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Health Risks and Benefits of Eating Raw Fish

Health + Wellness
Health Risks and Benefits of Eating Raw Fish

By Dr. Atli Arnarson

There are several practical reasons people cook fish before eating it, rather than simply serving it raw.

Most importantly, cooking kills bacteria and parasites that can cause disease.

Nevertheless, some people prefer the texture and taste of raw fish. It is especially popular in Japan as part of dishes like sushi and sashimi.

But how safe is raw fish? This article reviews the risks and benefits.

Types of Raw Fish Dishes

Raw fish dishes are growing in popularity. Here are a few examples:

  • Sushi: A category of Japanese dishes, sushi is characterized by cooked, vinegared rice and various other ingredients, including raw fish.
  • Sashimi: Another Japanese dish that consists of finely sliced raw fish or meat.
  • Poke: A Hawaiian salad traditionally made with chunks of raw fish that are seasoned and mixed with vegetables.
  • Ceviche: A lightly marinated seafood dish popular in Latin America. It typically consists of raw fish cured in lemon or lime juice.
  • Carpaccio: Common in Italy, carpaccio is a dish originally consisting of finely sliced or pounded raw beef. The term may also cover similar dishes consisting of other types of raw meat or fish.
  • Koi pla: A Southeast Asian dish consisting of finely chopped raw fish mixed with lime juice and various other ingredients, including fish sauce, garlic, chilis, herbs and vegetables.
  • Soused herring: Marinated raw herring that is common in the Netherlands.
  • Gravlax: A Nordic dish made up of raw salmon cured in sugar, salt and dill. It is traditionally eaten with mustard sauce.

These dishes are an important part of food culture around the world.

Summary: Raw fish is a major ingredient in various dishes from around the world, including sushi, sashimi and ceviche.

Parasitic Infections From Raw Fish

A parasite is a plant or animal that feeds off another living organism, known as the host, without offering any benefits in return.

While some parasites do not cause any obvious acute symptoms, many may cause serious harm over the long term.

Parasitic infections in humans are a major health problem in many tropical countries. Many of them are transmitted by infected drinking water or improperly cooked food, including raw fish.

However, you can minimize this risk by buying raw fish from trusted restaurants or suppliers that have properly handled and prepared it.

Below is an overview of some of the main parasitic diseases that can be transmitted to humans after eating raw or undercooked fish.

Liver Flukes

Liver flukes are a family of parasitic flatworms that cause a disease known as opisthorchiasis.

Infections are most common in tropical regions of Asia, Africa, South America and Eastern Europe (1).

Researchers estimate that around 17 million people worldwide, most in Southeast Asia, are affected by opisthorchiasis.

Adult liver flukes reside in the livers of infected humans and other mammals, where they feed on blood. They may cause an enlarged liver, bile duct infection, gallbladder inflammation, gallstones and liver cancer (2).

The main cause of opisthorchiasis seems to be consuming raw or improperly cooked fish. Unwashed hands and dirty food preparation surfaces and kitchen utensils also play a role (3, 4).

Tapeworms

Fish tapeworms are transmitted to people who eat raw or undercooked freshwater fish or sea fish that spawn in freshwater rivers. This includes salmon.

They are the largest parasite known to infect humans, reaching a length of up to 49 feet (15 meters). Scientists estimate that up to 20 million people may be infected worldwide (5, 6).

While fish tapeworms often don't cause symptoms, they may cause a disease known as diphyllobothriasis.

The symptoms of diphyllobothriasis are usually mild and include fatigue, stomach discomfort, diarrhea or constipation (7).

Tapeworms may also steal substantial amounts of nutrients from the host's gut, especially vitamin B12. This may contribute to low vitamin B12 levels or deficiency (8).

Roundworms

Parasitic roundworms may cause a disease called anisakiasis. These worms live in marine fish or fish that spend a part of their lives in the sea, such as salmon.

Infections are most common in regions where fish is frequently eaten raw or lightly pickled or salted, including Scandinavia, Japan, the Netherlands and South America.

Unlike many other fish-borne parasites, Anisakis roundworms cannot live in humans for very long.

They try to burrow into the intestinal wall, where they get stuck and eventually die. This may cause a severe immune reaction leading to inflammation, stomach pain and vomiting (9, 10).

Anisakiasis may also cause immune reactions even if the worms are already dead when the fish is eaten (11).

Another family of parasitic roundworms may cause a disease known as gnathostomiasis (12).

These worms are found in raw or undercooked fish, poultry and frogs in Southeast Asia, Latin America, India and South Africa. However, infection is rare outside of Asia.

The main symptoms are stomach pain, vomiting, appetite loss and fever. In some cases, it may cause skin lesions, rashes, itching and swelling (13).

Depending on where in the host's body the parasitic larvae migrate, the infection may cause serious problems in various organs.

Summary: Regularly eating raw fish increases the risk of parasitic infections. Many fish-borne parasites can live in humans, though most of them are rare or only found in the tropics.

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The ghoulishly named ogre-faced spider can "hear" with its legs and use that ability to catch insects flying behind it, the study published in Current Biology Thursday concluded.

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"So here comes a flying bug and somehow the spider gets information on the sound direction and its distance. The spiders time the 200-millisecond leap if the fly is within its capture zone – much like an over-the-shoulder catch. The spider gets its prey. They're accurate," coauthor Ronald Hoy, the D & D Joslovitz Merksamer Professor in the Department of Neurobiology and Behavior in the College of Arts and Sciences, told the Cornell Chronicle.

What the researchers wanted to understand was how the spiders could tell what was moving behind them when they have no ears.

It isn't a question of peripheral vision. In a 2016 study, the same team blindfolded the spiders and sent them out to hunt, Science explained. This prevented the spiders from making their forward strikes, but they were still able to catch prey using the backwards strike. The researchers thought the spiders were "hearing" their prey with the sensors on the tips of their legs. All spiders have these sensors, but scientists had previously thought they were only able to detect vibrations through surfaces, not sounds in the air.

To test how well the ogre-faced spiders could actually hear, the researchers conducted a two-part experiment.

First, they inserted electrodes into removed spider legs and into the brains of intact spiders. They put the spiders and the legs into a vibration-proof booth and played sounds from two meters (approximately 6.5 feet) away. The spiders and the legs responded to sounds from 100 hertz to 10,000 hertz.

Next, they played the five sounds that had triggered the biggest response to 25 spiders in the wild and 51 spiders in the lab. More than half the spiders did the "backward strike" move when they heard sounds that have a lower frequency similar to insect wing beats. When the higher frequency sounds were played, the spiders did not move. This suggests the higher frequencies may mimic the sounds of predators like birds.

University of Cincinnati spider behavioral ecologist George Uetz told Science that the results were a "surprise" that indicated science has much to learn about spiders as a whole. Because all spiders have these receptors on their legs, it is possible that all spiders can hear. This theory was first put forward by Walcott 60 years ago, but was dismissed at the time, according to the Cornell Chronicle. But studies of other spiders have turned up further evidence since. A 2016 study found that a kind of jumping spider can pick up sonic vibrations in the air.

"We don't know diddly about spiders," Uetz told Science. "They are much more complex than people ever thought they were."

Learning more provides scientists with an opportunity to study their sensory abilities in order to improve technology like bio-sensors, directional microphones and visual processing algorithms, Stafstrom told CNN.

Hoy agreed.

"The point is any understudied, underappreciated group has fascinating lives, even a yucky spider, and we can learn something from it," he told CNN.

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