A Fluke, a Tapeworm, and a Roundworm Walk into a Sushi Bar...

I know what you are thinking. You are thinking, "I can't read this blogpost because I LOVE sushi and she WILL NOT ruin it for me!" Don't worry, I'm a big fan of sushi too, and I certainly don't want to diminish the amazingness that is this Japanese delicacy. Let me start off by saying that most of the time, especially here in the U.S. or in countries with well-regulated sushi bars (such as is the case in Japan), you are not at risk for contracting parasites from eating sushi. I'll end this post with a brief discussion about raw fish regulations just to ease your troubled mind. That being said, let's talk about the parasites that you can get from eating raw fish that hasn't been properly processed.

There are many different species of parasites that use fish as one of their hosts. Any of these parasites has the potential to infect humans if accidentally eaten. You can pick up a variety of flukes, tapeworms, and roundworms from a variety of marine and freshwater fish. To keep this blogpost to a reasonable size, we will only look at one representative from each of these three groups. We will talk about the fluke Clonorchis sinensis, the tapeworm Diphyllobothrium latum, and the roundworm Anisakis simplex.

Because we are looking at three different worms under the theme of "can be in sushi", I won't go into the detail that I normally do. I've never blogged about C. sinensis or about A. simplex, but you can find a previous blogpost about D. latum here. (Perhaps I'll blog about the other two in later posts.)

Clonorchis specimens from a patient.

These three parasites not only represent three different classes of organisms (and two different phyla if you are keeping up taxanomically), they also represent parasites found in different types of fish. C. sinensis is typically found in freshwater fish or in fish that prefer brackish waters (a mix between freshwater and marine ecosystems). This parasite is really only found in East Asia, where it utilizes a snail for its first intermediate host and a fish as its second intermediate host. The parasite has been known to be problematic in regions that import fish from East Asia in addition to popping up in local populations where it is endemic. It is also interesting to note that this parasite has been identified in mummies and coprolites from Korea. This tells us that humans have a long history of association with this particular parasite.

The fish tapeworm, D. latum, also boasts a long association with humans. Coprolites from both North America and from South America have tested positive for this parasite. Some of the earliest New World human populations were infected with fish tapeworms, which makes sense given their proximity to water sources and diets that often integrated fish. A diet that included fish is evidenced by the existence of bones and scales in macroscopic remains from coprolites as well as in artifacts constructed from fish bones. This parasite infects freshwater fish, such as trout, and can be found just about anywhere in the world. It is often diagnosed in campers/fishermen who do not properly cook their catches and in sweet little old Jewish ladies who taste test tainted gefilte fish before the dish is fully cooked.

Anisakis worms embedded in a herring.

The last of the three, A. simplex, is by far the most notorious. This roundworm is cosmopolitan in nature, like D. latum, but prefers for its hosts to be marine fish as opposed to freshwater fish. It is most often associated with mackerel and herring, which has earned it the common name of "herring worm". Apparently it can infect many other marine fish and even things like squid. It has been contracted from dishes around the world including sushi/sashimi, cod livers, fermented herring, and ceviche. Though a person can experience mild to moderate abdominal pain after contracting one of the other two parasites mentioned here, a person contracting A. simplex will experience much more violent abdominal pains. These pains are sudden and severe by comparison because these worms actually die when they fail at their attempts to burrow into your intestines.  This often instigates an IgE-mediated immune response (i.e. an allergic reaction, sometimes even anaphylaxis), making this parasite by far the most dangerous of the three discussed in this blogpost.

Ceviche

There are a great variety of symptoms to look out for if you think you may have picked up one of these parasites. Because they affect the digestive system, you may experience things like abdominal pain, vomiting, nausea, loss of appetite, and diarrhea. The first of the three parasites, C. sinensis, primarily affects the liver and may lead to hepatomegaly (enlarged liver) and jaundice. The fish tapeworm, D. latum, can cause irritability or muscle weakness in addition to numbing or tingling of the skin. It may also manifest as an elevated heart rate. The most prominent symptom for an A. simplex infection is the sudden and severe abdominal pain. As the parasites die, they can also cause anaphylactic shock or they can leave behind intestinal granulomas, which many times mimic the symptoms of people with Crohn's disease.

Now that I have you thoroughly terrified, let's talk about how much effort we go through to prevent ourselves from being infected. In the U.S. (and probably many other places), we have regulations pertaining to the serving of raw fish. Raw fish, no matter where it comes from, must be processed to make sure that parasites are killed. This is done by freezing and/or treating with salt and/or chlorine. The FDA states that freezing temperatures and times vary with the nature of the fish to be frozen and the parasites to be killed. It seems that they recommend between -4 degrees F or less for 7 days and -31 degrees F or less for 15 hours for most cuts of fish. Thicker cuts need to be kept colder longer. The FDA goes on to say that brining and pickling are not safe ways to control for fish parasites as they are not effective methods for reducing parasite threats. Recent studies have shown that while not optimally effective alone, treatment of fish with chlorine in conjunction with ultrasound processing significantly reduces parasites in fish meat. Using an ultrasound for at least 30 minutes is another method for controlling for fish parasites that seems to work pretty well. The only other method that this author knows about is treating the meat with at least 15% NaCl (salt) after 7 days of storage. The paper I read about that bit pointed out that 20% NaCl was better and could be used after only 6 days of storage. I'm sure there are other methods, but these seem to be the most prominent as far as I can tell.

The risk of actually contracting these parasites in the United States is low. The liver fluke (C. sinensis) is extremely rare in the U.S. with most reported cases demonstrating patients who contracted the parasite in another country before coming to the US. Infections with D. latum are also rare in the U.S. despite once being common in people living around the Great Lakes. Recent cases have popped up from the West Coast, but are still not very prevalent. There are less than 10 cases of anisakiasis reported each year in the U.S.

The other bit of good news is that if you happen to contract any of these by some off chance of bad luck, they are almost never fatal. Additionally, they are rather easy to treat with drugs readily available here in the states.

The Moral of the Story
Don't ever let anyone make you feel bad for eating sushi...unless you are in a country with poor food regulations and the food looks sketchy, then you should definitely not eat the sushi. Use good judgement and know the warning signs just in case. You should be able to get plenty of good sushi, sashimi, gefilte fish, and ceviche here in the US of A since we make it a point to be especially careful when handling raw fish. Go celebrate with a dragon roll or some ebi!!!

Mmmmm!!!