Comparative histopathological and ultrastructural investigations were performed on the livers of 2 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 3 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 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 2 nematode species.

The importance of larval viability in relation to the success of recovery techniques and to the interpretation of histological and migration studies was tested using mice infected with Toxocara pteropodis, T. canis and T. cati. Little difference was found in total numbers of viable larvae recovered from tissues incubated for 18 hours in 0·85% saline, 2% trypsin or 1% pepsin, but the rates of larval egress did vary. The recovery of dead larvae by digestion techniques was much lower. Larvae migrated in tissues undergoing fixation, which influenced histological findings. Larvae of T. canis did not undergo significant peritoneal migration in acutely-infected mice, but did penetrate their intestinal walls in large numbers within 4 hours of death. The implications of these findings in experimental studies are discussed.

Migration by nematode larvae through the tissues of their mammalian hosts can cause considerable pathology, and yet the evolutionary factors responsible for this migratory behaviour are poorly understood. The behaviour is particularly paradoxical in genera such as Ascaris and Strongylus in which larvae undergo extensive migrations which begin and end in the same location. The orthodox explanation for this apparently pointless behaviour is that a tissue phase is a developmental requirement following the evolutionary loss of skin penetration or intermediate hosts. Yet tissue migration is not always necessary for development, and navigation and survival in an array of different habitats must require costly biochemical and morphological adaptations. Migrating larvae also risk becoming lost or killed by the host. Natural selection should therefore remove such behaviour unless there are compensating benefits. Here we propose that migration is a selectively advantageous life-history strategy. We show that taxa exploiting tissue habitats during development are, on average, bigger than their closest relatives that develop wholly in the gastrointestinal tract. Time to reproduction is the same, indicating that worms with a tissue phase during development grow faster. This previously unsuspected association between juvenile habitat and size is independent of any effects of adult habitat, life-cycle, or host size, generation time or diet. Because fecundity is intimately linked with size in nematodes, this provides an explanation for the maintenance of tissue migration by natural selection, analogous to the pre-spawning migrations of salmon.