What's In *Your* Sushi?

By Carl Zimmer | August 4, 2008 11:58 am

Martijn ter Haar, a person from the Netherlands I’ve never met, clearly knows what sort of movies I like to watch. The ones with big parasitic worms crawling around inside a sealed box of fish at the supermarket. Warning: Safe for work, not safe for lunch.

This bad boy is Anisakis, a worm that would much rather be off in the sea than in your gut. Its eggs float through alll the world’s oceans until they’re scarfed up by a crustacean, inside of which they mature into a new larval stage. Their next host is the fish that eats the crustacean–they escape from its gut and drill into its muscle. Finally, the fish is eaten by a dolphin or a seal, whereupon the worm becomes an adult and lives  harmlessly and happily, churning out eggs that the predators kindly squirt out with their poop. In other words, the good life.

But if some thoughtless fisherman trawls up an Anisakis-infected fish, and you end up eating that fish raw–as sushi or herring–or without cooking it thoroughly, the parasite is doomed. Inside a human host it will die–often without the host even knowing what happened. But sometimes things get a little nasty for the host, too. You may feel a tingle in your throat and cough the worms up. Or you may feel like your appendix just popped. What’s happening is that the poor confused Anisakis is drilling its way out of the gut and wandering around the abdominal cavity. Your immune system may go wild over this visitor and launch a frenzied allergic reaction. Even when there’s no worm crawling around in the fish, you can get an allergic reaction–a few loose proteins left behind are enough. If Anisakis makes you sick, a doctor may have to go in and fish the critter out (as seen in this even more disgusting video from Dr. Peter Kelsey of Harvard).

Out in the ocean, by contrast, Anisakis doesn’t do much harm to its hosts, and they don’t harm it. It’s a relationship fine-tuned by millions of years of coevolution. And when Anisakis populations drop, it’s a sign that the entire food web is not well–thanks to overfishing or pollution. Obviously nobody wants to get to be close friends with Anisakis (note: don’t eat sushi out of the back of a truck). But how about a little sympathy for the ocean-wandering worm trapped in the supermarket?

[Update:  A commenter asks, "What is the actual rate of 'things getting a little nasty' for humans"? Severe cases of Anisakiosis are rare, as detailed here. But in Madrid, where people eat a lot of raw fish, 12% of people test positive to exposure--either to the worm or to its proteins.]

 [Image from Dr. Simonetta Mattiucci's web page]

CATEGORIZED UNDER: The Parasite Files

Comments (17)

Links to this Post

  1. Around the Blogs | Bitesize Bio | August 15, 2008
  2. Elsewhere / Headspace | March 21, 2009
  1. So, what is the actual rate of “things getting a little nasty” for humans? I was unable to visit some of the links due to a corporate firewall. Is this one of those things that happens to less than one in a million sushi eaters, or is it an actual concern?

    I for one eat sushi as much as my budget will allow. I love the stuff! :)

  2. Oh, great. Thank you SOOO much for giving me nightmares about my last sushi dinner… :) With pictures, too!

  3. Joy

    I *knew* there was a reason why I didn’t like sushi! *ducks to avoid seaweed wrappers and rice* That’s one cool parasite, though.

  4. Michele

    I was going to have sushi for dinner but I think I will reconsider. I will have to put the images out of my mind before I have sushi again!

  5. sailor

    “But in Madrid, where people eat a lot of raw fish, 12% of people test positive to exposure–either to the worm or to its proteins.]”
    Yes, but how many of those who test positive do so without painful symptoms?
    Also how big are those critters? easy to see if you are catching and preparing the fish yourself?
    I practically live on tuna, quite a lot raw, some times of the year. So far I have not felt a thing.

  6. sushi san

    In the UK all fish to be served raw as in sushi, must of been previously frozen. Freezing kills the Anisakis. So really the only thing you need to do is make sure that the restaurant is serving previously frozen fish. Saying that – not many restaurants use previously frozen fish.

  7. Oh, grow up the lot of you. Years ago I ate sushi while reading Carl’s book Parasite Rex, and it hardly put me off my meal. (Though to be fair, IIRC the book was going on about tapeworms rather than Anisakis at that point.)

    Anyway, about the sushi worms. Lots of the fish used in most sushi joints was previously frozen. Maybe that means it’s not truly right-off-the-boat-fresh, but there’s an upside. Freezing kills the parasites… if the fish was frozen long enough… most of the time, anyway. So you’re probably OK. Itadakimasu!

  8. Neil Fiertel

    I believe that all sushi sold in North America is supposed to be flash frozen to some specific temperature and held that way for a predetermined number of days to prevent parasite infections. I hope I am right about that! I know that Japanese visitors to our sushi restaurants make faces at the fact that our sushi is not fresh as it were. I can live with it ,considering..

  9. sailor

    Well in case anyone else is interested and cannot easily freeze stuff (agreed the best solution if you can, but they have to be frozen to minus 20C), there was a photo of those little worms with a centimeter measure. The would appear to be about 2 cm long, so quite visible. Good inspection would help. Heating to 60C would also do it, but the sushi would not taste the same.

  10. trrll

    I’ve seen these squirming in a package of fresh fish that I brought home from the supermarket. I figured that they were probably harmless with cooking, but somehow, it kinda spoiled my appetite.

  11. Brian

    To sort out the risks from sushi, might be more relevant to have the data on how what percentage of people in Japan test positive to exposure to the parasite, as opposed to the Madrid ceviche eaters.

    In Canada, it is mandatory for all raw fish to be flash frozen if served in restaurants. Anyone who has ever caught a fresh halibut and tried to cook it without freezing will have experienced the worms crawling out of the cooking flesh.

  12. Ate one, had that.

    Just doing some post-traumatic Anisakis browsing and thought I’d share. If this ever happens to you, the ‘gross factor’ will be the least of your problems. I had excruciating, stabbing, crampy, bloating pain all over my belly for FOUR HOURS the morning after a sushi meal (U.S. restaurant). I couldn’t walk, was vomiting often, and was yelling in pain. I had no fever, no diarrhea. Ruled out gastroenteritis and suspecting a kidney stone, I ended up in the emergency room on anti-nausea meds, morphine, and dilaudid. CT scan negative, very, very high WBC, inconclusive findings… Six hours later, I’m home with some percoset. The next two days were spent in bed in a fog of slowly subsiding pain. Two days ago I really wanted to figure it out and started googling “sick sushi”, etc. 6 1/2 days after the sushi, I finally overcame the constipation from the narcotics and flushed the worm. (Unbelievably, I saw it and it looked like the one in the video, but dead.)

    So there’s a head’s up. Regardless, that much abdominal pain is always an emergency and can indicate other life-threatening conditions.

  13. Really am fond of this article. I’ll be linking back to this from my own website.

  14. Bree lynch

    To all of you who are trying to shut people up who are afraid by making comments about how rare it is to come across one of these little guys ill tell you for it being so rare I must be extra special because I’ve had TWO, on two separate occasions in a piece of white fish I got in a sushi dish at Hana Japanese steak house in leesville Louisiana. Needless to say i WON’T be going back there, I would still check every piece of fish I come across, I think everyone should. These guys were wiggly, definitely alive, worms!.

  15. Kim

    lol in Us or Canada, all the foods got security-checked before selling. if you consider salmon has tapeworms then other seafoods would have tapeworm. Tapeworms dont live if fish but if ur food, u dont keep it clean anything can happen to ur food. Your salmon has to be frozen then later u can actually eat them.

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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.

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