Probable association between Anisakis infection in the muscle of skipjack tuna (Katsuwonus pelamis) and human anisakiasis in Tokyo, Japan.

Anisakiasis is a gastrointestinal disease caused by parasitic anisakid nematodes, mainly Anisakis simplex sensu stricto (A. simplex). Anisakiasis is prevalent in Japan and approximately 40% of anisakiasis cases in Tokyo occur through the consumption of raw or marinated mackerel. However, in 2018, there was a sudden increase in the number of the food poisoning cases in Tokyo caused by consumption of skipjack tuna (Katsuwonus pelamis). Therefore, we investigated anisakiasis cases resulting from ingestion of skipjack tuna in Tokyo, and surveyed the presence of Anisakis larvae in skipjack tuna in 2018 and 2019. Nineteen samples from 15 patients (13 in 2018 and 2 in 2019) with anisakiasis surely caused by ingestion of skipjack tuna were all identified as A. simplex. The higher mean abundance of Anisakis simplex larvae in skipjack tuna muscle in May 2018 (1.30; 13 larvae/10 fishes) compared to that in the other periods was regarded as a contributing factor in the increase in anisakiasis cases by ingesting skipjack tuna in 2018. To verify whether Anisakis larvae migrate from the visceral organs to the muscle during the period from fishing on the boat until processing for sale, the number of Anisakis larvae in skipjack tuna caught from August to November 2018 was investigated by removing the visceral organs at three different timings, i.e., immediately after catching, after landing, and after transport to the laboratory. Anisakis larvae were detected in the muscle irrespective of the timings at which visceral organs were removed. All larvae from the muscle were detected only from the ventral part and were identified as A. simplex. We thus consider that avoiding raw consumption of the ventral muscle should be an effective measure to prevent anisakiasis.