Anisakis larvae in live-caught chub mackerel from Japan: tissue-specific distribution and sibling species identification

Abstract Anisakis simplex complex is a major causative agent of human anisakiasis, but species composition and tissue distribution, including intra-vitam muscle parasitism, remain unclear. We investigated the prevalence and tissue localization of Anisakis larvae in live-caught Pacific chub mackerel ( Scomber japonicus ) from four Japanese regions. A high-resolution melting (HRM) assay targeting the ITS region enabled high-throughput discrimination of A. simplex sensu stricto (s.s.), A. pegreffii , A. berlandi , and A. simplex s.s. × A. pegreffii hybrids. In Iwate and Shizuoka (Pacific coast), A. simplex s.s. was predominant in both viscera and muscle tissues, with a positive correlation between tissue burdens and a ventral bias of muscle larvae. In Fukui (Sea of Japan), overall prevalence was high, and the viscera showed a mixed sibling-species composition, whereas muscle larvae were predominantly A. simplex s.s. In Nagasaki (East China Sea), A. pegreffii dominated the viscera, but only A. simplex s.s. occurred in muscle. These findings indicate that A. simplex s.s. can parasitize muscle of live mackerel and that heavier visceral infections are associated with greater muscle involvement. This study underscores the importance of sibling-species monitoring, proper cold-chain and evisceration management, and HRM assays for effective surveillance to mitigate anisakiasis risk.