Eating raw or undercooked fish, including sushi and sashimi, can expose you to parasitic worms. The most common culprit is a roundworm called Anisakis, though tapeworms and other nematodes also turn up in certain species. The actual risk depends heavily on the type of fish, where it was caught, and how it was handled before reaching your plate, and the story gets more interesting when you learn that the number of these parasites in ocean fish has been climbing dramatically over the past half century.
The Parasites That End Up in Your Sushi
Several types of parasitic worms can infect the fish used in sushi, but three groups account for the vast majority of human cases. The first and most significant is Anisakis, a roundworm whose larvae live in the muscle and body cavity of many marine fish species. Anisakis normally completes its life cycle in whales and dolphins: adult worms in a whale’s stomach produce eggs that pass into the ocean, hatch into larvae, get eaten by tiny crustaceans, and eventually work their way into fish that eat those crustaceans. When you eat raw fish containing live Anisakis larvae, you become an accidental dead-end host. The larvae cannot mature inside you, but they can cause real problems before they die.
The second group involves tapeworms, particularly Diphyllobothrium nihonkaiense, which uses wild Pacific salmon as an intermediate host. Cases of this tapeworm have been rising globally as more people eat raw salmon, shifting what was once a rural disease into an urban one tied to the popularity of sushi and sashimi in major cities worldwide.
The third is the sealworm, Pseudoterranova decipiens, which follows a similar cycle but uses seals rather than whales as its final host. Sealworm larvae tend to migrate into fish muscle tissue rapidly after the fish is infected, sometimes within hours, making them harder to remove during processing.
Which Fish Carry the Most Risk
Not all sushi fish are equally dangerous. A study of sushi from restaurants in Seattle found that salmon was by far the most commonly infected fish, with almost 10% of salmon sushi pieces containing nematode larvae and up to three worms per piece. Mackerel showed a lower infection rate of about 5%, while tuna and rockfish sushi were completely free of nematodes in the same study.
This pattern makes biological sense. Salmon are heavily exposed to Anisakis because they feed on the small crustaceans and fish that carry larvae. Wild Pacific salmon also harbor tapeworm larvae in their muscle tissue. Researchers have detected Diphyllobothrium nihonkaiense larvae in wild pink salmon from Alaska, confirming that salmon from both the American and Asian Pacific coasts pose risks when eaten raw.
Tuna are less frequently infected for several reasons. They tend to live in warmer, open-ocean waters where the larval lifecycle is less concentrated, and their large body size and fast metabolism create a less hospitable environment for certain parasites. That said, “lower risk” does not mean “no risk,” and species like mackerel, herring, cod, squid, and anchovies are all known intermediate or transport hosts for Anisakis.
What Happens If You Swallow a Live Larva
When a live Anisakis larva reaches your stomach, it tries to burrow into the mucosal lining. It cannot complete its lifecycle in a human body and will eventually die, but the burrowing and your immune system’s reaction to it can cause intense symptoms. A study of patients presenting to emergency departments found that those with gastric anisakiasis (larvae in the stomach) developed symptoms quickly, with pain peaking within six hours of eating the contaminated fish. Patients whose larvae had migrated further into the small intestine took longer to develop symptoms, with pain peaking around 48 hours, and were far more likely to need hospital admission: 57% of intestinal cases required inpatient treatment compared to just 5% of gastric cases.
The hallmark symptom is severe abdominal pain, which was present in every patient in that study. Nausea, vomiting, and diarrhea are common companions. CT scans in these patients typically showed swollen, thickened intestinal walls, and in the intestinal form, fluid accumulation in the abdomen was frequent.
Tapeworm infections from Diphyllobothrium present differently. They tend to be less dramatic at onset and can persist for weeks or months since the tapeworm can actually grow inside you. Symptoms include abdominal discomfort, diarrhea, and sometimes vitamin B12 deficiency because the worm absorbs this nutrient from your intestine. Diagnosis often comes when a person notices segments of the tapeworm in their stool.
How Common Are These Infections
Anisakiasis cases have been reported in 34 countries, but the disease is far more common in places with strong traditions of eating raw seafood. Japan, Spain, South Korea, Italy, and the United States report the most cases. Japan leads by a wide margin due to its deep culinary tradition of raw fish. Spain might surprise some people, but raw or lightly pickled anchovies (boquerones en vinagre) are a beloved dish there, and they are a known Anisakis vehicle. Researchers estimated that the AndalucÃa region of Spain alone sees roughly 2,200 anisakiasis cases per year, with the Cantabria region recording about 35 cases per 100,000 people annually.
