Scientists Find Parasites in Virtually All Fish From a Recent Tuna Haul, With 96% Infected

Tuna is one of the most widely eaten fish in the world, whether it comes from a can, a sushi counter, or the seafood aisle. But a new study examining skipjack tuna off the coast of Brazil found something most people probably wouldn’t expect to find inside their lunch.

Researchers at the Federal University of Santa Catarina examined 53 skipjack tuna and discovered more than 1,600 parasitic worms throughout the fish. Ninety-six percent were infected, with parasites found in the stomach, intestines, liver, eyes, reproductive organs and, importantly, the muscle tissue that people eat. Only the heart and kidneys were consistently parasite-free.

The findings, published in Ocean and Coastal Research in 2026, sound alarming. But finding parasites in wild fish doesn’t necessarily mean the tuna on your plate is dangerous. What matters is which parasites are present, where they’re found, and how the fish is processed or prepared before it reaches consumers.

That’s particularly relevant for skipjack tuna (Katsuwonus pelamis), one of the world’s most commercially important tuna species and a common source of canned tuna. Parasites are also not a newly emerging phenomenon in seafood. Previous research has documented substantial changes in the abundance of Anisakis, a group of parasitic worms capable of infecting humans when contaminated seafood is eaten raw or inadequately prepared.

The Brazilian researchers identified two major groups of parasites in the tuna they examined: Anisakis and Trypanorhyncha. They don’t pose the same risks to people, and understanding that difference is important before deciding what these unsettling findings actually mean for the tuna we eat.

What the Brazil study found, and what it means

The Brazilian research team examined 53 skipjack tuna caught locally, using powerful microscopy tools to survey every major organ. The intestines were the most heavily infected tissue. The muscle – the portion sold to consumers – was the second most contaminated. Stomach tissue came third. Only the heart and kidneys were universally parasite-free.

The University of Washington meta-analysis synthesized published estimates of worm abundance in fish across time and geography. Between 1978 and 2015, Anisakis worms increased 283-fold, while a related worm genus, Pseudoterranova, showed no significant change. The analysis screened 56,778 fish from 215 species across 123 published papers.

The Brazilian sample is far smaller – 53 fish – and covers a single point in time and location. Its 96 percent infection rate, combined with the organ-level distribution of worms, suggests that parasitism in this commercially dominant species may be more widespread than routine inspection acknowledges.

The study authors noted that immediate evisceration of tuna after catch is a critical intervention: removing the intestines quickly reduces the risk of larvae migrating from gut tissue into the edible muscle. That step, already standard in high-quality seafood processing, takes on additional significance when intestinal parasite loads are this high.

The two worms: different risks, different biology

Anisakis is a parasitic nematode found in raw or undercooked fish and is the primary genus of public health concern. Anisakiasis is a gastrointestinal infection caused by ingesting raw or undercooked seafood containing anisakid larvae, which can attach to the lining of the esophagus, stomach, or intestines and cause abdominal pain, nausea, vomiting, and diarrhea. Allergic reactions, including rashes and intense itching, can also occur.

The worms reproduce in the intestines of marine mammals and are released into the ocean through fecal matter. After hatching, they first infect small crustaceans such as krill. When small fish consume the infected crustaceans, the worms transfer to their bodies, continuing up the food chain as larger fish eat smaller infected fish.

Cases of anisakiasis are reported worldwide, with the highest concentrations in Japan and parts of Western Europe where raw or undercooked seafood is regularly consumed. Cases are widely considered underreported because symptoms closely resemble ordinary food poisoning.

Trypanorhyncha, the second genus found in the Brazilian tuna, presents a different profile. According to a parasitology study, heavy Trypanorhyncha loads in edible fish tissue primarily cause consumer rejection rather than direct health risk – the worms are visually off-putting but are generally not pathogenic in humans. Accidental human infection with Trypanorhyncha larvae is rare.

Why humans are accidental hosts

Adult Anisakis worms live within the stomachs of marine mammals. Humans are a biological dead end for the parasite. When people eat live Anisakis larvae, the worms can invade the intestinal wall and cause symptoms similar to food poisoning, but in most cases the worm dies after a few days and symptoms resolve.

The health risks of these worms are relatively low for people but may have a larger impact on marine mammals such as dolphins, whales, and seals, in whose bodies the parasites can persist for years. Researchers are not certain what caused the large increase in Anisakis abundance, but marine mammal population growth is one of the most plausible explanations.

