The present study was carried out to determine the possible causes of an emergent event of respiratory distress with consequent mass mortalities among Nile fish, Oreochromis niloticus (O. niloticus) at Mariotteya stream, an intrastate tributary of River Nile, Egypt. The area of incident extended from Shabramant till Abouseer city (along 4 km distance) with the direction of water current. Field visits have recorded thousands of dead large sized fish on both sides of the stream while huge numbers of fish of different size accumulated on the water surface showing typical signs of asphyxia. It was also noted that Mariotteya water body is being subjected to multiple sources of pollution through the dumping of improperly treated organic and inorganic chemical wastes in addition to sewage materials. Results of field and laboratory investigations have revealed that, all the examined 60 fish samples were heavily infested with
different types of parasites including zoonotic species: Cichlidogyrus arthracanthus (monogenea), Lamproglena monody and Ergasilus sarsi (Copepod), Myxobolus dermatobia, Chilodonella hexastica, Trichodina truttae, Trichodina fultoni, Cryptosporidium spp. and Balantidium spp. (Protozoa), Acanthosentis tilapae. (Acanthocephala), Clinostomum spp., Euclinostomum spp., Heterophid and Prohemistomatid metacercarae (larval of Trematodes). The physical and chemical examinations of water samples and the analysis of heavy metals concentration indicated marked abnormal water quality parameters and environmental pollution which might be incriminated as a primary stress factor that promoted the invasion of parasites as a secondary stress factor. The study concluded that both factors would have interacted to produce this catastrophic intense case of respiratory distress and mass mortalities. The impact of the recorded parasitic infestation and environmental pollution was briefly discussed.