الفهرس 
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Abstract
A total number of 512 fish of different weights and sizes were randomly collected a live or freshly dead from River Nile and its turiburites (265) and Private fish farms in Manzala (247) in Dakahlia province in the period from July 2014 to June 2015.
The main clinical signs observed of the naturally infected fish were loss of appetite, emaciation, weakness, sluggishness, and less respond to external stimuli. Skin appeared pale or darker in color with some erosions, ulcers, scale loss with eroded fins and black spots on different parts of fish body. Gills in the postmortem examination appeared severely congested, sticky with excessive mucous accumulation on gill filaments and gill pouches.
The parasitological examination of the naturally infected fishes revealed the following parasites: (1) monogenetic trematodes including Cichlidogyrus arthracanthus, C. tilapiae and C. halli, Macrogyrodactylus clari, Ancylodiscoide longitubus, (2) crustacean parasites, Lamproglena mondi and Ergasilus elegans, and (3) protozoal parasites, Henneguya branchialis, Myxobolus niloticus, and Trichodina.
The prevalence of ectoparasites collected from River Nile and Manzala fish farms at Dakahlia Province revealed a total infection of 49.8 and 52.6% among the examined fishes respectively.
With the respect to C.gariepinus, infection rate of 53.1% and 60.6% were recorded from River Nile area. While, in Manzala, we recorded infection rate of 45.8% and 43.4% for O.niloticus. The most susceptible fish species to parasitic infection in both examined localities concluded to be C. gariepinus.
The peak of the total ectoparasitic infection in C. gariepinus and O.niloticus collected from River Nile was in winter (62.8%) and (60%) followed by autumn (60%) and (46.6), Summer (50%) and (40%) and spring (40%) and (36.66%), respectively.
Higher prevalence was detected regarding parasites collected from C. gariepinus in Manzala area. The prevalence rate was as follows, 72.5% in autumn, 61.1% in winter, 57.6% in summer, and 46.6% in spring. Meanwhile, autumn was the highest season in O.niloticus, which reached to 57.1%, but the spring was the lowest one 32%.while 40% in summer and 43.7% in winter
Regarding to the ectoparasites recovered from examined fish species collected from River Nile in Dakahlia Province, it was noticed that C. gariepinus was found to be infected with protozoa 27.5%, monogenea 22 % and no crustacea were recovered 0%. While O. niloticus showed protozoa of 12.5%, monogeneans of 33.3%, crustaceans of 25%. In Manzala, mongenea were the highest in infection in C. gariepinus (28.03 %) followed by protozoa (26.5%) and crustacea were not recovered as well. On the other hands,monogenea in O. niloticus recorded high infection (37.3 %) followed by crustacean and then protozoa which were 32.1 and 21.7%, respectively.
Concerning the seasonal prevalence of the detected parasites, The summer season recorded high prevalence of monogenetic trematodes in both C. gariepinus and O. niloticus collected from River Nile (34.7 and 39.1%) followed by spring (33.3 and 44.4%), respectively. Autumn and winter in both species was detected as (20 and 26.6%) and (16.2and 18.9%), respectively. Concerning Manzala area, summer was the highest season of infection in C. gariepinus (44.4%), while in spring was the highest of infection (40.9 %) in O. niloticus.
The highest seasonal prevalence of examined O. niloticus with curstacea
in River Nile and Manzala was in summer (39.1%) and (48.1%),respectively.
Crustaceans were not detected in C. gariepinus in both localities.
Protozoa species in C. gariepinus in River Nile recorded high infections in winter (32.4%), while in tilapia, the autumn season was the highest of infection (26.6%). Manzala area recorded higher rate of protozoal infection in C. gariepinus in winter season (36.1 %). But, autumn and winter were the highest seasons in O.niloticus (22.2%).