Anisakid larvae in the organs of Atlantic mackerel, Scomber scombrus (L.): Host or parasite superiority?

Members of the Anisakidae family, in particular the genus Anisakis are among the most common seafood-borne parasites which may lead to anisakidosis, a zoonotic disease. Anisakis simplex (s.s.) (Rudolphi, 1809) can infect humans by means of the ingestion of raw or undercooked fish harbouring the third-stage larvae of this nematode. Histopathological and ultrastructural investigations were performed on the visceral organs and flesh of Atlantic mackerel Scomber scombrus (L.) from the Bay of Biscay. Results showed the presence of a high infection level of third-larval stages of an anisakid species in the visceral organs and rarely in the flesh of the host. A subsample of 15 live nematode larvae from 5 fish were genetically identified by mtDNA Cox2 gene sequencing and found to belong to the species Anisakis simplex (s.s.). Of 41 Atlantic mackerel examined, 36 individuals (88 %) harbored A. simplex (s.s.) L3 larvae, with an intensity of infection ranging from 5 to 650 larvae per fish (86.97 ± 17.26, mean ± standard error). A total of 3131 larvae were counted and the number of calcified larvae was significantly higher than that of live ones (p < 0.01). The most parasitized sites were the serosa covering the intestine and the pyloric caeca as well as the mesenteries which encircled the gonads. In most infected organs, a granuloma surrounded the calcified and live larvae with a more intense response around the former. Within the granuloma epithelioid cells, fibroblasts and collagen fibres were recognized, outside the granuloma, mast cells (MCs), melano-macrophage aggregates (MAs) and macrophages were frequent and to a lesser degree rodlet cells (RCs). This is the first study that has investigated the cellular immune responses in the visceral organs and flesh of Atlantic mackerel against A. simplex (s.s.) larvae.