Marine mammals have been protected under the Marine Mammal Protection Act since 1972, allowing many populations of seals, sea lions, whales, and dolphins to recover. More marine mammals means more definitive hosts in which Anisakis reproduces, which in turn means more larvae entering the ocean food chain.

Chelsea Wood, an associate professor of aquatic and fishery sciences at the University of Washington, has framed this dynamic explicitly. According to coverage of her 2026 research, Wood views the presence of these parasites as a signal that a fish came from a healthy ecosystem with an intact food web. Anisakid parasites integrate multiple trophic levels, from crustaceans to small fish to large predators to marine mammals.

In the late 1970s, researchers found fewer than one Anisakis worm per 100 fish. Now, they find more than one worm per individual fish, on average. The Brazilian study suggests that, in at least one commercially important species in one Atlantic region, that average may be substantially higher.

The commercial supply chain: what processors do and don’t catch

Tuna sold for consumption passes through several inspection layers before reaching consumers. Commercial seafood processors and sushi chefs visually screen fillets for visible worms, and high-end operations use candling – shining a bright light through thin fish fillets to reveal larvae. These methods catch a large proportion of visible worms.

Anisakis larvae can be very small, and when embedded deep in muscle tissue rather than concentrated near the surface, they are easily missed. The Brazilian study found worms in the muscle of 96 percent of fish. Even with diligent inspection, some larvae in deep-embedded muscle will evade detection.

The standard industrial solution is freezing or thorough cooking. Appropriate freezing procedures kill anisakid larvae and render commercially frozen tuna safe for consumption. Cooking fish to a sufficient internal temperature also kills parasites reliably.

Parasites killed during processing are harmless to humans, which is why properly canned fish is safe regardless of what parasites the live fish carried. The risk is concentrated in raw preparations such as sashimi and ceviche, and in fish that has been inadequately frozen prior to raw service.

A parallel University of Washington study found that anisakid levels increased in both chum and pink salmon between 1979 and 2021, according to coverage of Wood’s 2026 research. Skipjack tuna is not typically thought of in the same high-risk category as salmon, but the 96 percent infection rate in the Brazilian sample challenges that assumption.

Global scale: how much tuna are we talking about

Skipjack tuna is one of the most important tropical tuna species, fished throughout its geographic range. According to the UN Food and Agriculture Organization’s 2024 fisheries data, skipjack was the third most-captured marine species globally in 2022 at 3.1 million tonnes. That figure makes it one of the most consumed fish on Earth, present in the majority of canned tuna products sold in supermarkets across North America, Europe, and Asia.

The Brazil study examined 53 fish from a single haul in a single southern Brazilian location and cannot be extrapolated to all skipjack globally. Parasite loads vary by ocean region, season, proximity to marine mammal populations, and water temperature. A skipjack caught off the coast of Santa Catarina may carry different parasite burdens than one caught in the central Pacific or the Indian Ocean.

High parasitic diversity in skipjack tuna has been documented across the Atlantic, Pacific, and Indian Oceans. The Brazilian result fits a consistent global pattern, even if the specific infection rate in this sample is unusually high.

Read More: Parasitic worm populations are skyrocketing in some fish species used in sushi

What to do with this information

The Brazilian study is a data point, not a verdict on the safety of tuna.

Properly cooked or commercially canned tuna is safe. The cooking and canning processes kill parasites reliably, and dead worms are harmless. Anyone eating canned skipjack tuna from a major commercial brand is not at meaningful risk from parasites found in the live fish.

The risk category is raw or inadequately frozen fish. Anisakiasis can be acquired by eating raw or undercooked saltwater fish, squid, and octopus, including preparations such as sashimi and ceviche, as well as fish that has been inadequately treated – salted, marinated, smoked, or improperly frozen. Risk scales with frequency: the more often a person eats raw or undercooked fish, the more likely they are to encounter a live larva.

For people who eat sushi or sashimi regularly, the most practical precaution is confirming that the restaurant or supplier freezes fish before raw service. Some worms make it past screening steps even with diligent inspection. Cutting pieces of raw fish in half before eating is a low-effort check that can reveal visible worms before ingestion.

Rising parasite counts in ocean fish are partly a consequence of successful conservation. Recovering marine mammal populations accelerate the Anisakis life cycle – an awkward trade-off, but one with a relatively manageable human health dimension. Cook your fish, or confirm it was properly frozen before being served raw.

Disclaimer: This information is not intended to be a substitute for professional medical advice, diagnosis, or treatment and is for information only. Always seek the advice of your physician or another qualified health provider with any questions about your medical condition and/or current medication. Do not disregard professional medical advice or delay seeking advice or treatment because of something you have read here.

AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.