Transcriptional analysis of the common carp (Cyprinus carpio L.) immune response to the fish louse Argulus japonicus Thiele (Crustacea: Branchiura).

In the present study we investigated changes in transcription levels of a panel of selected immune relevant genes in peripheral blood leucocytes (PBL) and skin samples collected from carp exposed to larval Argulus japonicus. We show that in skin up-regulation of gene transcription of the chemokine CXCa, and to a lesser extent the chemokine receptor CXCR1 and the cytokine TNFalpha, are good indicators of parasite-induced skin damage at 2 days post-parasite exposure. Up-regulation of gene transcription corresponded well with an increase in leucocytes, probably neutrophilic granulocyte numbers in skin samples collected at the sites of infection. We show that time-point controls are essential when studying gene expression, especially in peripheral blood leucocytes (PBL). In addition, we demonstrate that non-infected control samples isolated from the skin of infected fish are suitable autologous controls, at least until after larval A. japonicus have undergone their first moult and begun to demonstrate increased mobility over their host's integument. The observed results are indicative of A. japonicus affecting the skin as a whole organ, particularly after the parasites' first moult, a phenomenon which has a great impact on correct skin sampling for RNA isolation.