Introduction Parasitic diseases constitute the largest sector of fish diseases in warm water fishes in Egypt, reach- ing 80 % (Eissa, 2002). Although the majority of fish ectoprotozoa is commensally, but they induce serious diseases and mass mortalities especially in fry and fingerlings under stress condition (Khan, 2004; Eissa et al., 2013). Generally, fish parasites result in economic losses not only due to mortali- ties, tissue damage, and growth reduction but also from treatment expenses, which limited the expan- sion of aquaculture. Trichodinosis is one of the ectoparasitic proto- zoal diseases caused by Trichodina sp., they have a direct life cycle and they reproduce by binary fis- sion. Interestingly, most trichodinids are not pathogens, but under certain environmental condi- tions or when the fish are stressed by other factors, the parasite increases greatly its rate of infestation among fish and can become pathogenic and result- *Corresponding author: Amel M. El Asely E-mail address: amel.alaasly@fvtm.bu.edu.eg Journal of Advanced Veterinary Research Volume 5, Issue 3 (2015) 99-108 Journal of Advanced Veterinary Research http://advetresearch.com/index.php/avr/index 1 Department of Fish Diseases and Management, Faculty of Veterinary Medicine, Benha University, Egypt. 2 Department of Pathology, Faculty of Veterinary Medicine, Benha University, Egypt. ABSTRACT ISSN: 2090-6277/2090-6269/ © 2015 JAVR. All rights reserved. Studies on Some Parasitic Diseases in Oreochromis niloticus Fish Hatchery with Em- phasis to Life Stages Amel M. El Asely* 1 , Eman A. Abd El-Gawad 1 , Eman I. Soror 1 , Aziza A. Amin 2 , Adel A. Shaheen 1 This study was conducted on 210 Oreochromis niloticus (O. niloticus) of different life stages including (100 fry, 100 fingerlings and 10 broodstocks) obtained from a private fish hatchery at Kafer El-Sheikh Governorate, Egypt; during August 2014. The hatchery com- plains 30% mortality among fry and fingerlings while no mortalities was recorded among broodstocks. Parasitological examination revealed heavy infestation with Triochodina species (sp.) in all examined life stages at a prevalence rate 100%. In addition, Gyrodactylus sp. was recorded in gills of fry, fingerlings and broodstocks at a rate of 5, 12, and 10 %, respectively. Kidneys and gills of all examined life stages showed heavy infestations with Myxosporean sp., with 100 % prevalence rate. Haemogregarina sp. was described in the blood of fingerlings and gill tissues of broodstocks. Additionally, Encysted metacerceria was observed in gills of broodstocks. The recovered parasites were demonstrated hisopathologically in the gill and kidney tissues of the examined fish. The histopathological examination revealed that the infested gills exhibited serious lesions such as hyperplasia and hypertrophy of the lining epithelial cells of the gill filaments, fusion and necrosis of secondary lamellae and vasodilatation. The lining epithelium of the renal tubules showed degenerative and necrotic changes with the presence of various developmental stages of myxosporidia. In conclusion, fry and fingerlings exhibited high mortalities, while no mor- tality was recorded among broodstocks, regardless the intensity of infestation and severity of pathological alterations which was intense in broodstocks. . ARTICLE INFO Original Research Accepted: 11 May 2015 Keywords: Broodstocks Fry Histopathology Nile tilapia Parasitic infestation