Parasitic diseases of fish result in great economic loss due to effect on normal health conditions of fishes, reduction of growth, abnormal metabolic activities and even death. The factors that directly influence the parasitic fauna of fishes are age, diet, abundance of fishes, independent number of a parasite within fish and season. The characteristic of any water body can influence and determine its parasitic fauna and when environmental conditions become suitable for mass reproduction of parasites, the parasitic diseases may spread very quikly . Thus, proper identification of fish parasites, their prevention and correct therapy for treatable infestations dramatically improve the health and productivity of affected fish. In the present study, two species of catfish found in Biratnagar area of Nepal were examined for occurrence and burden of gastrointestinal helminth parasites.
Among 100 Clarias gariepinus examined, 84.0% were found to be infected with helminth parasites. Three parasites were detected from gut content of this fish i.e. one nematode species, one cestode and one digenean treamtode. This result is in conformity with some other studies. Dan-kishiya and Zakari  reported nematode, cestode and trematode in wild Clarias gariepinus in Gwagwalada, Nigeria. Aliyu and Solomon  and Salawu et al  also detected some nematodes, cestodes and trematodes from Clarias gariepinus. The prevalence was 75.0% in study of Salawu et al , 59.4% in the study of Aliyu and Solomon , and 67.5% in the study of Kawe et al . Difference in prevalence of parasites in fish may be due to many factors. Williams and Jones  suggested that parasitism is determined by the interaction between both biotic and abiotic factors and differs in various aquatic ecosystems. Reports have shown that helminths are generally found in all freshwater fishes, with their prevalence and intensity dependent on factors of parasite species, host and its feeding habits, hygiene of the water body, and presence of intermediate hosts for the parasites .
In the current study, nematode species i.e. Procamallanus laevionchus was detected from 46.0% Clarias gariepinus and have higher burden than cestode namely Proteocephalus species (21.0%) and digenean treamtode i.e. Allocreadium species (17.0%). The higher incidence of nematodes than cestodes and trematodes revealed that nematodes were the commonest cause of parasitic infection in Clarias gariepinus and this is in conformity with with the findings of Kawe et al  and Aliyu and Solomon . Kawe et al  also detected Allocreadium species from 3.6% and Procamallanus laevionchus from 32.5% of Clarias gariepinus. Barson and Avenant-Oldewage  reported Proteocephalus species from 14% of Clarias gariepinus. Multiple infections by the helminth parasites was commonly seen in Clarias gariepinus. Among total Clarias gariepinus, Procamallanus laevionchus and Proteocephalus species were concurrently detected from 15.0% fish. Ajala and Awole  also reported multiple infections of helminth parasites in the gastrointestinal tract of Clarias gariepinus.
The catfish culture in Nepal is of great significance because of highly nourishing and easy source of protein. Parasites attack the fish and destroy them and/or produce disease in their bodies, thus making them unedible. The stinging catfish i.e. Heteroponeustes fossilis was also examined and all of them were found to contain some helminth parasites in their gastrointestinal tract with prevalence of 100%. Five parasites were detcted from gut content of this fish. Two nematode species namely Procamallanus heteropneustes and Eustrongyloides species were detected from 37.8% and 30.0% fishes respectively. One cestode namely Lytocestus indicus (21.7%), one trematode i.e. Phyllodystomum folium (23.9%) and an Acanthocephalan i.e. Pomphorhynchus species (100%) were detected from the Heteroponeustes fossilis. Other studies from different countries also repoted variable prevalence of various parasites from gastrointestinal tract of Heteroponeustes fossilis. Yadav SN  also detected Pomphorhynchus species from gastrointestinal tract of 100% of Heteroponeustes fossilis, which is similar to the result of current study. In the study of Nimbalkar et al , the prevalence of Eustrongyloides larvae was 50% in this fish. Similarly, Ningthoukhongjam et al  identified cestode parasites from intestine of 50% of Heteroponeustes fossilis and Gupta R  reported Procamallanus heteropneustes from 31.3% and Lytocestus indicus from 5.7% of Heteroponeustes fossilis. Puinyabati et al  clearly discussed that the species and feeding activity of the fish host and the preference and composition of the food play an important role in the diversity of the helminth parasites in the gastrointestinal region of fishes.
The present findings confirm that helminth parasites are widespread in the gastrointestinal tract of catfish found in the Biratnagar area of Nepal and the prevalence of helminth parasites is higher with heavy parasitic burden. Since it has been stated that helminth parasitic infection of fish affects its productivity, marketability, palatability and death of a good number of fishes, it is necessary to detect the parasites and develop their effective control measures.