Eating Raw Fish – Scientific Safety Analysis On Dangerous Side Effects?

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From Japanese sushi to Italian carpaccio, the practice of eating raw fish spans many cultures and continents.

There are plenty of good reasons to cook fish before you eat it, namely the wide variety of parasites and bacterial pathogens that it carries.

The practice of cooking fish destroys these hazardous elements before they can infect your body, making it safe to eat.

There are, however, a number of culinary practices that make it possible to consume raw fish with little to no health risk.

The taste and texture of raw fish is regarded as a delicacy, and is extremely popular in countries such as Japan, where sushi and sashimi are integral cultural dishes.

Just how safe is eating raw fish, though? In this article, we’ll weigh up the available evidence to present a comparison of the health benefits and risks of consuming raw fish.

Eating Raw Fish Dishes

Dishes that incorporate raw fish can be found around the world, and are consumed in a variety of different ways. Let’s take a look at some of the most popular raw fish dishes:

Sushi

Sushi is one of the most popular dishes in Japan, and generally consists of cooked vinegared rice served with an assortment of raw fish.

Modern contemporary sushi originated in the late stages of the Edo Period of Japan, and can now be found in almost every country in the world.

Sashimi

Sashimi is another raw fish dish that is popular throughout Japan. Unlike sushi, sashimi does not typically include rice, focusing instead on very thinly sliced raw meat and fish.

Carpaccio

Carpaccio is a relatively new cuisine, originating in Venice in the 1950’s.

Carpaccio originally referred only to thinly sliced raw beef served with white truffle or parmesan cheese, but the definition has broadened in contemporary carpaccio to include various type of raw fish.

Gravlax

Gravlax is a Nordic dish that consists of raw salmon that has been cured in a combination of dill, salt, and sugar.

The salt curing process used to preserve salmon in gravlax doesn’t necessarily mean it’s raw, but it is uncooked still and may present some of the potential health risks of raw fish.

Ceviche

Ceviche is a raw seafood dish that originates from Latin America and the Caribbean.

Ceviche generally consists of raw fish presented on a platter with a number of other foods that possess complementary flavors, such as lettuce, corn, or avocado, and is cured in citrus juices and chili peppers.

Koi Pla

Koi Pla, also known as simply “Koi”, is a popular raw dish throughout Laos. Koi can be made from either raw meat or raw fish, and uses the high acidity level of lime juice to denature the protein in the meat and destroy bacteria.

Although popular, koi is an extremely common cause of parasitical infection.

Soused Herring

Soused herring is a popular Dutch dish that consists of herring fish that is semi-preserved in a vinegar marinade that contains cider, wine, herbs, or sugar.

Soused herring is made from young herring fish that are ripened in a barrel containing a salt solution for several days.

Poke

Poke, known in many contemporary restaurants as “Hawaiian fish salad”, is a raw fish salad popular in Hawaiian cuisine. Poke can be made with tuna, octopus, salmon, or shellfish, and is commonly mixed with raw onions and limu, a species of red algae.

The Health Risks of Consuming Raw Fish

Although these raw fish dishes are both delicious and culturally diverse, they can transmit a concerning number of health risks, pathogens and parasites that can severely damage the body.

Parasitical infections are a very serious threat presented by raw fish.

Unlike ectoparasites such as fleas, ticks, or leeches that live external to the body and are easily identified, the endoparasites that live within fish make their home inside the body, feeding off the nutrients in the bloodstream or digestive system.

Human endoparasite infestation caused by the consumption of raw fish is extremely common, and is a leading health condition throughout the developing world.

Raw fish can also host a broad spectrum of dangerous bacteria that are not destroyed in the cooking process.

These bacterial infections can play havoc with the immune system and even result in sepsis, an advanced stage of blood poisoning that can result in loss of limb or even death.

The preparation methods used to make raw fish play a large role in determining whether a dish will present a health risk or not.

