Anisakis: what it is, symptoms and how to protect yourself

That night the fish tasted clean. Fresh. Harmless.

He had taken it out of the water with his own hands and nothing in its smell or texture warned of danger.

Hours later, the pain began to rise from her stomach like a slow, dark tide. It wasn't indigestion. It was a foreign body trying to force its way through.
Unbeknownst to her, she had swallowed anisakis larvae, a microscopic parasite capable of turning a perfect dinner into a medical emergency. All from eating fresh fish.

What is Anisakis?

Anisakis is neither a bacterium nor a toxin. It is a parasitic worm, a microscopic nematode that lives naturally in the marine ecosystem. Its life cycle begins in small crustaceans, passes through fish and cephalopods, and ends in marine mammals such as dolphins or whales, which are its definitive hosts. There, the parasite reproduces and completes the cycle.

Anisakis is invisible and odorless in most cases. The fish may appear perfectly fine and still contain live larvae inside, coiled among the muscle fibers. It's not a problem of dirt or poor fishing practices.

How is Anisakis transmitted?

Anisakis doesn't appear in fish by chance. It follows a perfectly organized biological cycle that has been functioning in the ocean for millions of years. It all begins when tiny larvae are ingested by microscopic crustaceans. These, in turn, are eaten by fish and cephalopods such as hake, cod, or squid. Inside them, the larvae migrate from the intestine to the muscles, where they become encysted, alive and waiting for their next host.

When a marine mammal like a dolphin or whale eats that infected fish, the parasite finally reaches its destination. In its intestine, it matures, reproduces, and releases new eggs into the water, completing the cycle. For Anisakis, that's the plan, its life cycle.

Humans enter this story by mistake. When we eat fish or shellfish containing live larvae, we ingest them just as a marine predator would. The difference is that our bodies are not suitable hosts. The larvae cannot complete their life cycle, but they can attempt to embed themselves in the stomach or intestinal wall, triggering a violent inflammatory reaction and, in many people, an allergic response.

That's why infection isn't related to the fish being spoiled. It can be fresh, clean, and high-quality. The risk arises when a live larva manages to survive the journey from the sea to your plate. And that's surprisingly easy.

What symptoms does it cause and how long does it take for them to appear?

The first symptom is almost never an allergy. It's pain. Intense, deep pain, localized in the upper abdomen, that appears between 2 and 12 hours after ingesting the contaminated fish. It's not a vague discomfort. It's a clear sign that something is trying to penetrate the stomach or intestinal wall. The anisakis larva, unable to complete its life cycle in a human, tries to burrow into the digestive mucosa, triggering an acute inflammatory reaction that can mimic appendicitis, an ulcer, or even an intestinal obstruction.

This pain is often accompanied by nausea, vomiting, diarrhea, and abdominal bloating. In some cases, the larva becomes embedded in the stomach wall and can be seen directly during a gastroscopy as a whitish filament moving within the inflamed tissue. If the parasite dies inside, its breakdown releases highly immunogenic proteins that intensely activate the immune system.

This is where the second major mechanism of damage comes in: the allergic reaction. In sensitized individuals, even a small amount of anisakis protein can cause anything from hives, generalized itching, and swelling of the lips or eyelids to anaphylaxis, with difficulty breathing, a drop in blood pressure, and life-threatening complications. This reaction can occur even if the parasite is dead, because the problem isn't the live infection, but the antigens it leaves behind.

The time it takes for symptoms to appear depends on which of the two mechanisms predominates. When there is direct digestive invasion, symptoms usually begin within a few hours. When it is an allergic reaction, they can appear even minutes after ingestion. This is why anisakis is so treacherous: it can behave like a severe gastroenteritis, like a severe food allergy, or like both at the same time.

Why are so many people getting infected without realizing it?

Anisakis doesn't spread because people are careless. It spreads because our brains are poorly calibrated for this type of risk. Evolutionarily, we're designed to fear what we see, what smells bad, what looks rotten. But anisakis doesn't smell, doesn't taste strange, and doesn't change the appearance of the fish. It's an invisible danger, and the human brain is terrible at managing threats it can't detect with its senses.

