الفهرس 
AcknowledgementOnly 14 pages are availabe for public view
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Abstract
^a McCullough ( 41) mentioned that the biological control of freshwater snail hosts of schistosomiasis pad attracted much atte.nt ion in re,cent years, but the efficiency of such bio-control agents had rarely been tested outside laboratory conditions. Abdallah and Nasr (9) had been suggested that the effecttve biological control could be attained by taking advantage of the ecological competition between closely related species of snails. .. The closer any two species were ecologically related, the harder will be the competition if they were forced to live in tpe same habitat. If two species have exactlJ the same ecology, they simply cannot exist together for ~y length of time, the best fitted species will replace the weaker one. It was suggested on the besis of labor atory studies that snail Helisoma meets with the above requirements and it had been shown interesting potential as competitor 01, the intermediate host snails of schis tosomiasis. The present study aimed at 1. Study the competitive interactions between duryi ~d ~. alexandrina. 2- To elucidate the possible effect of li. duryi on the cercarial production of ~. mansoni infected ~.alexandrina snails. Intermediate host and control snails: . alexandrina stock was originally collected from irrigation ditches, located near Alexandria and maintained as a breeding stock in the laboratory. The control snail g. dur,yi was obtained from Sudan in 1986 and maintained at the laboratory till now. . The experimental design of the present study were classified into two main parts: Part 1- Dealing with the competitive interactions between .!!. duryi as a bio-control agent and Ji.alexandrina snails as a target snail of schistosomiasis. Besides to study the possible chemical changes of water conditioned by both snails. following eocperiments: It includes the I.A.l. The effect of .!!£ duryi with different proportions on the death rate and egg deposition of Jh alexandrina snails 0 I.A.2. The possible chemical changes of water conditioned by bot h snails. I.B. The effect of water conditioned by duryi (J5, 45 and 60 days) on the hatchability of egg masses and I the development of the target snails.! I.C. The effect of lie duryi snails on the egg hatchability of ]. alexandrina snails according to the availab llity of the food supply 0 Part II- Dealing with the effect. of .ti. duryi snails on the cercarial production of S. mansoni infected ~ alexandrina snails. A pilot study was: first tried to infect ].alexandrinE snails with miracidia newly hatched from eggs obtained from human patients infected with .§.. mansoni. The results indic ated that this methods was time consumed and gave a relatively small number of eggs and miracidia, therefore the collections of eggs was mainly obtained from intestines and livers of the infected mice. Also a pilot study was tried to infect !h. duryi snails each with 8 miracidia of h mansoni but all trials failed indicat ing that li.duryi snails were refractory to infection by schistosomiasis. The results obtained in the present study were divided int 0 : I.A.l. The effect of B£ duryi with different proportions on the death rate and egg deposition of ~ alexandrina snails: The results showed that the death rate of the target snails increased as the number of H. duryi in creased and was greatest ~t proportion 20:20. The egg deposit ion of ~ alexandrina snails was decreased till reached to the zero level at t he end of 3~ week, while egg deposit ion of Helisoma snails was numerous and cont inued till the 4th week. 1.A.2. The possible chemical changes of water conditioned by bot h snails: The results of the present study indicated that some chemical changes in the water conditioned by B.dur.yi were occurred in the form of significant reduction in calcium concentrat ion and pH and a significant increase in~gnesium, sodium potassium and chloride leading to significant reduction in Ca/Mg, Ca/Na and CalK rat ios. These ionic imbalance make the habitat unfavourable for the survival of vector snails and the hatchability of their Besides calcium depletion interferes with successful penetration and maturation of the invading schistosome cercariae in the definitive host. 1.B, The effect of water condit ioned by H. dur.\Ci (35, 45 and 60 days) on the hatchability of egg masses and development of the target snails: The obtained results demonstrated that water conditioned by .!it. duryi decreased the egg hatchability of snails, and the development of the newly hatched snails to t he adult stages. The least egg hatch observed among egg masses in water contioned for 60 days followed by 35 and 45 days respec The effect of B£ duryi snails on the egg hatchability of ~ alexandrina snails according to the availability of food supply: The results of the present study showed that the nwnber of the hatched egg masses of ]h alexandrina snails in the presences of H. duryi depends on two main factors: 1. Number of .!it. duryi snails present in association with egg masses of &t. alexandrina. 2- Availability of food supply i.eo presence of food. or absence R was noticed that the number of the hatched egg masses of the target snails decreased as the number of .t. duryi e.g increased. duryi ,e.g the egg deposited on Elodea leaves ~d then began to eat the leaves themselves. This behaviour was more manifested when the aquira were devoid of food supply. Paz:t II- The effect of H. duryi snails on the cercarial production of ~. mansoni infected ~ alexandrina snails: The obtained results revealed that the average cercarial production per single B. alexandrina snail was decreased as the number of 1h duryi increased. Also the reduction in cercaria1 production of the infected snails increased as the number of duryi increased and these reduction was greatest at proportion 10:10 then 10: 5 and 10:2 respectiveiy. The total cercaria1 production per 100 exposed ~ alexandrina snails was increas ed as t he number of !it. dur,yi decreased and was greatest at proportion 10:2 then 10:5 and 10:10 respect ively. The infection rate of alexandrina snails was increased as the number of H.duryi decreased. It was greatest at proportion 10:2 then followed by 10:5 and 10:10 respectively. The death rate of the target snails was increased aa the number of duryi increased.^leng