Raw Seafood

On the Half Shell

Analyze vibrio bacteria data from the Centers for Disease Control to identify trends in outbreaks.

For many along the coast, oysters are a holiday tradition. Revelers feast on oysters on the half shell, roasted oysters, oyster stuffing, and oyster stew. And, since December is an "R" month it's safe to eat oysters, right? Seafood handled and cooked properly is very safe to eat and is an excellent low fat source of protein and Omega-3 fatty acids. But improperly handled seafood and seafood eaten raw can pose a health risk, especially for those who already have health problems, regardless of whether or not it's a month with an "R" in it (one of the many myths associated with oysters).

In November 2004, two men in Florida became very ill from eating raw oysters. One died and the other had his leg amputated. The culprit of the oyster-related illnesses was Vibrio vulnificus, a naturally occurring bacterium found in warm, coastal waters. The Gulf of Mexico is particularly susceptible to high Vibrio concentrations. Because they like warm water, the bacteria are more plentiful in summer months, but are still present throughout the year.

A healthy person who ingests V. vulnificus may suffer no side effects at all or may suffer gastrointestinal problems such as vomiting and diarrhea. If the bacteria make their way into the bloodstream through the digestive tract, the person will suffer a more severe infection called septicemia. Symptoms of septicemia include gastrointestinal problems, fever, and a drop in blood pressure, which can lead to shock or even death. Septicemia is 80 times more likely to occur in people with health problems such as liver disease than in healthy people.

It is important to keep in mind that Vibrio infections are extremely rare. A 2003 survey showed that nearly 75% of Americans (approximately 210,000,000 people) ate some kind of seafood at least once a month, with shellfish ranking high on the list. In 2002, there were 452 reported cases of Vibrio infections (Center for Disease Control Vibrio Outbreak Summary). That means only ~ 0.0002% of people who ate seafood became ill from Vibrio bacteria. That's only slightly higher than your odds of getting struck by lightning (~ 0.00004%).

Rare as it may be, it's wise to take precautions to prevent Vibrio infections. Vibrio cannot be seen or smelled, so even hard-core raw oyster connoisseurs cannot tell an infected oyster from an uninfected oyster. And despite what some believe, adding hot sauce to a raw oyster will not kill Vibrio. Currently, the best way to kill the bacteria is to thoroughly cook your seafood. Researchers with Virginia Sea Grant are experimenting with a variety of treatments for oysters including high pressure, irradiation, and microwave, that may kill the bacteria without cooking the oyster. If successful, this will allow seafood lovers to safely enjoy raw oysters.

In the following data exercise, we will analyze Vibrio data to identify trends in outbreaks.

Data Activity
Download the Vibrio data table with data from the Center for Disease Control's Vibrio Outbreak Summaries. The total number of reported cases of Vibrio (a total of 9 species including V. vulnificus) was divided into two groups: Gulf Coast States (Florida, Alabama, Mississippi, Louisiana and Texas) and non-Gulf Coast States (the remaining 45 states). For seafood consumption data, percentages were based on known patients who ate one seafood item in the week prior to getting sick.

Use the data table to answer the following questions. To better visualize trends, graph the different variables (e.g., number of Vibrio cases over time).

  • Looking at the number of Vibrio cases and related deaths for 1999-2006 for Gulf Coast versus non-Gulf Coast states, what are the trends occurring over time? What are the changes over time in the total number of states with Vibrio? What might be some reasons for an apparent increase in the number of states with Vibrio cases, and the increase in the number of cases in non-Gulf Coast states? (Hint: think about changes in seafood consumption in non-coastal areas.)
  • Looking at seafood consumption, are people who eat raw or cooked seafood more likely to contract Vibrio? (For the percent who ate cooked seafood, subtract the percent who ate raw from 100%).
  • Is there a seafood type(s) that is more likely to cause a Vibrio infection? (No one seafood item is more likely to have the Vibrio bacteria than another. For instance, oysters are not more susceptible to the bacteria than shrimp.) So, why do certain seafood items appear more likely to cause a Vibrio illness? (Hint: think about how various seafood types are served.)

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Read Me

Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science


Grade Level



Lesson Time

45 min.-1.75 hr



  • Describe the vibrio bacterium.
  • Explain how vibrio effects humans, and how to avoid getting sick from vibrio.
  • Compare outbreak data and discuss outbreak differences among locations and types of seafood.


Vibro vulnificus, Septicemia


Materials Required

Virbrio data table


Natl. Science Standards

IK-1 IK-2 L5-1L5-5 L9-4 PSK-1 PS5-4 PS9-1



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