Risk factors for human Anisakis infection and association between the geographic origins of Scomber japonicus and anisakid nematodes

It has been reported that nearly all cases of anisakiasis in Japan are caused by Anisakis simplex sensu stricto. To elucidate this further, we investigated the presence of Anisakis type I larvae and Pseudoterranova decipiens in 218 Scomber japonicus fish collected from the seas of Japan. Anisakis type I larvae were detected in 74.3% (162/218) of the fish, and 99.8% of the Anisakis type I larvae comprised A. simplex sensu stricto and Anisakis pegreffii. Molecular identification techniques were used for 7.5% (360/4806) of the Anisakis type I larvae. The larvae found in the fish of the Pacific stock (the Pacific coast of Japan) and the Tsushima Warm Current stock (the East China Sea and the Sea of Japan) were primarily A. simplex sensu stricto and A. pegreffii, respectively. In addition, for the first time in Japan, Anisakis simplex C and Anisakis ziphidarum were detected in the fish of the Pacific stock. The average number of A. pegreffii and A. simplex sensu stricto larvae per fish was 47 and 6, respectively. However, the average number (0.61 larvae) of A. simplex sensu stricto in the muscle per fish was 12 times the average number (0.05 larvae) of A. pegreffii. When fish on the purchased day were compared with those held at 4 °C and 20 °C for 20 h, the penetration rates (ratio of the number of larvae detected in the muscle to the total number of larvae detected) of A. pegreffii and A. simplex sensu stricto were as high as 1.8% and 5.8%, respectively. In conclusion, we suggest that anisakiasis in Japan is mainly caused by A. simplex sensu stricto because it penetrates the muscle of the fish at a higher rate than A. pegreffii.