Innate immune encapsulation allows gonadal tolerance to Anisakis simplex (s.s.) infection in Atlantic mackerel (Scomber scombrus)

Most of the publications on Anisakis simplex (Rudolphi, 1809) focus upon systematic aspects and zoonotic risk but there is lack of detailed information on infection and cellular immune response of the gonads. The current investigation detected whether gonadal infection by A. simplex affects reproductive development and how Atlantic mackerel, Scomber scombrus (L.), responds immunologically. Thirty-four specimens out of 62 S. scombrus (55%) individuals had the larvae on or in the gonads, and live and calcified dead larvae often co-occurred. Nematode larvae were found in both mature and immature gonads. The intensity of infection varied from 1 to 31 nematodes per gonad (5.68 ± 5.53, mean ± standard deviation). In ovaries that harboured the nematode (inside the organ and in the mesenteries around it), in close vicinity to the larvae, mast cells (MCs), rodlet cells (RCs), macrophages, and macrophage aggregates (MAs) occurred. Fewer of the above cells were observed in the mesenteries of parasitized testes. In the gonads of infected fish, within the granulomas that surrounded the calcified and live larvae, putative epithelioid cells, fibroblasts, MCs, MAs and collagen fibres were present. The absence of a correlation between the intensity of infection and the host gonadosomatic index in female and male gonads might indicate that A. simplex larvae do not affect the production of gametes and gonad development in Atlantic mackerel. Our results suggest that localized encapsulation represents an effective tolerance strategy allowing parasite persistence while preserving host reproductive function.