A portrait of 2 nematodes in liver of their paratenic fish hosts, illustrating different immunological approaches

Comparative histopathological and ultrastructural investigations were performed on the livers of two fish species, namely, flounder (Platichthys flesus (L.) naturally infected with the nematode Anisakis simplex (s.l.) (Rudolphi, 1809) larvae (L3) and tuvira (Gymnotus inaequilabiatus) (Valenciennes, 1839) harbouring the nematode Brevimulticaecum sp. (L3) (Shikhobalova and Mozgovoi, 1952). The intensity of infection by A. simplex (s.l.) larvae (L3) in flounders ranged from 3 to 10 parasites per organ. The worms were encapsulated by the peritoneal visceral serosa on the external surface of the liver. Infected P. flesus livers showed hepatocyte cytoplasmic rarefaction and cell swelling. A few immune cell types, such as macrophages, limited numbers of mast cells (MCs), lymphocytes, and some epithelioid cells, were observed within the granuloma. The intensity of infection by Brevimulticaecum sp. (L3) in G. inaequilabiatus ranged from 4 to over 340 larvae per organ, and the nematode larvae were encircled by round-to-oval granulomas. Each granuloma possessed three concentric layers of cells and tissue: an inner layer in close proximity to the Brevimulticaecum sp. (L3) cuticle, formed by densely packed layers of epithelioid cells showing several desmosomes between each other; a middle layer of numerous MCs entrapped in a thin fibroblast-connective mesh; and an outer layer of fibrous connective tissue with thin, elongated fibroblasts. High numbers of macrophages and macrophage aggregates (MAs) were scattered within the granuloma. This is the first study to compare the cellular nature of granulomas and the immune responses in the livers of paratenic fish hosts of two nematode species.