“He was a bold man that first ate an oyster."
- Jonathan Swift
“Oysters are top of the mountain for dangerous foods to eat. I have eaten them my entire life, and will continue. But you are putting yourself at risk when you do it.”
- Gary Oreal, manager, Rustic Inn Crabhouse, Fort Lauderdale
On August 18, 2022, the Florida Department of Health reported the deaths of two customers of the Rustic Inn Crabhouse in Fort Lauderdale who had consumed raw oysters contaminated with Vibrio vulnificus. Every year an estimated 80,000 Americans contract vibriosis—an intestinal disease caused by small bacteria called vibrios—and 100 die from it. Many people contract this disease from consuming raw oysters. In an effort to avoid getting sick, some oyster gourmands follow the age-old rule of thumb and only eat raw oysters in months that contain an “r.” In this Germ Gems post, I highlight vibriosis and norovirus infections, the main reasons I always give raw oysters a pass from May through August.
A few words about oysters. Oyster is the common name for a number of different families of salt-water bivalve mollusks that live in marine or brackish habitats. They feed on phytoplankton and are most active at temperatures ranging from the high 60s to the high 70s (20–26 °C). They are filter feeders, drawing water in over their gills through the beating of cilia. During warmer months (those not containing the letter “r”), oyster filtration increases and so does the oyster’s acquisition of microbes, such as, bacteria and viruses. These microbes can be passed on to humans who consume contaminated raw oysters.
Vibrios. In the 1970s, new threats associated with bacterial pathogens belonging to the genus Vibrio emerged. V. cholerae, V. vulnificus, and V. parahaemolyticus are the three most important vibrio species associated with human illness.
V. cholerae is the cause of cholera, an acute, severe diarrheal illness caused by infection of the intestine with the toxigenic bacterium V. cholerae serogroup O1 or O139. An estimated 1.3 to 4 million people around the world get cholera each year, and 21,000 to 143,000 people die from it. Drinking water contaminated by human feces is the main means of transmission of this species. (I discussed this in my December 1, 2019 Germ Gems post, “Cholera: When Water Turns Deadly.”)
Consuming raw shellfish contaminated with V. parahaemolyticus and V. vulnificus causes the illness vibriosis. Because oysters containing these bacteria don’t look, smell, or taste different from any other oysters, a person has no warning that the oyster is contaminated. Fortunately, most Vibrio infections from consuming contaminated oysters result in mild illness, including diarrhea and vomiting and specific treatment isn’t necessary. Instead, drinking plenty of liquids to replace fluid lost through diarrhea is advised.
Some people infected with V. vulnificus, however, can get very sick. In fact, about 20% of those with V. vulnificus infection die. The risk of fatal V. vulnificus infection is increased in people with any of the following health conditions:
Liver disease
Excessive alcohol intake
Diabetes
HIV infection
Chronic bowel and stomach diseases
Cancer (including lymphoma, leukemia, Hodgkin’s disease)
Hemochromatosis/hemosiderosis (abnormal iron metabolism)
Steroid dependency (as used for conditions such as emphysema and chronic obstructive pulmonary disease)
Achlorhydria (a condition in which the normal acidity of the stomach is reduced or absent) - or
Any illness or medical treatment that results in a weakened immune system.
Older adults are more likely to have one or more of the above conditions, and if so, they should be advised against ever eating raw oysters or clams.
In fatal cases, bloodstream invasion (septicemia) and severe skin blistering occur. (In addition to acquiring V. vulnificus by eating contaminated oysters, bacteria may enter the body via skin wounds.) If severe diarrhea or fever persists, contact your primary care doctor as hospitalization and antibiotic therapy may be required.
Virus infections. In the past, hepatitis A virus was commonly acquired by eating raw shellfish. Routine immunization with hepatitis A vaccine has all but eliminated this pathogen from consideration of infections from contaminated oysters. Now, another group of viruses belonging to the genus Norovirus has stepped in to fill this vacuum.
Globally, norovirus infections are a huge problem (researchers estimate that at least half of all outbreaks of gastroenteritis around the world are caused by noroviruses (NoVs). And NoVs are the leading cause of nonbacterial illnesses in shellfish consumers.
NoV illnesses from contaminated shellfish are seasonally related, occurring with higher frequency from late fall through winter. (Increased stability of viruses at lower water temperatures appears to favor this seasonality.) Typical symptoms of a NoV infection include diarrhea, vomiting, fever, headache, and muscle aches. The illness is usually self-limiting, with symptoms lasting between 24 to 48 hours.
On April 7, 2022, the Centers for Disease Control and Prevention (CDC) reported a multi-state outbreak of NoV infections in 103 people traced to raw oysters coming from a distributor in British Columbia. By June the number of cases increased to 192 detected in 13 states. The CDC advised restaurants and retailers not to serve potentially contaminated raw oysters from Canada. In addition, it urged people to thoroughly cook oysters before eating them along with employing rigorous hand-washing and sanitizing potentially contaminated surfaces and laundry.
Raw appeal. What is it about raw oysters that make them so appealing? A recent study suggests it all comes down to their basic savory taste, better known as umami. Researchers from the University of Copenhagen say perfect pairings like champagne and oysters can result in “uncanny umami synergy,” one of the five basic tastes along with sweet, bitter, salty, and sour. While it might not be “uncanny umami synergy,” I’d substitute beer for champagne. And since it’s soon to be September (a month with an “r”), I’ll indulge and have a few raw oysters with my beer.
