الفهرس 
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Abstract
Biomphalaria alexandrina snails belonging to the phylum ~llusca, class Gastropoda, subcliss Pulmonata, family Plano rbidae, subfamily Planorbinae. It is the intermediate host of Schistosoma mansoni in Egypt. It” is more common in Lower Egypt especially in the lowlands in the northern part of the Nile D~lta. They are more abundant in drains than in irrigation cannals. In the water channels they are more plentiful towarcls the distal ends of the network due to slower current. The class Trematoda is classified into two subclasses Digenea and Aspidogastrea. D:tgenea is further classified into two superorders; Anepitheliocystidi~ and Epithe1iocystidiae. The infection by the trematodes causes many diseases to man and animals so they ~e constitute an important subject of many investigations allover the world. In Egypt many members of order Strigeata and order Echinostomata belonging to the superorder Anepitheliocystidia and also members of order Plagiorchiata, Opithorechiata belonging to the superorder Epitheliocystidia are still under investigationJ The present study aimed to investigate the trematode para sit e in f e c t ion in B. a le x and r i n a s n a i I s coIl e c t e d fro m 11 I vater bodies in a locality near Alexandria and to study the infection in different sites depending on the distance from the villages in different seasons. The chosen study area lies within an area east of AI e x and r i a . It in c 1 u d e st h r e e villages: E 1- M an s h i a, K hat a b and Nasr Saleh. The chosen water channels were selected according to human water contact activities, density of ~ alexandrina snails and the feasibility of snails collection. Maping of the studied area was done. The present study focused on the morphological charaLters of the trematode species of B.alexandrina snails, their classification, prevalence and seasonal abundance. The snails were collected monthly from different watar bodies chosen in the proximi ty or at a distance from the villages. The collected snails were counted, measured and examined by light exposure for cercarial shedding, then they were grouped according to the type of the obtained cercariae. Snails not shedding cercariae were maintained and investigated successively for cercarial shedding.) Snail still not shedding cercariae were crushed and examined for any detectable para site. The prevalence of infection by the different species us recorded monthly throughout the study period which extended for one year from September 1985 to August 1986. The obtained larval stages of the various trematodes were examined a1i ve and after using vital stain. Rediae and sporocysts were examined after using vi tal stain, measured, and drown by camera lucida. The cercariae were measured after being placed in a DROP of distilled water on a slide. The cover slip was applied and a slide was gently heated over a flame for 1-2 seconds to kill the cercariae. Besides, the cercariae were fi~ed by using a hot solution of lactophenol and stained by acidified carmine as permanent stain and mounting the stained cercariae in glycerine jelly as semi permanent preparations and then drown by the help of camera lucida. The establishment of the ~dult Echinostome ~as succeeded under laboratory conditions. This was achieved by the encyst ment of Echinostome cercariae in laboratory bred ~.alexandrina snails. The albino mice were inoculated by the obtained encystecl metacercariae orally by stomach tuhe. After eight days the eggs were a.ppeared in the stool, measured and drown by camera lucida. The mice were then sacrificed and the adult Echino stome worms were obtained from the small intestine. They ’were fixed, stained by using Semichon carmine and drawn by camera lucida. The obtained data were subjected to statistical analysis and interpretation. The results obtained in this study could be summarized as follows: !)Identification of the various trematode infections detected in the examined B.alexandrina snails: Four forms of larval trematode fauna were found para sitizing the target snails;’ A-Cercaria and sporocysts 0.£ Schi.stosoma mansoni belonging to order Strigeata, superfamily Schistosomatidae. The cercaria was characterised by forked tail, absence of eye spots and has no redial stage. B-Larval stages of Echinostome (rediae, cercariae, metacer-= cari~. They were belonging to order Echinostomata, super family Echinostoma t idae. The cercar ia was charac t,er i sed by single tail, collar spines and has no eye spots. C-ParamphistDme cercariae which were belonging to order Strigeata, superfamily Paramphistomoidea. The cercaria was characterised by the presence of two eye spots and has a single tail. D-Strigeoid. (Holostome) cercariae which were belonging to order Strigeata, superfamily Strigeoidea. The cercaria has ? long tail fruci and no eye spots. II)Distribution of the trematode infections in B.alexandrina and among the examined villages: II)1.Prevalence of larval trematodes in B.aolexandrina snails: A-Echino’stome infection (rediae, cercariae, metacercariae) showed the highest prevalence followed by ~.mansoni (sporo cysts and cercariae) and the least prevalence was encoun tered among the snails infected with Paramphistome and Strigeoid . B-One snail was harbouring double infection. It shed Schis to some cercariae and at the’sametime harbouring rediae of Echinostome. C-Four snails were found infected with Echinostome metacercaria and at the sametime shedding S .mansoni cercariae. II)2.Prevalence of larval trematodes in different sites (far and near sites): A-The present study revealed that the prevalence of ~.mansoni and Echinostome infections were higher in far sites than in near sites~ This could be due to human water contact activities pollution by domestic and farm animals which are more pre vailed in far than in near sites, besides the snails which may get the inf ec tion near de~elling places swep t by wa t er current to”far sites. B-No significan~ di f f erence was found between t he in f ec t i on in near and far sites as regards to Echinostome metacercariae. (-No significante difference was recorded between the infection in near and far si tes in case of both Paramphistome and Stri~eoide cercariae. This could be due to low prevalence rate of infection among the collected snails. II)3.Snail size and trematode infections: The infected B.alexandrina snails with various larval trematode stages were mostlyfownd at the size between 9-16 mm with high frequency between 12-15 mm. III)Effect of seasonal variations on the prevalence of larval trematode infections in B.a1exandrina snails: --I-The infection of B.a1exandrina with S.mansoni was observed allover the year. The maximum infection rate was recorded during June