Cellular responses in rainbow trout Oncorhynchus mykiss to experimental Anisakis simplex infection

Fish in wild and cultured populations may be infected with numerous types of pathogens but the host responses vary dependent on both host and parasite species. The present study demonstrates how an experimental infection with endoparasitic nematode larvae (Anisakis simplex) induces cellular and humoral immune responses in rainbow trout Oncorhynchus mykiss. The nematode larvae invaded the peritoneal cavity of the fish following oral administration and became encapsulated by a range of host cells including macrophages, neutrophils, mast cells, fibroblasts, and lymphocytes. The main part (92.7%) of the recovered larvae was located in the body cavity and 51.3% along the pyloric caeca with only few in or on the stomach, liver, spleen, swim bladder, and musculature. The cellular reaction was documented by transmission electron microscopy (TEM) and histochemistry. Real time quantitative PCR (qPCR) showed that a series of immune-relevant genes in the host spleen became regulated by the infection. Thus, A. simplex induced downregulation of immune-genes (encoding IgD and lysozyme) and upregulation of the gene encoding the immune-regulating cytokine IL-10. Nematode molecules influencing the antiparasitic host reactions are discussed.