Anisakiasis is a parasitic infection where worms attach to parts of the gastrointestinal tract. It occurs when a person ingests raw or undercooked seafood.

Anisakid nematodes are a type of roundworm that may be present in certain fish and squid. Anisakiasis is a parasitic infection that occurs after consuming raw or undercooked and improperly prepared fish or squid that contains these worms.

As the worms are unable to live in human hosts, a person will eventually excrete them as waste. However, they may still cause a person to experience gastrointestinal distress for a few days.

In this article, we will discuss anisakiasis, including its symptoms, causes, and prevention tips.

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Anisakiasis, or herring-worm disease, is an infectious disease that can transmit from fish and squid to humans.

Adult anisakis worms live within the stomach of marine mammals. The eggs of the worm pass through the marine mammal when it excretes feces.

When larvae from these eggs develop and hatch in the water, small marine animals, such as crabs, ingest them and become their host. When a fish or squid eats the smaller creature, the larvae transfer to them. The larvae can then transfer into humans if they consume raw or undercooked fish or squid.

Marine animals that commonly carry anisakids include:

  • mackerel
  • salmon
  • herring
  • squid
  • sardines
  • cod
  • trout

A 2020 systematic literature review suggests the worldwide prevalence of anisakiasis is 0.32 per 100,000 of the population. Infection is most common in locations where people traditionally consume raw fish as part of their diet.

According to the Centers for Disease Control and Prevention (CDC), a person may experience the following symptoms with anisakiasis:

  • abdominal pain
  • nausea
  • vomiting
  • swelling of the stomach (abdominal distention)
  • diarrhea or blood and mucus in the stool
  • mild fever

Additionally, some people may also experience an allergic reaction to the parasite. This can result in hives, breathing problems, skin rashes, and teary eyes. In more severe cases, it may also result in anaphylaxis.

Allergic reactions can occur after 1–12 hours, and initial symptoms of intense stomach pain usually present within 8 hours of consumption. Symptoms such as diarrhea, nausea, vomiting, and abdominal pain may persist for up to 5 days.

Evidence suggests it is not possible for a person to transmit anisakiasis to another person.

There are three species of Anisakis that typically cause infections in humans:

After a person consumes raw or undercooked fish or squid that contains anisakid nematodes, it is possible for the larvae to invade and penetrate the gastrointestinal tract.

As the human body is not a suitable environment for them, they cannot transition into their adult form and they eventually die.

A person will normally excrete them as part of their bowel movements. However, the larvae can produce an inflamed mass in the esophagus, stomach, or intestine. This is why a person may experience symptoms of gastrointestinal distress.

As the demand for fish products increases, so does the potential risk of exposure to fish-borne parasites such as anisakids.

Evidence notes that health experts consider anisakiasis to be an endemic disease in countries such as Japan, Spain, Mexico, Canada, and the United States. This term means that anisakiasis is consistently present but limited to these particular regions.

For example, a 2019 article indicated that 33.7% of samples of fresh fish that people commonly consume in Spain contained Anisakis larvae. Similarly, a 2022 systematic review suggested that 45.5% of fresh fish in China contained anisakid nematodes.

However, because eating such fish products is becoming more common, anyone who eats undercooked or raw fish or squid is at risk.

To diagnose someone with anisakiasis, a medical professional will likely start by asking questions about a person’s recent consumption of seafood, as well as assess any symptoms.

In some cases, a medical professional may suggest a radiography or endoscopy, which can help show if the larvae are present. Research suggests that a computed tomography (CT) scan may also be a useful tool for the diagnosis of anisakiasis.

An allergist can determine if a person is allergic to Anisakis through tests such as a skin-prick test and detecting specific IgE antibodies in blood tests.

According to the CDC, anisakiasis usually does not require the removal of the worms as they will eventually die. In cases when treatment may be necessary, there are a few options:

  • Endoscopic foreign body removal or endoscopy: During an endoscopy, a medical professional will pass a long, thin tube through a natural opening, for example, a person’s mouth. The tube contains a light and mediums for inserting instruments via the scope to help remove foreign bodies.
  • Medication: Evidence suggests that taking 400 milligrams of albendazole orally, twice a day for 6–21 days, is a successful treatment for anisakiasis. Albendazole is a medicine for the treatment of worms that works by impairing glucose utilization, leading to a decrease in the worms’ glycogen stores. This causes the worms to starve and eventually die.
  • Surgery: In cases where a person is experiencing obstruction, appendicitis, or swelling of the abdomen, a medical professional may decide surgery is the most appropriate form of treatment.

To avoid anisakiasis, it is best for a person to avoid eating raw or undercooked fish or squid. According to the U.S. Food and Drug Administration (FDA), a food thermometer is the only way to guarantee the safety of seafood. It suggests that a person should cook seafood in general to an internal temperature of at least 145°F.

When purchasing seafood for cooking, a person should also consider the following tips:

  • A person should only purchase refrigerated fish or fish that is on display on a thick bed of ice.
  • Refrigerated seafood sometimes lists time and temperature indicators on the packaging, which details if the product has undergone proper storage procedures. A person should check this information and only purchase the item if the indicator suggests that the product is safe to eat.
  • When purchasing fish sold as “previously frozen,” a person should check that it smells fresh and mild. If it smells fishy, sour, or rancid, a person should avoid eating it.
  • A person should avoid frozen seafood if the package is open or torn.
  • A person should avoid packages that appear frosty or contain ice crystals. This may indicate that the fish underwent thawing and was then refrozen.
  • After purchasing the seafood, a person should store it in a refrigerator or freezer. If a person will be using the seafood within 2 days of purchase, they should store it in a refrigerator at 40°F or below. Otherwise, the seafood will require wrapping and freezing.

If planning to eat raw fish, it is important to kill the parasites within it. A person can achieve this by using the following freezing methods:

  • freezing and storing at a temperature of -4°F or below for 7 days
  • freezing at a temperature of -31°F or below until solid and storing at a temperature of -31°F or below for 15 hours
  • freezing at a temperature of -31°F or below until solid and storing at a temperature of -4°F or below for 24 hours.

In some cases, after eating raw or undercooked fish or squid, a person may notice a tingling sensation. This is the worm moving in their mouth. In this situation, a person may be able to manually remove the worm or cough it up to prevent infection.

Some people may experience vomiting as a symptom, which can often help expel the worm from the body.

Lastly, when planning to eat at a restaurant, a person should check the hygiene rating of the establishment, as well as reviews, to help reduce the risk of eating contaminated seafood.

The general outlook for anisakiasis is positive. Most infections are self-limiting as the larvae are unable to survive for long periods in humans. Symptoms typically resolve in a few days, but associated tissue damage can cause a longer duration.

Some people may also experience an allergic reaction to the larvae. However, treatment options are available to help relieve symptoms in these cases.

Anisakiasis is an infection that occurs following the consumption of parasites known as anisakids. These roundworms may be present in several types of fish and squid. As such, a person may develop anisakiasis after eating raw or undercooked fish or squid.

Due to the infection, a person may experience symptoms such as abdominal pain, vomiting, and diarrhea. As humans are not suitable hosts for the worms, they are unable to survive for long periods and a person will typically excrete them as waste.

To prevent infection, it is advisable to follow proper seafood preparation and storage guidelines to kill any parasites.