Unwashed hands, poor kitchen hygiene, unclean drinking water, and poor handling methods all increase the risk of becoming sick from consuming raw fish.

Let’s take a look at some of the most concerning health risks presented by consuming raw fish:

Tapeworm

Tapeworm, or Cestoidea, are long, flat worms that colonize the digestive system of many animals. There are six different types of tapeworm, which are generally categorized by the source from which they are able to infect the human body.

The type of tapeworm that can infect raw fish is called Diphyllobothrium latum, and, while relatively harmless in the early stages of infection, can lead to severe health complications if left untreated.

Tapeworm is an especially insidious parasite, as their three-stage life cycle means that food is often infested with eggs or larvae that cannot be seen with the naked eye.

These eggs and larvae are consumed as part of a raw fish dish, and proceed to colonize the intestines and digestive system. Tapeworm infestation in the human body usually causes the following symptoms:

Some of the more severe symptoms of a tapeworm infestation in the digestive system occur when tapeworm specimens that infest the digestive system migrate to other parts of the body and colonize the eyes, liver, heart, and brain, which is life-threatening and can potentially result in death.

Concerningly, tapeworms that colonize the human body can grow to extreme sizes, with some cases reporting worm lengths of over 45 feet.

If you believe that you may be suffering from a tapeworm infestation, it’s essential to consult a medical professional immediately.

Tapeworm Fast Facts:

  • Tapeworm larvae and eggs are undetectable in raw fish
  • Tapeworm colonize the digestive system and cause vitamin deficiency, growing up to 15 meters in length
  • Tapeworm can infest the brain if left untreated

Roundworms

Roundworms are another parasite that can be transmitted via the consumption of raw fish.

Roundworm infection results in a condition called ascariasis that can result in a variety of health complication, although the parasites themselves find it difficult to survive in the human body[1].

According to statistics released by the World Health Organization, roundworm infestation affects roughly 10% of the population of the developing world[2].

There are many different types of roundworms, but the specific genus that is transmitted via raw fish consumption is called Anisakis. This species of roundworm causes a condition of the same name that is also referred to as herring worm disease.

Anisakis roundworm nematodes attach to the walls of the throat, stomach, or intestine, where they live only for a short time until dying[3].

Although anisakid roundworm die inside the human body, the resulting mass of dead nematodes cause an immune system response that results in widespread inflammation, vomiting, stomach pain, and allergic reactions[4].

The key signs of anisakid roundworm infection are:

  • Blood, mucus, and worm fragments in stool
  • Rash, itching, and red skin
  • Diarrhea and abdominal distension
  • Fever

In some cases, anisakid roundworm infection can induce anaphylactic shock, a severe immune system reaction that can be fatal if not treated immediately.

Treatment for roundworm infection generally requires the removal of worms from the body via surgery or endoscopy[5]. Roundworm eggs and larvae can also cause immune system responses even if they are ingested after they have died.

Roundworm Fast Facts:

  • Roundworm live within fish and are able to colonize the human body, but die quickly
  • Dead roundworm masses inside the body can cause extreme immune system reactions
  • Roundworm infestation is difficult to diagnose, and is typically only discovered once nematode fragments or blood are found within the stool

Liver Flukes

Liver flukes are another group of nematodes, or parasitic worms, that are able to colonize the human body.

These tiny parasites, called Clonorchis sinensis, or the Chinese Liver Fluke, infest more than thirty million humans and are the third most common worm parasite in the world[6].

Most common throughout Southeast Asia, China, and Korea, liver flukes cause liver fluke disease, which is a serious and life-threatening health condition.

These dangerous parasites enter the body when raw fish is consumed and enter small bile ducts in the liver and gallbladder.

Living within the human body for up to 30 years, liver flukes cause constant irritation and scarring, with one adult fluke laying over 3000 eggs every day.