Still, given the enormous volume of sushi consumed worldwide, the per-meal risk is quite low. Most commercially sold sushi in developed countries has been frozen before reaching the restaurant, which kills parasites if done properly. The cases that do occur tend to involve freshly caught fish that bypasses the commercial freezing chain, home-prepared sushi from fresh catches, or traditional preparations like lightly marinated fish where the acid is not enough to kill larvae.
How Freezing and Cooking Protect You
Freezing is the primary safety measure for fish intended to be eaten raw, and it works well when done correctly. Research shows that the critical conditions are either rapidly freezing the fish to a core temperature of −35 °C, or maintaining a core temperature of −20 °C for at least 24 hours. Both methods reliably kill Anisakis, Pseudoterranova, and tapeworm larvae.
The catch is that not all freezers perform equally, and the nature of the fish product matters. An experimental study that froze whole herring and skinless cod fillets at various temperatures found that while all parasites in thin cod fillets died at −15 °C or lower, some Anisakis larvae in whole herring survived 24 hours at −15 °C, −18 °C, and even −20 °C in a standard single-compressor freezer. The larvae were still moving after thawing. The reason is straightforward: a whole fish takes much longer to freeze through to the core than a thin fillet, so the parasites deep inside may not reach lethal temperatures quickly enough in a basic freezer.
This is why food safety regulations in the EU, the United States, and Japan specify either blast-freezing at very low temperatures or extended holding times. A typical home freezer set to −18 °C might not be cold enough or consistent enough to guarantee all larvae are dead in a whole fish within 24 hours. If you are preparing sushi at home from fresh fish, the safest approach is to buy fish that has already been commercially frozen to these standards, or to freeze thin fillets for at least seven days at standard home-freezer temperatures to build in a wide safety margin.
Cooking to an internal temperature of 63 °C (about 145 °F) also kills all relevant parasites. For people who are not committed to the raw preparation, this eliminates the risk entirely. Marinating in vinegar, lemon juice, or soy sauce does not kill Anisakis larvae. Salt-curing at certain concentrations can work, but the required salt levels and curing times are far beyond what most home cooks would apply.
The Allergy Problem That Survives Cooking
Here is where things get counterintuitive. Even if you cook fish thoroughly enough to kill every parasite, proteins from Anisakis larvae that were present in the fish can still trigger allergic reactions in sensitized individuals. Some Anisakis allergens are heat-stable, meaning they survive normal cooking temperatures. Research using mass spectrometry has identified multiple allergenic proteins in heat-treated Anisakis larvae, including known allergens designated Ani s 1 through Ani s 5, along with over a dozen additional potential allergens. Even after substantial heat degradation, these proteins retained the ability to react with antibodies in sensitized individuals.
Laboratory tests have also confirmed that extracts from fish known to contain Anisakis can activate the immune cells responsible for allergic symptoms, even after the parasites themselves are no longer alive. For most people, this is not a concern. But for the subset of people who have become sensitized through previous exposure to Anisakis (which may have been asymptomatic), eating fish that once harbored these larvae can cause hives, swelling, or in rare cases anaphylaxis. In Spain and other countries with high anisakiasis rates, Anisakis allergy is a recognized clinical entity that allergists screen for.
Freshwater Fish Carry Different Parasites
The conversation about sushi safety tends to focus on saltwater parasites, but freshwater fish bring their own risks and arguably more serious ones in certain parts of the world. Liver flukes, particularly Clonorchis sinensis and Opisthorchis viverrini, infect people who eat raw freshwater fish. These parasites are a major health concern in East and Southeast Asia. Unlike Anisakis, which causes a self-limiting infection because it cannot mature in humans, liver flukes can establish long-term infections in the bile ducts, causing chronic inflammation and, over years or decades, significantly increasing the risk of a bile duct cancer called cholangiocarcinoma.
