VIBRIO // the local food report

Oysters are big business on Cape Cod. Since 2005, oyster production in Massachusetts has grown by almost 300 percent. This year, somewhere between 15 and 20 million oysters are expected to be grown here. Mostly, we eat them raw, on the half shell. (Caviar and sour cream don't hurt.) And because of our tourism season on the Cape, we do a lot of this eating during the summer months.

In recent years, though, there's been an increase in the number of reported gastrointestinal illnesses linked to eating raw oysters. It isn't clear if this represents an actual increase in the number of people getting sick—for one thing, health officials estimate that for every reported case of Vibrio parahaemolyticus, or Vp, the culprit behind these illnesses—142 cases go unreported. For another, when almost 3 times as many oysters are being consumed, it's not surprising the number of people getting sick would increase accordingly. Furthermore, doctors are more aware of Vp, so they're testing for it more. 

But the rise has prompted state, federal, and local parties to start studying Vp a little more carefully. While we've known about the family of bacteria called vibrio for a while, we actually don't know all that much about our local species. There are other vibrios in other parts of the world—Vibrio vulnificus makes people much sicker and is common in warm saltwater climates, and Vibrio cholerae is the culprit behind cholera and is spread through contaminated drinking water. Vp is found in saltwater, and while it's possible to get infected with the bacteria from simply swimming with a cut, it's very unlikely. The vast majority of cases are associated with eating raw seafood, and roughly 70 percent occur between May and October, in the warmer months. High levels of bacteria are linked to warmer weather—studies show that the parts per million double every 15 minutes at air temperatures above roughly 70-75 degrees. People who ingest Vp are more likely to become ill if they have a compromised immune system.

Beyond this, we don't know all that much about Vp. We know that there are different strains of Vp, but we don't know if they all make you sick, or if only some of them do. The FDA recently hosted its biannual shellfish sanitation conference in Texas, and one major decision that came out of it was to put more resources toward studying the bacteria. 

In the meantime, because of the increase in the number of reported Vp cases in the past few years, last year the state changed the regulations for how long oysters can be out of the water between harvesting and icing, and this spring they are expected to come out with an updated Vp control plan for the summer season. I'll keep you posted once the new regulations come out.

And just to put the odds in perspective: it's still much less risky to eat a raw oyster from local waters than it is to get in a car.