The immune response of rainbow trout (Oncorhynchus mykiss) against Aphanomyces invadans

Aphanomyces invadans, also referred to A. invaderis or A. piscicida, is the aetiological agent of epizootic ulcerative syndrome (EUS). Immune response s of rainbow trout (Oncorhynchus mykiss) against this fungus were examined, including both antibody and macrophage responses. The fish antibodies to t he fungus, assessed by an enzyme-linked immunosorbent assay (ELISA) were ge nerally non-specific, but sera from fish Vaccinated with A. invadans reacte d with bands at 55 and 40 kDa by Western blot analysis. Macrophage activity was examined in vitro by culturing rainbow trout head kidney macrophages w ith fungal spores. The macrophages were able to phagocytose spores in vitro and, in some cases, hyphae were observed growing from germinated spores wi thin the macrophage. When low concentrations of spores (1 x 10(2) and 1 x 1 0(3) spore ml(-1)) were cultured with macrophage monolayers, no germination of spores or growth of hyphae was observed. At concentrations of 1 x 10(4) spore ml(-1) or higher, macrophages were unable to kill all the spores and fungal mycelium eventually overgrew the culture. Transmission electron mic roscopy (TEM) was used to examine the ultrastructural interaction between m acrophages and the fungus. The possible role of rainbow trout as a model fo r A. invadans infection in salmonids is also discussed. (C) 1999 Academic P ress.