The present study was aimed to know the present status of parasitic fauna and its prevalence, intensity and abundance in the northeast part of Bangladesh and to evaluate the histopathological alterations of different parts of the fish organ due to parasitic infestation. The present status of parasitic fauna was studied on 17 different fish species collected from Singari Beel, Erail Beel situated at Golapganj Upazila, Kawadighi haor situated at Sadar Upazila and Surma River from October 2020 to March 2021 (six months).
During the investigation 189 individuals of different fish species were examined. Among them 91 fish were found infested with 860 parasites from 7 groups namely Digenea, Nematode, Acanthocephala, Crustacea, Protozoa, Monogenea and Cestode. A total of sixteen different parasitic genera were identified from the hosts examined as Myxobolus sp., Tetrahymena sp., Dactylogyrus sp., Djombangia sp., Argulus bengalensis, Euclinostomum multiceacum, E. heterostomum, Camallanus intestinalus, Procamallanus viviparous, Gnathostoma spinigerum, Camallanus pearsei, Pallisentis ophiocephali, Pallisentis nandai, Pallisentis goboes, Lytocystus indicus, Senga ophiocephalina. The highest prevalence (100%) was observed in Channa striatus and the lowest (30.76%) was observed in Mystus cavasius. The highest abundance and mean intensity were observed at 27 and 43.2 percent respectively in Mastacembelus armatus. The lowest abundance (0.77%) was observed in Nandus nandus and lowest mean intensity (1.8%) in Clarias batrachus. Parasite infected fishes were euthanized and histological samples from different organs showed multiple pathological alterations in gills, kidney, liver and gastrointestinal tract. Moreover, for the first time in Bangladesh, S. ophiocephalina were identified by Polymerase Chain Reaction (PCR) and generated PCR product size around 700 bps. The obtained 28s RNA F and 28s RNA R sequence of S. ophiocephalina matched with 1121– 1463 bps (Identity- 100%) of the 28s RNA gene of S. magnum, S. lucknowensis and S. visakhapatnamensis strain 28S ribosomal RNA gene, partial sequence (GB Accession number KR780913.1, KR780891.1 and KR780890.1).