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The danger lurking in your sushi -

The World Health Organization (WHO) estimates that some 56 million cases of parasitic infections associated with the consumption of fish products occur annually.

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Health consequences -

When eating an infected piece of sushi, for example, you could develop a disease called Anisakiasis, running the risk of having the parasite attach to your gut lining and having to undergo surgery to remove it.

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What is Anisakiasis disease? -

Anisakiasis, also known as herring worm disease, is a disease caused by a parasite that lives on or inside another organism. It gets its name from the type of nematode larvae that causes the disease, which belongs to the genus Anisakis.

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Types of parasite -

While the genus Anisakis comprises nine species, three (Anisakis simplex, Anisakis pegreffii, and Anisakis physeteris) have been confirmed as zoonotic pathogens. This means they can cause diseases in humans and animals.

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How do they spread? -

Adult worms are found in the mucous membrane that lines the stomachs of marine mammals, like whales. The parasite’s eggs are expelled along with the animal’s feces and hatch in seawater. These eggs hatch into larvae in the water, which are eaten by crustaceans (think crabs and shrimp).

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How can they reach your sushi? -

By feeding on these crustaceans, fish and squid are infected with the parasite's larvae. They infect the fishes' guts, becoming embedded on their organs and eventually in their muscles.

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How can they reach your sushi? -

When we consume fish, squid, octopus, or other seafood containing third-stage Anisakis larvae that’s raw or undercooked, we can become accidental hosts.

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Human infection -

Once ingested, the worms can invade the gastrointestinal tract and settle in our stomach and sometimes the small intestine. Eventually, the parasite dies and produces an inflamed mass in the esophagus, stomach, or intestine.

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Human infection -

The infection can only occur by consuming raw or undercooked infected fish or squid. People cannot spread Anisakiasis to one another.

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Study -

A recent study published by the University of Washington highlighted the dramatic increase in Anisakis parasites since the 1970s. Researchers analyzed data from various locations around the world and found a staggering 283-fold increase in the worm's prevalence.

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What caused this increase? -

While scientists aren't entirely sure why Anisakis populations have exploded in recent decades, possible factors include climate change, increased nutrients from fertilizers, and growing populations of marine mammals.

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Most affected regions -

Japan has been particularly hard hit by Anisakis infections. According to estimates, the country experienced around 20,000 cases each year in 2018 and 2019.

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Most affected regions -

However, since sushi and other dishes featuring raw seafood, like poke bowls, became hugely popular in the West, cases have been on the rise in the US, Europe, and South America.

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You might not even know you’re infected -

Anisakiasis or anisakidosis is rarely diagnosed because most people assume they merely suffered a bad case of food poisoning.

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Symptoms -

People with Anisakiasis can experience symptoms such as abdominal pain, nausea, vomiting, abdominal distention, diarrhea, presence of blood and mucus in stool, and mild fever. Infection of the small intestine is less common, but when it occurs it can result in an inflammatory mass and symptoms similar to Crohn’s disease, which develops one to two weeks later.

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Symptoms -

If experiencing a tingling sensation during or after eating raw or undercooked fish or squid, beware. This sensation may actually be the worm moving in the mouth or throat! Individuals can often take the worm out of their mouths themselves or cough it up to prevent infection. Some people experience vomiting, which can often expel the worm from the body.

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Diagnosis -

A healthcare provider can diagnose Anisakiasis by endoscopy, radiography, or surgery if the worm is embedded. If feeling any of the symptoms, remember to let healthcare providers know if you ate undercooked fish or squid.

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Allergic reactions -

Rashes and itching can also occur. More infrequently, so can severe life-threatening allergic reactions leading to anaphylactic shock.

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Treatment and recovery -

Thankfully, most Anisakis infections resolve themselves on their own after a few days. However, in some cases doctors may need to remove the worm via endoscopy, which consists of the insertion of a long, thin tube with a camera into the stomach. In rare cases, surgery might be necessary.

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Complications -

In 2017, an expert team were summoned to treat a 32-year-old man in Lisbon, Portugal, after he had been suffering stomach pain, vomiting, and fever for a week.

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Complications -

They found parasite larvae on his gut lining after a blood test showed mild inflammation in the area. But it was only when the man said he had recently eaten sushi that doctors suspected he might have Anisakiasis.

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Complications -

The doctors performed an endoscopy on the man. They found the larvae firmly attached to an area of swollen and inflamed gut lining. A special kind of net had to be used to remove the larvae.

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Most affected seafood species -

Species that are most frequently parasitized include salmon, tuna, squid, cod, hake, mackerel, blue whiting, sardines, and anchovies. In the UK, Atlantic salmon and sea trout caught at sea or in rivers are also known to be at risk of being infected with the same species of parasite, NHS Choices says. According to WHO, cage-cultured sea bass, grouper, and snapper may harbor anisakis parasites.

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Other types of food to be careful with -

When we think about dishes made of raw or undercooked fish, sushi and sashimi are usually what comes to mind. But don’t forget that other delicacies also include raw seafood, such as ceviche and carpaccio.

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How to avoid them -

To make sure you keep getting your seafood fix, there are simple steps you can take. Find out how in the following slides.

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Clean before eating -

If cooking at home, make sure to remove the guts of the fish and clean it thoroughly, inspecting it for worms. Whenever possible, buy already cleaned and gutted fish and perform an extra check before consuming it.

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Cook your seafood thoroughly -

The US Food and Drug Administration (FDA) recommends cooking seafood to an internal temperature of at least 145°F (63°C). Other methods such as frying, baking, or grilling are also guaranteed to eliminate the parasites.

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Freeze seafood before eating it raw -

According to WHO, the parasites that cause Anisakiasis are killed by normal cooking and by freezing at -20°C (-4°F) for at least 24 hours. It’s important to note that cooking or freezing still may not protect against allergic reactions to ingesting the parasites. If you’re not sure that your freezer can go that low, it’s prudent to buy frozen fish. To increase consumer food safety, in some countries commercially prepared sushi is frozen before being sold.

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Avoid eating raw or undercooked fish and squid -

Bad news for sushi lovers, but abstaining from consuming any type of raw or undercooked seafood may be the best way to avoid getting infected with the parasites.

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Inspect sushi for worms before eating -

If cutting sushi out completely is not an option, worry not. Chelsea Wood, an assistant professor at the University of Washington and co-author of the study that found the staggering increase in parasites, admitted she enjoys eating sushi regularly. According to Wood, seafood processors and sushi chefs are well-practiced at spotting the worms in fish and picking them out before they reach customers in grocery stores, seafood markets, or sushi bars. But, some worms can make it past these screening steps. She recommends cutting each piece in half and looking for worms before eating it.

Sources: (CDC) (National Science Foundation) (University of Washington) (BBC) (Food Safety News)

See also: Is it still safe to eat tuna?

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