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 b 2 microglobulin (b 2 M). Both tumour necrosis factor a (TNFa) 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 b 2 M genes being the exceptions which were significantly up-regulated at 12 h and 3 days 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.

The study was undertaken to determine the modulation in innate immune response of rohu (Labeo rohita) during experimental freshwater lice Argulus siamensis infection. Results showed that serum a-2 macroglobulin (a-2M) activity, ceruloplasmin level and alternative complement activity were signi¢cantly (Po0.05) lower in ¢sh at di¡erent degrees of lice infection in comparison with uninfected control. No signi¢cant di¡erence (P40.05) in haemagglutination titre was observed in ¢sh with low-and high-degree lice infections as compared with uninfected control. The serum lysozyme level was signi¢cantly (Po0.05) lower in low degree of lice infection as compared with control ¢sh. The total serum antiprotease, myeloperoxidase activity and total protein level were not signi¢cantly di¡erent (P40.05) in di¡erent degrees of lice-infected ¢sh with respect to the control ¢sh. The study indicated that A. siamensis infection modulated the immune system of rohu by suppressing the a-2M, serum complement activities and ceruloplasmin level and through induction of stress response. The baseline data obtained in the present study have tremendous importance in understanding the susceptibility of rohu to di¡erent degrees of parasitosis and might be useful in controlling this dreaded ectoparasitic infection in ¢sh.

Argulus is one of the most important fish parasites that cause heavy economic loss to aquaculture industry. The present investigation was undertaken to study the genetic diversity of the Argulus sp. collected from 13 locations representing major aquaculture zones in India by RAPD analysis and to develop species-specific markers. Thirteen random decamer primers were used to amplify DNA fragments from three individual parasites of each location. Of the 172 bands scored by the primers, 168 were polymorphic. The per cent polymorphic loci and gene diversity values varied within a range of 8.14-43.02 and 0.0342-0.1727 respectively. Nei's genetic similarity between populations across all the primers ranged from 0.363 to 0.969. The dendrogram based on Nei's genetic distance showed two clusters; Bangalore and Mandi populations forming one cluster, and the rest in another cluster. The clusters also revealed strong correlation with the species identified as A. japonicus and A. siamensis respectively by morphological method. The study thus indicated A. siamensis as the major prevalent species in carp culture farms in India. Species-specific primers were designed from unique sequences cloned from RAPD fragments that could able to identify A. siamensis and A. japonicus separately.

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