1. What Are Anisakis, the Thin White Worms Found in Seafood?
According to public information from Japan's Ministry of Health, Labour and Welfare, Anisakis larvae are 2–3 cm (about 0.8–1.2 inches) long and 0.5–1 mm wide [2]. They look like white threads and can be seen with the naked eye [2]. The adult worms live in the digestive tracts of marine mammals such as whales and seals, while fish and shellfish contain only the larvae [1].
If a person eats raw fish containing these larvae, they may experience severe pain in the upper abdomen or nausea several hours later, according to national infectious disease information [1]. In 2024, Anisakis was the most common cause among all food-poisoning cases, with a total of 1,037 reported incidents [5]. Data from Gunma Prefecture shows 330 cases, which is about 30% of that total [5]. Separately, the Ministry's public information lists a patient count of 337; for this article, we will consider this separate figure as "about 300 people" [2].
To answer the main question early: freezing causes the larvae to lose their infective power, and heating also kills them. We will look at the specific conditions in later sections. First, to understand why freezing is important, let's see if common kitchen items work.
2. Why Vinegar, Salt, Soy Sauce, and Wasabi Cannot Stop Anisakis Larvae
We usually eat sashimi with soy sauce and wasabi. We use vinegar for marinated mackerel (shime-saba). These items all seem like they might kill bacteria or parasites.
However, the Ministry of Health, Labour and Welfare states that Anisakis larvae are not killed by treatment with food vinegar, salting, or eating them with soy sauce and wasabi [2].
This means you should not think that your usual seasoning can get rid of the worms. So, what are the specific conditions for freezing, which is considered a reliable method?
The intuition that "seasoning or pickling makes it safe" does not hold true.
3. How Long and How Cold Must Fish Be Frozen to Stop Anisakis? (-20°C for 24 Hours)
The Japan Institute for Health Security (JIHS) states that Anisakis larvae lose their infectivity when frozen at -20°C (-4°F) for 24 hours or more [1]. The Ministry's public information lists the same conditions, adding that heating kills them instantly at 70°C (158°F) or in 1 minute at 60°C (140°F) [2]. Materials from Aichi Prefecture use these same numbers, showing agreement between national and local government sources [3].
The important point is that it is not just about reaching -20°C; the length of time (24 hours or more) is also a condition.
A footnote in materials from the European Food Safety Authority (EFSA) lists other examples for freezing to deactivate larvae: -15°C (5°F) for 96 hours or more, -20°C (-4°F) for 24 hours, and -35°C (-31°F) for 15 hours [4]. In these examples, lower temperatures require shorter times. However, these are alternative conditions listed in European materials; the Japanese standard is -20°C for 24 hours or more. For homes in Japan, use the national guidelines as your guide.
Can this "-20°C and 24 hours" rule be followed in a home freezer?
4. Why the Cold Must Reach the Deep Center of the Fish for Freezing to Work
Freezing only works if the cold actually reaches the deep places where the worms are. Materials from Aichi Prefecture note that caution is needed for large fish, as it may be difficult to heat or freeze the Anisakis located in the deep parts [3].
With thick cuts of fish or solid blocks, the surface freezes first, and the center freezes later. Even if your freezer display shows -20°C (-4°F), that is different from whether the cold has reached the deep part of the fish. The displayed temperature alone does not tell you if the center of the fish was kept at -20°C for 24 hours or more.
This raises another question: if we choose fresh fish, are the worms simply not in the meat?
Because of this, you cannot rely only on numbers to think "it is safe because I put it in the freezer."
5. Why Even Fresh Fish May Have Anisakis Larvae in the Meat
Aichi Prefecture materials state that it is known that when fish freshness declines, larvae move from the internal organs to the muscle (meat) [3]. Therefore, removing the internal organs as soon as possible after catching is one effective method.
However, there are cases where larvae are already in the muscle while the fish is still alive. You cannot say "it is fresh, so it is not in the meat." Freezing becomes a decisive factor if the conditions reach the deep part of the fish, regardless of freshness.
Since larvae may be in the muscle even while the fish is alive, you cannot claim safety based on freshness alone.
6. How to Check Your Freezer Temperature and Sashimi Labels for Anisakis Safety
If your home freezer has a temperature display, check what it says, but remember that the display does not confirm whether the center of the fish has met the required conditions. It takes time for the center of the fish to reach the displayed temperature. Thick blocks and thin slices should take different amounts of time for the center to freeze.
Look at the labels on store-bought sashimi or fish packs to see if they say "frozen," "thawed," or "farmed," and make a note. This article does not cover what those label terms mean in detail.
If you want to read more, the Ministry of Health, Labour and Welfare public information [2] and JIHS explanations [1] are available in Japanese. If you experience severe abdominal pain after eating, tell an adult early and consult a medical institution.
It is safer to assume that the time required changes depending on the thickness of the block.