Added to this is the normality bias. If you've eaten raw or undercooked fish many times without getting sick, your brain learns that it's safe, even if statistically it isn't. Each experience without consequences reinforces a false sense of control. It's the same mechanism that makes people stop wearing their seatbelts after a hundred trips without an accident.

Furthermore, culinary culture plays a huge role. Dishes like sushi, ceviche, marinated anchovies, or fish carpaccio are perceived as sophisticated, healthy, or clean. This aesthetic further reduces the perception of danger. No one associates something so carefully prepared and beautiful with a live larva capable of penetrating their stomach wall.

The result is a perfect storm: a real parasite, a visually appealing food, and a human brain that trusts precisely when it shouldn't. And that's how most anisakis infections occur not from ignorance, but from overconfidence.

How to act in the event of an Anisakis infection?

Once anisakis is inside the body, there are no home remedies or tricks. If you experience severe abdominal pain after eating raw or undercooked fish, you must go to the emergency room and explicitly state this. That information makes all the difference.

If the larva is still alive and attached to the stomach wall, the most effective treatment is a gastroscopy. Using a flexible camera inserted through the mouth, the doctor can locate the parasite and remove it directly with forceps. When this happens, the pain usually disappears almost immediately. It's literally removing the foreign body that was causing the inflammatory reaction.

Antibiotics are ineffective because anisakis is not a bacterium. Common antiparasitic drugs are also ineffective, as the larva is usually embedded in the tissue and dies on its own, but leaves behind an intense inflammatory reaction. In these cases, treatment focuses on controlling pain and inflammation with analgesics and, sometimes, corticosteroids.

If an allergic reaction is the predominant factor, the priority is to control the immune response. Antihistamines, corticosteroids, and, in severe cases, adrenaline are used. This is why someone can have an allergic reaction even when the parasite is no longer alive, because the immune system reacts to its proteins.

Anisakis is not expelled like ordinary diarrhea. It must be removed endoscopically, or the body encapsulates and slowly destroys it, at the cost of pain and the risk of complications. The earlier it is detected, the less damage it causes.

How can I eat fish safely?

Eating fish without fear of anisakis doesn't mean giving up sushi or anchovies. It means understanding what kills the parasite and what doesn't. Anisakis larvae only die in two ways: sufficient heat or sufficient cold.

Cooking is the most direct method. When the fish reaches a temperature of over 60°C throughout, the larvae die in seconds. This includes frying, boiling, baking, or thoroughly grilling the fish. The risk arises when the inside remains raw or lukewarm, as happens with thick cuts, tataki, or fish that has been seared briefly.

The other method is freezing. According to European food safety legislation, all fish intended for consumption raw or nearly raw must undergo a preventative freezing treatment designed to inactivate parasites such as anisakis. This requirement applies to restaurants and operators that market products ready for this type of consumption, such as sushi, marinated anchovies, fish carpaccio, or ceviche.

The established technical requirement is that the product reaches a temperature of at least -20 °C throughout its entire mass for a minimum of 24 hours, although many operators apply longer periods as a safety margin. This process is not a health recommendation, but a legal requirement designed to reduce the risk of parasites before the food reaches the consumer.

In retail, fish that has been frozen and then sold thawed must be labeled as such, either with a mention of "thawed" or equivalent wording. This information does not necessarily imply that the product is intended to be eaten raw, but it does inform the consumer that the fish has already undergone an industrial freezing process. Conversely, fish sold as fresh is not required to have been frozen, as regulations assume it will be cooked before consumption.

This has an important practical consequence: if a consumer buys fresh fish without any indication of thawing and plans to eat it raw or undercooked, the responsibility for applying the preventative freezing treatment then falls to them. Safety against anisakis does not depend on the origin of the fish or its appearance, but rather on whether it has been subjected, at any point in the supply chain, to sufficient cold or heat to inactivate the larvae.

Lemon, vinegar, salt, and cold smoking are ineffective. They can kill bacteria, but they don't destroy anisakis larvae. Therefore, marinated or pickled dishes that haven't been frozen beforehand remain risky, even if they taste acidic or are "cured."

