Worms found in 96% of fish tested, some of them exactly in the part we eat

Sushi, sashimi, and canned food lovers might find it hard to digest the following findings: a new study published in the scientific journal Ocean and Coastal Research reveals that one of the most common and best-selling tuna species in the world is swimming while infested with parasitic worms. A team of researchers from the Federal University of Santa Catarina in southern Brazil analyzed 53 tuna fish caught in the area and discovered that almost all of them, without exception, were infested with hundreds of internal parasites.

During the microscopic examinations, the researchers counted a total of more than 1,600 small worms. Out of the 53 fish tested, 96% were found to be infested. The only organs found to be completely free of parasites in all fish were the kidneys and the heart, while in the other tissues, the presence of worms was particularly prominent.

The intestines were found to be the most infested organ, followed immediately by the muscle tissue, which is exactly the part intended for human consumption, and then the stomach. The researchers identified mainly two types of parasites in the fish: Trypanorhyncha and Anisakis.

The species tested in the study is the skipjack tuna, Katsuwonus pelamis – also known as bonito or katsuo, a fish of immense commercial value that is the third most caught species in the world, with 3.1 million tons caught and sold in 2022 alone. In light of the findings, the researchers emphasized the critical importance of sanitary inspection and immediate gutting after fishing to prevent the migration of larvae from the intestines into the fish meat.

Is there a reason for panic? Experts clarify that the risk depends on how the fish is consumed: in canned food and with full cooking, the heating and industrial sterilization processes kill the parasites, so they do not pose a health risk to humans. At the same time, finding worms in canned food harms the quality of the product, consumer trust, and could cause heavy economic damage to the fishing industry.

In raw or uncooked fish (sashimi and sushi): eating live parasites can lead to them attaching to the human intestinal walls and causing acute symptoms in the digestive system, including severe abdominal pain, nausea, vomiting, and diarrhea. In restaurants and food factories, it is customary to carefully check the fish fillets and remove visible parasites, but it is difficult to completely prevent the penetration of tiny larvae.

In terms of kashrut, the very presence of parasites in a fish raises a halachic question separate from the health risk. Tuna itself is a kosher fish, but the Torah forbids eating creeping things and worms, so when infestation exists, there is significance to the location of the parasite and the way it reached the fish meat. In halachic rulings, there are discussions and differences between types of worms and their locations, and in particular regarding Anisakis parasites found in fish.

Therefore, the study's findings do not make tuna "not kosher" across the board, but they may strengthen the need for proper inspection, cleaning, and supervision in accordance with the guidelines of the kashrut body being relied upon. It is also important to distinguish between the health and halachic aspects: freezing or cooking may kill the parasite, but killing the worm itself does not necessarily permit it for consumption in terms of kashrut laws.

Despite the public aversion, some parasitologists see the phenomenon as a positive sign. Previous studies from the University of Washington pointed to a 280-fold increase in the amount of Anisakis worms in food fish since the 1970s. According to researchers, this increase may indicate the recovery of marine mammal populations, such as whales and dolphins, which serve as final hosts for these parasites, following conservation laws and the ban on whaling.

The researchers emphasize that further studies are needed in other fishing areas around the world to determine whether the infestation rate found in Brazil is characteristic of the oceans in general, and remind that full cooking neutralizes any biological danger.