The immune response in rohu, Labeo rohita (Actinopterygii: Cyprinidae) to Argulus siamensis (Branchiura: Argulidae) infection: kinetics of immune gene expression and innate immune response

Argulus siamensis is a major pathogen in freshwater aquaculture. The immune responses of Indian major carp, Labeo rohita to experimental infection of A. siamensis was evaluated by quantitation of immune-relevant gene expression in head kidney and skin, and serum innate immune parameters through the course of infection. In skin of infected fish, antioxidant genes like natural killer cell enhancing factor (NKEF-B) and superoxide dismutase (MnSOD) were significantly up-regulated in addition to lysozyme G and (2) microglobulin (M-2). Both tumour necrosis factor (TNF) and toll-like receptor 22 (TLR22) genes were significantly down-regulated in skin during early phases of the infection. Most of the genes exhibited significant down-regulation in head kidney; immunoglobulin (IgM) and M-2 genes being the exceptions which were significantly up-regulated at 12h and 3days post infection. Most of the innate immune parameters like serum complement activity and ceruloplasmin levels showed significant reduction in infected fish. The observed results are indicative of A. siamensis modulating the immune response of rohu by down-regulation of many immune factors which may explain the susceptibility of rohu to A. siamensis infection. The interaction of this parasite with the host need to be further explored to understand its pathogenesis.