Infestation with metacercarial stage of Isoparorchis hypselobagri (Billet, 1898) in cage cultured Ompok bimaculatus vis-a-vis host and environmental interaction in a large tropical reservoir

The present study reports the occurrence of metacercariae of Isoparorchis hypselobagri in a food fish Ompok bimaculatus cultured in cages at Maithon reservoir, India. The metacercarial parasite was initially identified morphologically. Molecular identification through partial amplification of 18S rRNA and cytochrome oxidase subunit 1 (COI) gene and sequencing further confirmed the parasite species. The patterns of infection in terms of intensity, mean intensity, and prevalence of metacercariae in O. bimaculatus was recorded. Influence of host size, season and water quality on infection rate was tested through Generalized linear model (GLM). The prevalence varied from 42.2% to 88.8% and intensity of infection ranged from 1 to 10. PCA analysis showed that the metacercarial parasite was mostly located near the air bladder, around kidney and near kidney of the host. GLM results showed that the size of the fish influenced the intensity of infection. Higher water temperature reduced the parasitic growth whereas dissolved oxygen was positively related with intensity of the parasite infection. The occurrence of the first intermediate host snails indicated that they might be the possible source of cercaria in the cages. This study is the first report of I. hypselobagri infections in inland cage culture from India and has implications for understanding the ecological and taxonomic patterns and pathology in caged fish in tropical reservoirs. The high infestation rate and large size of parasite presents serious threat for production of this high value fish species in cages. The presence of parasites not only affects growth and survival but also leads to reduced consumer acceptance. The control of snail vectors might play a significant role in prevention of parasitic diseases in cage culture system. Hence, before selection of candidate fish species and site for cage culture, management of such parasitic disease should be duly considered. This will not only prevent economic losses due to parasitic diseases but also ensure consumer safety.