Safety doesn't depend on the origin of the fish or how attractive it looks at the fishmonger's. It depends solely on whether it has been exposed to sufficient cold or heat. That's the real line between a safe meal and a medical emergency.

Countries with high consumption of raw fish, such as Japan, Spain, Italy, and the Nordic countries, strictly enforce these measures because the epidemiology of anisakis requires it. In Japan, for example, where raw fish consumption is widespread, visual inspection, deep freezing, and product traceability are standard industry practices. In Europe, these requirements are harmonized by food safety legislation, which treats anisakis as a priority biological risk.

Which animals are infected?

Anisakis is widely distributed in the marine ecosystem and affects a huge variety of species. The most common intermediate hosts are fish and cephalopods that are a regular part of our diet: hake, cod, whiting, horse mackerel, sardines, anchovies, mackerel, wild salmon, squid, and octopus, among many others. In these animals, the larvae first lodge in the intestine and then migrate to the muscles, which is the part we end up eating.

The definitive hosts, where the parasite reaches maturity and reproduces, are marine mammals. Whales, dolphins, seals, and sea lions harbor the adult Anisakis in their digestive tract and release the eggs into the water, restarting the cycle. The larger the population of these animals in an area, the greater the circulation of the parasite in that food chain.

This explains why some fishing grounds have higher rates of anisakis than others, and why large predatory species, which eat many small fish throughout their lives, tend to be more heavily parasitized. It's not a fault of the fish. It's a direct consequence of how the ocean works.

There are also clear geographical differences in the presence of Anisakis. The cold and temperate waters of the North Atlantic, the North Sea, the Bay of Biscay, and the North Pacific show particularly high infestation rates, partly because they harbor large populations of marine mammals, which are the definitive hosts of the parasite. In these areas, species such as hake, cod, and herring show significantly higher prevalences of Anisakis.

In the Mediterranean, although the parasite exists, infestation rates are generally somewhat lower, which has historically reduced the risk in certain fisheries. However, the increase in dolphin populations and the globalization of the fish trade are blurring these differences. Today, a fish caught in waters with a high concentration of anisakis can end up in any market in the world within hours.

Therefore, safety cannot be based on geographic origin. A fish from the Atlantic, the Pacific, or the Mediterranean can be equally safe or equally dangerous if it hasn't undergone the appropriate freezing or cooking treatments. Anisakis doesn't recognize borders, only biological cycles.

Frequently asked questions about anisakis

Does lemon or vinegar kill anisakis?

No. Lemon, vinegar, or salt can reduce bacteria, but they don't guarantee the inactivation of anisakis larvae. To make it safe, you need sufficient heat (thorough cooking) or preventative freezing.

Can I eat marinated anchovies without freezing them first?

It's not recommended. Marinated anchovies are a classic risk if they haven't been previously frozen. Marinating is no substitute for pre-freezing.

Does the "fresh" fish from the supermarket already come frozen?

Normally not. If fish is sold thawed, it must be labeled as "thawed" (or equivalent). If it's sold as fresh, it doesn't necessarily have to have been pre-frozen, because it's assumed you'll cook it.

How long should I freeze fish at home to avoid anisakis?

For home consumption, it's advisable to freeze any fish you plan to eat raw or undercooked for at least 48 hours. The goal is to ensure the product reaches sufficiently low temperatures for the time needed to inactivate any larvae.

What are the symptoms of anisakis and when do they appear?

It can cause severe abdominal pain (usually within a few hours), nausea, vomiting, or diarrhea. In sensitive individuals, it can also cause hives, swelling, or even anaphylaxis, sometimes within minutes of ingestion.

What should I do if I get a stomachache after eating raw or undercooked fish?

If the pain is severe or accompanied by persistent vomiting or allergic symptoms (hives, difficulty breathing), go to the emergency room and mention eating raw or undercooked fish. This information is key to diagnosis.

Does cooking "just flip it over" eliminate anisakis?

Not always. The risk arises when the inside remains raw or lukewarm. To inactivate larvae, the fish needs to reach a sufficiently high temperature throughout, not just on the surface.