Parasites were found in the vast majority of skipjack tuna, a fish widely eaten raw as sashimi. Researchers who examined 53 skipjack tuna caught off Brazil detected parasites in 96% of them, identifying more than 1,600 parasites in total. Many were found in the flesh that people eat, prompting warnings about consuming raw fish.
Researchers at the Federal University of Santa Catarina in Brazil published their findings on parasitic infection in skipjack tuna in the international journal Ocean and Coastal Research, according to the New York Post and other outlets on the 7th.
The team conducted visual inspections of 53 skipjack tuna caught in waters off southern Brazil, then used stereo microscopes, optical microscopes and scanning electron microscopes to identify the parasites.
Parasites turned up in 96% of the fish examined, with roughly 1,600 specimens identified in total. The researchers found parasites capable of affecting humans, including nematodes of the genus Anisakis and tapeworms of the Trypanorhyncha order.
The parasites were concentrated in the internal organs. The intestines held the most at 596, followed by 441 in muscle tissue and 408 in the stomach. That means a substantial number were present in the flesh that people consume. No parasites were found in the kidneys or heart.
The researchers noted that while finding parasites in wild fish is not unusual, this survey stood out for its high infection rate and the sheer number of parasites identified. They stressed that removing the internal organs promptly after the catch is important to prevent parasites from migrating from the organs into the muscle.
Skipjack tuna is consumed widely around the world. Global catches of the species reached about 3.1 million metric tons in 2022, according to the journal. The fish is also a common raw material for canned goods and other processed seafood products.
The findings cannot be applied wholesale to all tuna or to tuna sashimi sold in South Korea. The study was limited to 53 skipjack tuna collected in Brazil at a specific point in time. On that basis alone, it is difficult to conclude that skipjack tuna worldwide, or tuna sashimi on the market, carry a high probability of parasitic infection.
The concern centers on eating the fish raw. Consuming fish or squid raw or undercooked while live Anisakis larvae are present can cause anisakiasis. Symptoms may include abdominal pain, nausea, vomiting and diarrhea, and the larvae can attach themselves to the walls of the esophagus, stomach or intestines.
Cooking fish thoroughly is the safest form of prevention. The U.S. Centers for Disease Control and Prevention recommends heating fish to an internal temperature of at least 63 degrees Celsius (145 degrees Fahrenheit). For fish eaten raw, proper freezing is needed to reduce the parasite risk. The Food and Drug Administration sets out conditions such as freezing fish at minus 20 degrees Celsius or below for seven days.
