Experimental infection with Anguillicola crassus alters immune gene expression in both spleen and head kidney of the European eel (Anguilla anguilla).

Invasive parasites have been implicated in the declines of several freshwater species. The swim bladder nematode Anguillicola crassus was introduced into Europe in the 1980s and is considered a threat to the European eel (Anguilla anguilla). Infection affects stress resistance and swimming behaviour. European eels produce an immune response against the parasite during the late stages of infection and after repeated infections. We used RNA-seq to examine the molecular response to infection during the poorly understood early stage and identify expression of genes and associated processes that are modified in two immune organs of European eels 3 days post infection with A. crassus. In the spleen, 67 genes were differentially expressed, 32 of which were annotated. Most of these were involved in immune processes and their regulation. Other differentially expressed genes in the spleen were important for heme metabolism and heme turn-over. In the head kidney, 257 genes (134 annotated) were differentially expressed. Several of these were associated with immune functions. Other differentially expressed genes in the head kidney were related to renal function, in particular osmoregulation and paracellular flow. We conclude that the early response of European eels to A. crassus is complex and involves various processes aside from the immune system. We identified molecular changes occurring early during the infection and identified candidate genes and processes which will facilitate future studies aimed at determining the factors affecting European eel viability in the face of this invasive parasite.