Published: Fri, April 07, 2017
World | By Lorena Waters

Beyond Salmon: Parasites in Fish, Part 2 - Anisakis and Tapeworm

Beyond Salmon: Parasites in Fish, Part 2 - Anisakis and Tapeworm

Now that we know all about cod worms, we are going to venture into the world of the most dangerous parasites: anisakis simplex and tapeworm. If you are cooking fish, you need not worry. According to FDA, you are 100% safe if the fish reached an internal temperature of 140F. Surviving the human intestinal track is not easy and requires that anisakis and tapeworm be at full strength. So, if you "only" raise the internal temperature to 120F, a parasite might survive (if it is positioned in the middle of the fish fillet), but it will be so weak that it will most likely die shortly after reaching your stomach.

Anisakis simplex is most common in fresh water and anadromous fish, like wild salmon, which are born in fresh water, migrate to the ocean, then return to fresh water to reproduce. It is also common in certain small salt water fish, such as herrings and sardines. However, anisakis is rare in other salt water fish, such as tuna, swordfish and farm-raised salmon. Just like cod worm, it originates in seals. Tapeworm is mostly found in Pacific wild salmon and fresh water fish. It originates in bears and land mammals. They are fascinating organisms and you can read all about the anisakis life cycle and the tapeworm life cycle on wikipedia.

What used to bother me is the possibility of eating the eggs of these worms. Would you like to see them in Spanish? Dr. Palm, from the Institute for Zoomorphology, Cell Biology and Parasitology in Düsseldorf, Germany, put my worries to rest by explaining the life cycle of these worms. While anisakis and tapeworm are in fish, they are in larvae form (not egg form). They can not reproduce until they find a mammal host (in the case of anisakis and cod worm, it has to be a marine mammal like to seal, so they can not reproduce in a human), and tapeworms rarely make it into humans .

Are you ready to swear of sushi yet? Not so fast. If you are a US resident, keep in mind that you live in a country that just threw away every single bag of spinach because of E.Coli threat. You do not think FDA would allow anything remotely dangerous to be served to the US public, do you? That's why FDA requires all fish with a potential hazard of parasites that is intended for raw consumption to be previously frozen. Freezing fish to -20ºC [-4ºF] or below for 7 days or -35ºC [-31ºF] or below for 15 hours will kill the parasites. Since the restaurants do not want to take any risk and want to avoid supply and demand fluctuations, most go even further and freeze all fish (not only the ones that could be infected) before serving them raw. So, all that "fresh" sushi you've been eating is frozen.

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Does salting fish like gravlax or curing it in acid like for ceviche kill the parasites? Maybe The salt or acid used for curing prevents bacteria from growing. It may also weaken or kill parasites. However, it's not a full-proof method. Opinions in the scientific literature vary to the degree to which salt / acid harms parasites. Most sources say that salting is more effective than curing in acid. Also, according to Dr. Gardner from Harold W. Manter Laboratory of Parasitology at the University of Nebraska, the acids in your stomach and intestines are at least as strong as lemon / lime juice. So, if you are making ceviche, I would suggest taking the same precautions as you would for the fish raw.

To put all this in perspective, the risk you take downhill skiing is an order of magnitude greater Than the risk of eating raw, not previously frozen fish. Whether that risk is worth it to you. I hate downhill skiing and I love raw fish, so you can guess which risks I choose to take. In fact, the risk of driving or just walking down the street is probably higher than the risk of eating raw fish. I know plenty of people who were in life-threatening car accidents, and I am still to meat to person who got infected by anisakis simplex or tapeworm. And let me tell you, I get more pleasure from a bowl of sashimi than my morning commute.

Do parasitologists eat sushi in spite of their intimate familiarity with parasites? Both Dr. Palm and Dr. Gardner said "yes". In fact, Dr. Palm just got back from Japan where he had really yummy not-previously-frozen sashimi.

What if you want to freeze your fish to eliminate even the slightest chance of getting sick from parasites? What's the best way to freeze fish? Is all frozen fish equal? Can you buy frozen tuna from Trader Joe's, defrost it, and voila - $ 5 / Lb sashimi is served? In my next post, I'll answer all these questions and more.

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