Localized leucocyte response to Ichthyophthirius multifiliis establishment in immune carp Cyprinus carpio L

O-group carp (Cyprinus carpio) which had been immunized against Ichthyophthirius multifiliis by controlled infections were challenged by topical application of theronts to the caudal fin. The parasites which established were examined ultrastructurally, and host leucocyte responses were compared with those observed in primary infections. In the primary exposure group eosinophils and (to a lesser extent) basophils were the predominant cells infiltrating infection sites. In contrast, parasite development in immunized fish initiated localized leucocytic infiltrations which were dominated by eosinophilic granular cells (EGCs) and basophils. Greater localized phagocytosis was recorded in immunized fish by neutrophils, macrophages and resident epidermal filament cells. In vitro studies indicated that pronephric leucocytes from immunized fish displayed enhanced non-specific phagocytosis. In the skin, leucocytes were observed in close proximity to the trophozoite surface in both immunized and primary exposure fish, often undergoing lysis and release of cellular contents. However, there was no evidence of active cell adherence nor of any cell-mediated damage incurred to the parasite in either case. These observations are discussed in relation to the possible role of leucocytes in mediating pathogenesis and immune responses.