industry.  Attendees resolved to control “the beds on which shellfish are grown” and “the plants in which the shellfish are shucked” (shucking refers to removing the animal from its shell)(11).  In the late 1800s and early 1900s, most outbreaks of seafood food-borne illness resulted from sewage contamination in the areas where shellfish were grown (11).  From the late 1970s through today, there has been an increased incidence of disease associated with naturally occurring shellfish pathogens (11).  There are some that think this trend is suggestive of emerging environmental problems such as increasing water temperatures associated climate change.  While the species of pathogen isolated in bivalves may vary with salinity of the water the oysters or clams grow in, it is noted that all of these pathogens increase in numbers with an increase in water temperature (11).

How often do we see illness? In a 10 year study on mollusk shellfish food-borne illness, a total of 2795 cases, with 96 deaths, were documented (11).  Oysters accounted for the highest proportion of cases (49%) and death (97%).  Shellfish from the East and Gulf costs were equally likely to cause disease.  Most from the East Coast were viral contaminated clams, where as on the Gulf Coast, most cases were oysters infected with non-cholera bacteria in the family Vibronaceae.

The naturally occurring bacterial pathogens of concern include Vibrio vulnificus, V. parahaemolyticus, V. mimicus, V. hollisae, and V. furnissi.   When eating raw oysters, one might become exposed to V. vulnificus.  Incubation can take 1-5 days, though the median time is around 28 hours (3).  Symptoms include high fever, chills, nausea, vomiting, diarrhea, and abdominal pain.  The ill person can become rapidly dehydrated, and the infection can become systemic (body wide) if bacteria enter the blood.  V. vulnificus in some cases can multiply so rapidly that blood vessels may become infected (known as vasculitis), and blood clots can develop which may lead to digit or limb amputation, or even death (3)!

How about from contaminated sources? Viral and bacterial enteric pathogens of public health concern are caused by fecal contamination of the waters from which molluscan shellfish are harvested and of the environment in which they are processed.  Pathogens of concern that have been associated with disease include human enteric viruses; hepatitis A, non-A, non-B enteral hepatitis (hepatitis E), unclassified viruses; and such bacteria as Salmonella, Shigella, Campylobacter jejuni, and pathogenic Escherichia coli (11). The group of unclassified viruses includes the Norwalk and Norwalk-like viruses.

Do we stop eating seafood?  NO!!!  But measures to increase the safety of raw shellfish could be, and are being implemented (11).  These measures might include policies on when and where to collect (season, time/temperature), requiring harvest areas near sewage treatment facility outflows to be more closely evaluated (9), and continuing to improvement the technology of sewage treatment.  Some aquaculture techniques have included lowering temperatures and controlled purification which involves disinfecting (or purifying) the water in which the bivalves are grown. This may reduce gut bacterial levels but it does not entirely remove them.   When processing shellfish, facilities can employ techniques such as rapid chilling and cold storage to reduce overgrowth and contamination.  Irradiation of the harvested product has been suggested but has yet to be approved. Laboratory tests to detect contamination are moving beyond traditional culture techniques and include identifying pathogens based on their individual DNA using molecular methods (6).   Focusing on identifying groups at risk such as those individuals who are immunocompromised (young, old, ill) and concentrating on the more common agents could help resources go further.

Consumer education is also very important. Prevention strategies should focus on preparation and consumption.  Cooking oysters to an internal temperature of 85-90°C will destroy both viruses and bacteria of public health concern. But what if you want to eat them raw?  Maybe think about cooking oysters during periods of warm weather and go for raw during colder times of the year!  An excellent, comprehensive web site about food safety issues is .  It addresses a variety of audiences including consumers, food professionals and industry workers, and provides the latest news, safety alerts, recalls, and health warnings.   The site also allows for individuals to report cases, and it has information for physicians on how to diagnose and manage food-borne illnesses.  Other useful sites include and

With any food product, it is probably wise to stop and ask “To Eat or Not To Eat?”(4). Stay safe!


  1. Butt, A.A., Aldridge, K.E., and Sanders, C.V.  2004.  Infections Related to the Ingestion of Seafood Part I: Viral and Bacterial Infections.  Lancet Infect. Dis. 4: 201-212.
  2. Butt, A.A., Aldridge, K.E., and Sanders, C.V.  2004.  Infections Related to the Ingestion of Seafood Part II: Parasitic Infections and Food Safety.   Lancet 4: 294-300.
  3. Daniels, N.A.  2011.  Vibrio vulnificus Oysters: Pearls and Perils.  Clin. Infect. Dis. 52 (6):788-792.
  4. Galson, S.K.  2009.  To Eat or Not to Eat:  Food Safety in the United States: Practical Applications from the Surgeon General.   J. Am. Dietetic Assc. 5(20): 1142.
  5. Park, G.M., Kim, J.Y., Kim, J.H., Huh, J.K.  2012.  Penetration of the Oral Mucosa by Parasite-Like Sperm Bags of Squid: A Case Report in a Korean Woman.  J. Parasit. 98 (1): 222-223.
  7. Huss, H. H., Reilly, A., and Ben-Embarek, P.K.  2000.  Prevention and control of hazards in seafood. Food Control 11:149-156.
  8. Iwamoto, M., Ayers, T., Mahon, B.E., and Swerdlow, D.L. 2010. Epidemiology of Seafood-Associated Infections in the United States.  Clin. Microbiol. Reviews 23 (2): 399-411.
  9. Rabinowitz, P.M., Gordon, Z., Holmes, R., Taylor, B., Wilcox, M., Chudnov, D., Nadkarni, P. and Dein, F.J. 2005. Animals as Sentinels of Human Environmental Health Hazards: An Evidence-Based Analysis. EcoHealth 2: 26-37.
  10. Vinh, D.C., Mubareka, S., Fatoye, B., Plourde, P., and Orr, P.  2006.  Vibrio vulnificus Septicemia after Handling Tilapia sp. Fish: A Canadian Case Report and Review.  Can. J. Infect. Dis. Med. Microbiol. 17(2): 129-132.
  11. Wittman, R.J. and Flick, G.J.  1995.  Microbial Contamination of Shellfish: Prevalence, Risk to Human Health, and Control Strategies.  Annu. Rev. Public Health 16: 123-40.


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