الفهرس 
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Abstract
It is well recognized that the most feasible method for the control of schistosomiasis is through combating its intermediate hosts and their larvae, since these are the weakest link of the parasite’s life cycle (WHO, 1993). Thus, the present study aimed to evaluate the molluscicidal, ovicidal and larvicidal activity of Euphorbia peplus and praziquantel against Biomphalaria alexandria and S. mansoni miracidia and cercariae. In additin effects on egg laying, neurosecretory cells and internal defense system were evaluated after the chronic exposure of the snails to the tested molluscicides.
The results of the present study can be summarized as follows
1- The water suspension of E. peplus is more toxic to the tested snails nearly about two folds after 24-hour of exposure than praziquantel water solutions. It was also found that the molluscicidal activity of E. peplus was time independent where extension the exposure duration to 48-hour revealed insignificant effect of mortality values. In contrast, the molluscicidal activity of praziquantel displayed time-concentration dependence.
2- Investigation of the stability of the molluscicidal activity of E. peplus under various conditions showed that the plant extract activity stored at 4 C° for 2 days slightly affected, while the extract solution from stored powder for 2 years and heating the powder for 1 hour reduced its molluscicidal polency by
approximately 68%. Moreover, boiling and sunlight exposure of the extract increased its molluscicidal potency by about 50%.
3- E. pepfus water suspension concentration had no effect on various embryonic developmental stages even after 8 days of contionous exposure. While, praziquantel showed an ovicidal potency after 4 days of exposure. The calculated LCso concentration value was40 ppm. Moreover, the LCso value was time dependent when the exposure duration increased to 6 and 8 days, the LCso value were decreased to 8 and 5 ppm resectively. This may be due to the ability of praziquantel to penetrate the protective layers of the egg masses.
The embryonic developmental stages appeared to be more susceptible to praziquantel solution at a concentration as lowes 30 mg/1. The most obvious alterations in the blastula stage were the degeneration of ectodermal cells. The post-trochophore stage showed irregualr shell shape and shrinkage of head foat mass, while, the pre-hatch stage were progressively disrupted ’’ structurally.
4- Praziquantel solution has very toxic effect against S. mansoni miracidia and cercariae than E. peplus water suspension. However, both activites were time-dependent where, LCso values for the plant extract after 120 min exposure were 107 and 70 ppm for miracidia and cercariae respectively. Meanwhile, LCso of praziquantel solution against miracida and cercariae decreased
from 2.6 to 1.7 ppm and from 10.4 to 6.8 when the exposure duration was increased from 30 to 60 min.
5- Scanning electron microscopic observation of exposed miracidia and cercariae of S. rnansom to sublethal concentrations of the plant and praziquantel showed that.
i. The effect was restricted to the anterior part of mircidia, while the cilia of the posterior one were not effectd. The most obvious changes were reduction of the anastomosing folds of apical papilla and deforming and degeneration of the cilia of the anterior part. This may be due to the effect of the plant extract and the durg on the nervous tissue of miracidia.
ii. Treatment of cercariae with plant water suspension caused a severe effect on the tegument. It appeared ridged with numerous pits and recurved spines. In the tail region, the base of spines was swollen and their tips was recurved. The anterior end became swollen and the sensory papillae became retracted and swollen. This observation probably reflects a dysfunction of fluid balance in the body wall. Also, treatment with praziquantel affected on the tegument. It become more or less smooth with numerous pits and retracted and fewer spines. The spines in the tail region were deformed, Further more, the anterior end was retracted and had telecscopic shape with deformed sensory papillae- These findings showed that the drug affected not only the tegument but also the underlying muscle.
6- The mean number of both egg masses and eggs/ snxil/ week were reduced significantly after exposure to 10,15,20 ppn of £. peplus water suspension during the whole period of exposure. On the other hand, the chronic exposure of snails to 5 and 7 ppm of praziquantel solution caused a slight decrease in the mean number of egg masses and significant reduction in. txie
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mean number of eggs. While, at a concentration of 10 ppm the snails stopped egg laying especially after the first week. These results may be due to the effect of plant extract and praziquantel on the neurosecretory cells of cerebral ganglia which produce the gonadotrophic hormone.
7- Semithin sections of the meurosecretory cells of the cerebral ganglia of control snails showed that the cells are arranged in 1 -3 layers at the periphery of the ganglia and represented by two centers, the medio- and laterodorsal centers (Acells) and the caudodorsal center (Bcells).Exposure of snails to E. peplus at a concentration of 20 ppm, caused vacuolation and negative ability to staining. These varations may be due to accelerated liberation of the neurosecretory material present in these cells after treatment and the stopping of the synthesis of these products. Meanwhile, the treatment with praziquantel resulted in strong reduction in the cell number and sizes. The cells have an irregualr shape and darkly stained with toluidine blue. Such cells may be inactive where accumulation of neurosecretory materials in cell bodies would indicate their limited releasing.
- The ultrastructural examinations of neurosecretory c^ls of control snails have a special typical characteristics. They possess an extensive granular endoplasmic reticulum, Golgi apparatus and large amount of secretory granules, the elementary granules are transported via the axon towards the periphery of cerebral commissure and released by exocytosh. chronic exposure of snails to E. peplus caused mam pathological changes in neurosecretory cell bodies. A cells showed sowllen or/rupture of endoplasmic cisternae, deformation of mitochondria and Golgi apparatus. While, the B cells showed karyolsis of nuclie, diliation of endoplasmic reticulum, deformation of Golgi tubules and rupture lysosomes. The periiieurium were devoid of granules. Moreover, the treatment with praziquantel caused similar pathological changes. These changes are discussed as general changes of the cells response to the molluscicidal toxication of the tested molluscicides. In addition, after treatment with praziquantel the neuropile between the cells was rupture. Some cells have lost their contour and autolysosomal activity was observed by the presence of deformed organells inside large lysosomes.
8- The chronic effect of sublethal concentrations of E. pepJus water suspension and praziquantel water solution on the internal defense system indicated that:
i. The mean number of haemocytes decreased insignificantly after the first week of exposure to the plant extract. Thereafter a significant increase was occured after the second.