The cellular response of whiting, Merlangius merlangus (L.), to Contracaecum and Anisakis larvae parasitic in the liver was characterized by the presence of neutrophils, macrophages and proliferating fibroblasts. A capsule formed around the larvae after the removal of all necrotic debris by phagocytic cells. Melanin granules were deposited around the capsule. Four types of blood leucocytes were identified; thrombocytes, lymphocytes, monocytes and neutropbils. Phagocytic activity of the neutrophils and macrophages was suggested by the presence of dark stained bodies within them.
The fate of freshly hatched larvae of the marine ascaridoid nematodes Contracaecum osculatum and Pseudoterranova decipiens (from grey seals, Halichoerus grypus, from Scotland and the Canadian Atlantic) was investigated following oral or intraperitoneal introduction into rainbow trout, Oncorhynchus mykiss, maintained at 7–13 °C in fresh water. Neither species appeared to survive for long in the trout alimentary tract following oral introduction; a few larvae were found alive after 2 d but none after 21 d. intraperitoneally, P. decipiens did not survive beyond 21 d, but some C. osculatum exsheathed and developed over several months to lengths over 13 mm, and morphologically and morphometrically resembled third-stage larvae from naturally infected whiting, Merlangius merlangus, from the northern North Sea; no moult was detected. Thus, freshly hatched free-living larvae of C. osculatum are able to develop directly to the third stage in the body cavity of a fish without earlier passage through a crustacean or other invertebrate host.
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