With a squeeze of lemon and a dash of hot sauce, raw oysters are a winter delicacy. Unfortunately, a recent letter  in the New England Journal of Medicine warns, strains of Vibrio parahaemolyticus are contaminating some of them harvested from parts of the Atlantic Ocean.

What is Vibrio parahaemolyticus?

Vibrio is a group of bacteria that cause a variety of digestive issues. Other types of Vibrio cause severe illnesses such as cholera or blood infection. The bacteria noted in the letter, Vibrio parahaemolyticus, causes comparatively milder symptoms: diarrhea, vomiting, abdominal cramping, nausea, fever, and chills. Oysters and other shellfish become contaminated because V. parahaemolyticus naturally occurs in the waters where they live. These shellfish are the usual cause of the illness. Vibriosis can be especially dangerous for people with weakened immune systems but is rarely fatal for healthy people. The symptoms usually go away on their own within three days. (People taking antacids are more susceptible because stomach acid can help destroy bacteria, and antacids weaken the stomach acid.) The Centers for Disease Control and Prevention estimates there are around 4,500 cases of vibriosis caused by the species parahaemolyticus each year.

How can I avoid getting sick?

Simple: cook your shellfish. Instead of slurping them down raw, try oysters Rockefeller or an oyster po’boy instead.

Is Vibrio parahaemolyticus changing?

There are many strains of V. parahaemolyticus. Until last year, two of the stronger strains, O4:K12 and O4:KUT were problematic in the United States only along the coast of the Pacific Northwest. In 2012, however, O4:K12 and O4:KUT contaminated oysters in Oyster Bay Harbor, N.Y., off the coast of Spain, neither of which had been affected in the past. This summer, oysters and clams harvested from locations on the coast of Connecticut were recalled due to possible presence of V. parahaemolyticus.

Researchers aren’t sure how the bacteria arrived at these new locations. Perhaps contaminated oysters from somewhere else infected healthy beds. Or the bacteria may have traveled in ballast water released by ships.  Contamination also may be related to global climate change. Bacteria thrive in warmer waters. As ocean temperatures rise, the prevalence of V. parahaemolyticus increases.

Why is this important?

There's an adage about oysters: Only eat them in months with an R in the name. As the weather cools down, raw oysters become popular. But cooking is the only way to kill V. parahaemolyticus. Although vibriosis from this species generally is dangerous only for people with weakened immune systems, no one enjoys getting food poisoning.

In the mid-1990s, another strain of V. parahaemolyticus circuluated and caused a vibriosis pandemic that sickened people in India, Bangladesh, Taiwan, Laos, Japan, Thailand, Korea, and the United States. At this point it’s unclear whether O4:K12 and O4:KUT will remain in the Atlantic Ocean or spread to different places. Epidemiologists should be keeping an eye on its movement over the next few years. Consumers ought to be on the lookout for recalls. Ask where oysters were harvested.

And when you see a tray of plump oysters, think twice before you slurp them down. Maybe go for the shrimp cocktail instead.

Teagan Keating is a second year master's student at Drexel's School of Public Health. Find her on Twitter.

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