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.
Becoming a host to anisakis worms by eating them live can make you very sick---this disease is known as anisakiasis. According to wikipedia, its symptoms include violent abdominal pain, nausea, and vomiting. "Occasionally the larvae are coughed up. If the larvae pass into the bowel, a severe eosinophilic granulomatous response may also occur 1 to 2 weeks following infection, causing symptoms mimicking Crohn's disease." This isn't a life threatening disease (unless you have a weakened immune system) and is quite common in Japan where consumption of raw fish (that isn't previously frozen) is more wide spread. Unlike anisakis, tapeworms don't always manifest themselves with clear symptoms and can live in humans for decades if untreated, resulting in weight loss, Vitamin B12 deficiency, and potential anemia.
What used to bother me is the possibility of eating the eggs of these worms. Wouldn't they be too small for me to see, and what would happen if I eat them? 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't reproduce until they find a mammal host (in the case of anisakis and cod worm, it has to be a marine mammal like a seal, so they can't 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 don't 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 don't want to take any risk and want to avoid supply and demand price 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 previously frozen.
What's counter-intuitive to most cooks is that farm-raised salmon is much safer to eat raw than wild salmon. Farm-raised salmon is served pellet food, which is ground-up, processed fish meat. Any parasites in the fish meat are killed in the processing and grinding stages. Since salmon only obtains dangerous to humans parasites via food, farm-raised salmon simply isn't exposed to them. So, next time you use salmon for gravlax, tartar, or sashimi, go for the farm-raised stuff. When the Food and Agriculture Organization of the United Nations tested various fish for parasites in 2003, no parasites were found in any farm-raised salmon species , whereas parasites were frequently found in wild salmon (section 5.1.4 of Huss et al., 2003).
Does salting fish like for 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 as 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 eating 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 is up 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 yet to meat a person who got infected by anisakis simplex or tapeworm. And let me tell you, I get way 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 does all this mean to the home cook who wants to make sushi and ceviche? Buy your fish from a reputable source and use it that day if serving raw. Regardless of the parasite issue, fish that was not stored properly, or for too long, will grow bacteria and make you sick. Freezing fish that is not fresh will not help with the bacteria issue, but it will kill parasites. If you are not planning to freeze fish and want to eat it raw, I would limit your purchases to:
- Large Tuna (Yellowfin/Ahi, Big-eye, Bluefin)
- Arctic Char
- Kampachi (farm-raised)
- Farm-raised Altantic salmon (relatively low, but not insignificant risk of parasites)
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.
H. H. Huss, L. Ababouch, L. Gram. Assessment and Management of Seafood Safety and Quality. 2003. Food and Agriculture Organization of the United Nations. FAO Fisheries Technical Paper 444.