Liver flukes commonly infest both the liver and the bile ducts of a human host, resulting in a broad spectrum of symptoms that include:

  • Abdominal pain
  • Nausea and vomiting
  • Diarrhea
  • Fever, decreased appetite and weight loss

A severe liver fluke infection can have even more dire health consequences. Recent medical investigations into the effects of liver fluke infestation has discovered that the parasites are more than capable of causing cancer[7].

The International Agency for Research on Cancer classified liver fluke infestation as a category 1 biological carcinogen in 2009, placing it on the same severity scale as asbestos and Benzene[8].

Liver fluke infestation also causes a health condition called opisthorchiasis, which is extremely hard to diagnose.

Up to 80% of individuals suffering from liver fluke opisthorchiasis present no symptoms, which can eventually result in fibrosis, fluid accumulation in the legs, and malignant cancer.

The most common cause of this health condition is the consumption of incorrectly handled and prepared raw fish[9].

Liver Fluke Fast Facts:

  • Liver fluke disease is extremely hard to diagnose and often present no symptoms in early stages
  • Liver fluke nematodes have a strong potential to cause cancer in hosts
  • A liver fluke can live within the body for up to 30 years

Bacterial Infections

Aside from the endoparasitic infections that occur due to the consumption of raw fish, a broad spectrum of bacterial infections can infect the body via the same vector.

The most common form of bacteria infection caused by the consumption of raw fish is commonly known as food poisoning, which typically causes nausea, vomiting, gastrointestinal inflammation, and diarrhea.

Most cases of food poisoning last only 24-48 hours, but raw fish carries a variety of bacterial pathogens that can have far more negative health effects.

Just some of the harmful bacteria that can be transmitted by raw fish include Salmonella, Staphylococcus, and Clostridium botulinum, which causes botulism.

One of the most concerning bacteria that is carried by raw fish is Vibrio vulnificus, which is exceptionally virulent and commonly causes sepsis, a condition in which the entire body becomes infected[10], resulting in a mortality rate of between 40% and 80%[11].

One particularly troubling investigation conducted by the U.S. Food and Drug Administration found that nearly 10% of the seafood products that are imported into the United States test positive for salmonella[12], a bacterium that causes diarrhea, cramps, fever, and worse conditions such as potentially-fatal serious systemic infections[13].

A healthy immune system is generally able to fight off the less severe forms of bacterial pathogens that are found in raw fish, but the rising prevalence of antibiotic-resistant pathogens makes consuming raw fish a high-risk endeavor for anybody that possesses a weakened immune system.

Elderly individuals, young children, and individuals with immune system disorders such as Crohn’s disease or other autoimmune disorders should refrain from consuming raw fish.

Raw fish can also transmit a dangerous bacteria called Listeria. This bacteria has been observed to be present in up to 1.3% of all smoked and cured fish products[14] and causes a condition called Listeriosis, which is fatal in 24% of all cases[15].

Listeriosis is especially dangerous in pregnant women, in which it causes fetal death. According to statistics released by the CDC, 1600 diagnoses of listeriosis occur in the ES annually, resulting in 260 deaths on average yearly[16].

In light of this information, it’s critical for pregnant women to avoid consuming raw fish products.

Bacterial Pathogens Fast Facts

  • Almost 10% of all imported seafood tests positive for dangerous bacteria
  • Bacterial infections caused by ingesting raw fish can cause sepsis, a dangerous blood infection
  • Pregnant women should avoid raw fish to protect their unborn children

Raw Fish Pollutant Levels

Raw fish has been observed to contain high levels of pollutants called Persistent organic pollutants, or POPs.

These pollutants are organic compounds that don’t break down through natural degradation processes, and persist throughout the entire food chain.

These toxins present a significant health risk. Some POPs, such as polychlorobiphenyls, are carcinogenic, while others, such as polybrominated diphenyl esters, are able to inhibit cognitive function and endocrine function[17].

