Variation in susceptibility pattern of fish to Argulus siamensis: Do immune responses of host play a role?

Branchiuran ectoparasites of the genus Argulus can have extensive damaging effects on cultured fish. There exist no systematic studies that evaluate susceptibility or resistance of various carp species to Argulus sp. and the underlying mechanisms. The present study aimed at identifying the most susceptible and resistant cultured species, studying settlement and survival of parasite on these species, and finally unravelling the variations of immune response in both resistant and susceptible species. Fish from eight species (Labeo rohita, Cirrhinus mrigala, Catla catla, Hypophthalmichthys molitrix, Cyprinus carpio, Ctenopharyngodon idella, Carassius auratus, Labeo fimbriatus) were individually challenged with metanauplii of A. siamensis (100 metanauplii/fish) before rearing them in single tank in triplicate for 45 days. Based on the observed parasite load on each species, L. rohita was found to be the most susceptible and C. idella the resistant species. The settlement and survival of the parasite on L. rohita and C. idella was compared at 24, 48, 72 and 96h post experimental infection. Survival was significantly low at 72h onwards in C. idella indicating it is an unsuitable/poorly preferred host for A. siamensis. The inflammatory responses which are known to be related to susceptibility were analysed. Individuals of both the species were exposed to A. siamensis (100 parasites/fish), and after 24h and 3 d, skin samples directly from the attachment site and non-attachment sites were assessed for transcriptomic profiles of selected innate defence genes. Artificial skin abrasion permitted comparisons between abrasion associated injury and louse-associated injury. The inflammatory responses varied significantly between both species indicating their role in determining susceptibility of a host to A. siamensis. The expression of major histocompatibility class II and matrix metalloproteinase 2 was significantly higher in C. idella compared to L. rohita and therefore appeared to be involved in the early protective response against A. siamensis. It is essential to study the expression pattern of more participatory genes of the inflammation related pathways to understand species specific susceptible patterns.