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

Members of the Anisakidae family, in particular the genus <i>Anisakis</i> are among the most common seafood-borne parasites which may lead to anisakidosis, a zoonotic disease. <i>Anisakis simplex</i> (<i>s.s.</i>) (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 <i>Scomber scombrus</i> (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 <i>Cox2</i> gene sequencing and found to belong to the species <i>Anisakis simplex</i> (<i>s.s.</i>). Of 41 Atlantic mackerel examined, 36 individuals (88 %) harbored <i>A. simplex</i> (<i>s.s.</i>) 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 (<i>p</i> < 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 <i>A. simplex</i> (<i>s.s.</i>) larvae.