Opisthorchis viverrini infection is particularly entrenched in northeastern Thailand, where raw or undercooked fish dishes are traditional and popular. The fluke has been formally classified as a carcinogenic agent. Freshwater fish used in sushi-style preparations in regions where these flukes are endemic carry a qualitatively different and more serious risk than typical marine sushi fish, since the infection can become chronic and life-threatening rather than acute and self-resolving.
Diagnosis and Treatment
If you develop sudden severe stomach pain within hours of eating raw fish, anisakiasis should be on the list of suspects. The standard diagnostic and treatment approach is endoscopy: a doctor passes a camera into the stomach, spots the larva (which is usually visible burrowed into the mucosal lining), and removes it with forceps. This procedure is both the definitive diagnosis and the definitive treatment. Removing the larva brings rapid relief. As one early clinical analysis of 178 cases put it, endoscopic removal is essential to relieve the otherwise intolerable pain, and early endoscopy is strongly recommended for anyone suspected of having the condition who ate raw fish within the previous 12 hours.
In most cases, the procedure is straightforward, but larvae that have burrowed especially deep can be harder to grasp. A recent case report described using larger “jumbo” biopsy forceps to successfully remove a larva that had invaded deep into the gastric wall, beyond the reach of standard instruments. After removal, antacid treatment helps the damaged stomach lining heal.
For tapeworm infections caused by Diphyllobothrium, the approach is different. Diagnosis typically relies on finding tapeworm eggs or segments in stool samples. Treatment with the antiparasitic drug praziquantel is standard and effective. In some cases, the entire tapeworm can be extracted endoscopically from the intestine, which guarantees complete elimination and prevents relapse.
Parasite Levels in Fish Are Rising
There is a broader ecological trend worth knowing about. A large meta-analysis covering data from 1962 to 2015 found that the average abundance of Anisakis worms in fish increased 283-fold over the 37-year period from 1978 to 2015. By contrast, sealworm (Pseudoterranova) levels showed no significant change over the same period. The likely explanation involves rebounding marine mammal populations: as whale and seal populations have recovered from historic hunting, the final hosts that the parasites need to complete their lifecycle have become more abundant, driving up parasite numbers throughout the food chain.
Climate change adds another dimension. A meta-analysis of how warming affects parasite-host relationships in fisheries found that higher temperatures generally increase parasite-induced mortality in fish hosts, though the effect varies among different systems. Warmer oceans could accelerate parasite development, expand their geographic range, and stress fish immune systems simultaneously. For sushi consumers, the practical implication is that fish from waters where parasites were historically uncommon may carry increasing risk in the coming decades.
The link between local marine mammal populations and fish parasites is quite direct. In the Baltic Sea, researchers found a clear correlation between the number of grey seals in a given area and the prevalence of sealworm in local cod. Areas with more seals had more infected fish, and the relationship was strong enough to serve as a predictor. Environmental factors matter too: in Baltic waters with salinity below about 7 parts per thousand, almost no sealworm infections were found in fish, suggesting the parasite’s lifecycle requires a minimum salt concentration.
Supply Chain Gaps and What You Can Do
Modern sushi supply chains include commercial blast-freezing specifically designed to kill parasites, and in most developed countries, fish sold for raw consumption must be frozen before sale. But the chain is not airtight. Research applying DNA-based testing to European processed seafood has revealed significant mismanagement and poor transparency at various stages of the supply chain, with species mislabeling and inadequate traceability creating gaps that could undermine safety protocols.
For you as a consumer, the practical steps are fairly simple. Eating sushi at reputable restaurants that source from commercial suppliers is the lowest-risk option, because those fish have almost certainly been frozen to parasite-killing standards. If you buy fish to prepare sushi at home, ask your fishmonger whether it has been previously frozen and to what standard. “Sushi-grade” is a marketing term with no legal definition in most countries, so the freezing history matters more than the label. If you catch your own fish and want to eat it raw, freeze it yourself for at least a week in a home freezer, and prefer thin fillets over whole fish to ensure the cold penetrates all the way through.
Certain fish choices also reduce your risk. Farmed salmon raised on processed feed pellets have much lower parasite exposure than wild-caught salmon, because they never eat the infected wild crustaceans that carry larvae. Tuna is generally lower-risk than salmon or mackerel. And if you are in a part of the world where freshwater fish are served raw, the stakes are higher because of liver fluke risk, so proper freezing or thorough cooking becomes even more important.