Sashimi is a popular Japanese dish made from raw fish, but eating seafood without cooking also means consumers lose one of the most effective ways of destroying parasites.
Health authorities including the World Health Organization (WHO), the US Centers for Disease Control and Prevention (CDC) and the US Food and Drug Administration (FDA) recognise parasites as a potential hazard associated with raw or undercooked fish. However, that does not mean every slice of sashimi contains worms or that eating sashimi automatically results in infection.
One of the best-known parasites associated with raw marine fish is Anisakis, a type of parasitic roundworm. According to the CDC, people can become infected when they eat raw or undercooked fish or squid containing the larvae. Anisakiasis is particularly associated with countries where raw fish dishes are commonly eaten. (CDC)
The WHO has also identified raw fish as a route of transmission for several foodborne parasites, although the specific parasites involved depend heavily on the species of fish and where it comes from. (World Health Organization)
So, How Many Parasites Could Be In One Slice Of Sashimi?
There is no scientifically valid number of parasites per slice of sashimi.
This is an important distinction because parasite studies generally examine an entire fish, fillet or packaged fish product rather than individual restaurant-sized sashimi slices.
Research demonstrates just how variable the numbers can be. A study of Atlantic herring found that 84% of 75 freshly caught fish were infected with Anisakis, with an average of 11.3 parasites per infected fish and a range of 1 to 38 parasites per fish. However, only 5% of the overall worm population was found in the muscle in that study. (ScienceDirect)
Another Japanese study examined 218 Japanese chub mackerel and found Anisakis larvae in 74.3% of the fish. Among the infected fish, the researchers reported an average of 47 A. pegreffii larvae and six A. simplex sensu stricto larvae per fish. Importantly, only a fraction were found in the muscle, which is the part potentially used for sashimi. (PubMed)
This means it would be misleading to take a figure such as “47 parasites per fish” and claim that a slice of sashimi could contain a particular number.
One slice could contain zero detectable larvae. Another piece from a heavily infected fish could theoretically contain one or more. There is no fixed parasite count.
Some Fish Can Carry Dozens Of Parasites
The numbers found in scientific surveys can be considerably higher depending on the fish species and geographical location.
For example, researchers examining Atlantic mackerel in a 2025 study found Anisakis larvae in 36 of 41 fish — an infection prevalence of 88%. The infected fish carried between 5 and 650 larvae per fish, with an average of about 87 larvae per infected fish. Most of the parasites were found in internal organs rather than the flesh. (ScienceDirect)
Another study of horse mackerel sold in Morocco detected 4,815 larvae in 294 infected fish, with an average intensity of 16.38 larvae per infected fish. The researchers found a very high overall prevalence of 98.33% in the fish examined. (ScienceDirect)
These figures sound alarming, but they should not be interpreted as meaning that a sashimi serving contains dozens of worms.
Parasites are not distributed evenly throughout a fish. Many remain in the abdominal cavity and organs, while only a proportion migrate into the muscle. The amount reaching edible flesh can also depend on the fish species, size, fishing location, handling and time between catching and processing. (ScienceDirect)
Why Freezing Fish Is Important For Sashimi
This is why commercial controls are so important when fish is intended to be eaten raw.
The FDA identifies parasites as a potential hazard in fish intended for raw consumption and states that freezing is a practical and effective control for parasites in fish intended to be eaten raw. Its guidance specifies freezing at -20°C or below for seven days, or certain shorter combinations involving temperatures of -35°C. (U.S. Food and Drug Administration)
The CDC gives the same basic parasite-control options and recommends avoiding raw or undercooked fish if consumers want to eliminate the risk of anisakiasis. (CDC)
This does not mean simply putting a piece of fresh fish into an ordinary household freezer for a short period necessarily makes it safe. The temperature, duration, thickness of the fish and freezing process all matter.
Cooking is another effective control. The CDC recommends cooking seafood to an internal temperature of approximately 63°C (145°F). (CDC)
Can You See A Parasite In Sashimi?
Sometimes, but visual inspection is not a guarantee that raw fish is parasite-free.
An interesting study of pelagic fish found that the commonly used technique of “candling” — examining fillets against a light — detected only 7% to 10% of the nematode larvae actually present in the fillets in that particular study. The researchers concluded that visual inspection alone was insufficient to detect the majority of larvae in the fish examined. (ScienceDirect)
That is one reason why parasite control cannot rely solely on a chef looking carefully at a piece of fish.
Proper sourcing, processing, freezing and food-safety controls are considerably more important.
What Happens If Someone Eats An Anisakis Larva?
According to the CDC, anisakiasis can cause abdominal pain, nausea, vomiting, abdominal distension and diarrhoea. Some people can also experience allergic reactions, including, rarely, anaphylaxis. (CDC)
Symptoms can occur relatively quickly because the larvae can attach to or penetrate the gastrointestinal tract. In some cases, medical professionals can remove the parasite through endoscopy. (CDC)
The WHO has also documented anisakiasis as a foodborne disease associated with raw fish dishes including sushi and sashimi. (WHO IRIS)
However, it is important not to confuse parasite presence in fish with human infection. A fish can contain parasites without a person becoming infected, particularly when appropriate parasite-control measures have been applied before the fish is served.
The Bottom Line: Sashimi Does Not Mean “Worms In Every Slice”
Scientific evidence does show that some marine fish can carry substantial numbers of parasites. Studies have found anything from a small number of larvae to hundreds in individual fish, depending on the species and circumstances. (ScienceDirect)
But there is no evidence-based rule saying that one slice of sashimi contains “X number of parasites”. The scientifically accurate answer is that a slice can contain zero, while contaminated fish can potentially have larvae in its edible muscle.
For consumers, the key issue is therefore not counting worms in every slice but understanding how the fish was sourced and processed.
The CDC categorises raw or undercooked fish, including sashimi and sushi, as a riskier food choice compared with properly cooked seafood. (CDC)
So the next time someone claims that “every piece of sashimi contains parasites”, that is an exaggeration. But claiming that raw fish can never contain parasites would also be incorrect.
The factual position is somewhere in between: parasites naturally occur in some fish populations, their numbers vary enormously, and proper freezing or cooking is used to control the risk before the fish reaches the consumer.