First report of Dactylogyrus scorpius infection in Indian major carp, Labeo rohita from India: Host specificity and kinetics of immune gene expression in gills

Dactylogyrus is one of the major parasites ubiquitously present in the aquatic environment causing severe economic loss. The present study documented a new species of Dactylogyrus from India infecting the Indian major carp, i.e. rohu (Labeo rohita) both in natural and experimental conditions. The 18 s and 28 s rDNA sequence analyses revealed the present isolate of having 99.29% and 96.21% sequence homology, respectively with D. scorpius belonging to African Dactylogyrus lineage III. A co-habitational challenge study further confirmed that catla (Catla catla) and mrigal (Cirrhinus mrigala), two other species of Indian major carps, were resistant to this parasite in a 20-day infection trial. On the other hand, co-habitational challenge study showed 100% prevalence in rohu from 1-day post-infection (dpi) onwards with a gradual increase in average parasitic load of 4.8, 40.84, 61.50, 91.20 and 81.40 numbers on 1, 3, 5, 10 and 20 dpi of sampling, respectively. This study forms the first report of D. scorpius infection in India and rohu was found to be the preferred host of this parasite. The gill tissues collected from rohu fingerlings at the above time points were also subjected to expression analysis of different immune-related genes viz., genes involved in specific immunity (IgM, IgZ, MHC I), recognition molecule (TLR 22), pro-inflammatory cytokines (IL-1β, IL-6, IL-8, IL-15, TNF α), anti-inflammatory cytokine (IL-10), antioxidant molecules (MnSOD, GPx, catalase) and antimicrobial peptides (apolipoprotein A-I, lysozyme G). A significant up-regulation of all immune-related genes except MHC I and IL-15 was observed in gill tissues of infected rohu at different time points, thus indicating the role of both specific and non-specific immune responses against D. scorpius infection. The progressive rise in parasitic load in gill tissues corresponding to days of infection clearly indicated immune evasion mechanism for its co-existence in the advent of up-regulation of an array of immune-related genes in the host. The results obtained in this study could be useful for understanding host-pathogen interaction and further development of any prophylactic measures against this important parasite.