Farmed fish such as salmon are known to contain high levels of these dangerous elements[18], and have been attributed to the spread of diseases such as type 2 diabetes and cancer[19][20].

Cooking fish has been demonstrated in multiple clinical trials to significantly lower the amount of persistent organic pollutants in fish.

A 2003 clinical trial determined that cooking salmon, for example, results in a decrease in the overall mercury level of the food by up to 35%[21].

Another clinical investigation in 2004 determined that cooking fish results in a decrease of 62% in pollutant levels[22].

The scientific investigation that has resulted in these insights seems to point to the presence of pollutants in the lipid deposits of the fish as responsible for the reduction in pollutant content[23].

Cooking fish reduces the fat level of the tissue, which carries away some of the toxin content.

Raw Fish Pollutants Fast Facts:

  • Raw fish contains higher levels of pollutants such as mercury
  • These pollutants can cause cancer and diabetes as well as impair cognitive function
  • Cooking fish reduces overall toxicity levels

Raw Fish Health Benefits

Now that we’ve covered the most important health risks presented by consuming raw fish, let’s take a look at the potential benefits offered by raw fish dishes.

Raw Fish Contains Omega Fatty Acids

Fatty acids, such as omega-3 and omega-6, which have been demonstrated to protect the body from heart disease, improve cardiovascular function, reduce inflammation, and boost immune system function[24].

Fish is exceptionally high in omega-3 fatty acids, which are able to reduce blood pressure[25] and cholesterol levels[26].

Several clinical trials have determined that raw fish and cooked fish have different nutritional profiles, and different amounts of fatty acids[27].

A 2010 study found that frying tuna lowers its overall omega-3 nutritional profile[28], and another 2013 study found that frying and baking lowers the fatty acid content of sardines and anchovies[29].

There is a large amount of evidence to suggest that, as the cooking process oxidizes the fat content of fish, raw fish may offer a superior fatty acid nutritional profile.

Raw Fish May Minimize the Risk of Cancer

Clinical investigations performed in Japan in 2003 found that frequent consumption of raw fish is linked to statistically lower rates of colorectal cancer diagnoses[30].

Another study performed by the Japanese Department of Public Health in 1996 found that regular raw fish consumption results in a reduced risk of developing pancreatic cancer[31].

The high fatty acid content of raw fish also helps to prevent the body from cancer[32].

Raw Fish is a Rich Source of Micronutrients

Fish is extremely nutritionally dense, containing high levels of protein, selenium, iron, zinc, and calcium, as well as omega fatty acids and vitamins.

Fish is one of the best food sources for vitamin D, an essential micronutrient that is deficient in up to 30% of the US population[33].

Clinical trials suggest that the cooking process may lower the overall amount of micronutrients in fish, as well as increasing fat content[34].

Raw Fish Offers a Unique Flavor Profile

The primary reason raw fish is consumed in many cultures is due to the unique flavor profile that is destroyed through the cooking process.

A 2006 clinical investigation found that cooking processes significantly alter the levels of odor-active volatile compounds in fish, dramatically changing the effect they have on the olfactory system[35].

Cooking methods also alter the texture profile of fish[36], which can drastically alter the gastronomic experience.

Minimizing Raw Fish Health Risks

While there are many risks presented by the practice of consuming improperly prepared raw fish, many individuals enjoy the flavor and texture of raw fish dishes, as well as the rich cultural history of the practice.

Here are a few safety tips on enjoying raw fish while minimizing the health risks it presents:

Only Consume Raw Fish From Reputable Suppliers:

Avoid consuming raw fish while in geographic regions that are statistically prone to raw fish parasite infections. When purchasing raw fish from either a restaurant or supplier, always investigate their handling and hygiene processes.

Assess Fish Before Eating:

Always thoroughly inspect raw fish before eating it. Don’t eat fish that smells or appears “off”, spoiled, or bad.

Never leave fish out of the refrigerator for an extended period of time: Bacteria multiply quickly at room temperature, so always consume fish within one hour of taking it out of the refrigerator.

Maintain Proper Kitchen Hygiene:

Always wash your hands and kitchen utensils before handling raw fish, and be sure to avoid cross-contamination when cooking.

Always Eat Fish That Has Been Frozen:

Gordon Ramsay may disagree, but freezing fish is the only way to destroy parasites in raw fish.

It’s fine to eat fresh fish if you’re planning on cooking it, but raw fish should be frozen for at least one week at -4 degrees Fahrenheit to destroy any parasites that may lurk in it.

Freezing won’t kill bacteria, but will inhibit their growth and minimize the risk of bacterial infection.

Don’t Eat Raw Fish If Pregnant:

Pregnant women are at risk of causing severe harm to their unborn children when they consume raw fish.

Eating Raw Fish Conclusion

Eating raw fish can be an enjoyable and delicious experience, but comes with a number of risks that must be taken into account.

By following simple health and safety guidelines it’s possible to significantly decrease the chance that you will fall prey to an intestinal parasite or bacterial infection.

Always be sure to purchase raw fish dishes only from trusted suppliers, and remain aware of the hygienic practices of any restaurant you may order a raw fish dish from.

Combining raw fish with antibacterial foods such as soy sauce, salt, spices, or wasabi is generally the best (and tastiest) way to go, so don’t be afraid to try sashimi next time you go out to dinner- just make sure the chef washes his hands!

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[1] http://www.nhs.uk/conditions/Roundworm/Pages/Introduction.aspx

[2] http://www.who.int/water_sanitation_health/diseases/ascariasis/en/

[3] https://www.cdc.gov/parasites/anisakiasis/faqs.html

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2292572/

[5] https://www.cdc.gov/parasites/anisakiasis/faqs.html

[6] https://www.ncbi.nlm.nih.gov/pubmed/23052782

[7] https://www.ncbi.nlm.nih.gov/pubmed/23052782

[8] https://www.cancer.org/cancer/cancer-causes/general-info/known-and-probable-human-carcinogens.html

[9] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2705081/

[10] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2863362/

[11] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3488423/

[12] https://www.ncbi.nlm.nih.gov/pubmed/10826714

[13] https://www.ncbi.nlm.nih.gov/books/NBK8435/

[14] https://www.ncbi.nlm.nih.gov/pubmed/14968966

[15] https://www.ncbi.nlm.nih.gov/pubmed/1943998

[16] https://www.cdc.gov/listeria/

[17] http://www.sciencedirect.com/science/article/pii/S0026265X0400195X

[18] https://www.ncbi.nlm.nih.gov/pubmed/15866762

[19] https://www.ncbi.nlm.nih.gov/pubmed/22323980

[20] https://www.ncbi.nlm.nih.gov/pubmed/20872674

[21] https://www.ncbi.nlm.nih.gov/pubmed/12746129

[22] https://www.ncbi.nlm.nih.gov/pubmed/15106675

[23] https://www.ncbi.nlm.nih.gov/labs/articles/15799450/

[24] https://www.ncbi.nlm.nih.gov/pubmed/12480795

[25] https://www.ncbi.nlm.nih.gov/pubmed/22591893

[26] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3875260/

[27] https://www.ncbi.nlm.nih.gov/pubmed/24996368

[28] https://www.ncbi.nlm.nih.gov/pubmed/23572621

[29] https://www.ncbi.nlm.nih.gov/pubmed/23239760

[30] https://www.ncbi.nlm.nih.gov/pubmed/12671534

[31] https://www.ncbi.nlm.nih.gov/pubmed/8872522

[32] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4304705/

[33] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4581139/

[34] https://www.ncbi.nlm.nih.gov/pubmed/24191621

[35] https://www.ncbi.nlm.nih.gov/pubmed/16637700

[36] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4519516/

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