الفهرس 
Susceptibility of snails to infectionOnly 14 pages are availabe for public view
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Abstract
Fasciola gigantica, a digenetic trematode, depends on Lymnaea cailliaudi a molluscan gastropod, as an intermediate host for asexual reproduction in its complex life cycle. This study was conducted to provide an overview to the effect of the infection of L.cailliaudi snails with the iniracidia of the liver fluke F. gigantica. The laboratory reared snails were divided into two categories the first category was of 3-5 inm. shell height and the second was of 7-9 mm. shell height. Snails were individually exposed to 10 iniracidia and the susceptibility and the mortality of each category to infection was determined. Alternations in the elements of the internal defense system (imln~lnes ystem) due to infection at the cellular and hunioral levels were also investigated. At the cellular level, attention was paid to inorphological, numerical and functional aspects of circulating haemocytes. Histological reaction of the snails induced by the Infection with the parasite was determined especially in the headfoot, the invading site of the miracidia, and the digestive gland, the localization site of the rediae. The snails were fixed in Bouin’s fluid and were sectioned serially at 8-1 0p and stained with haeinatoxylin-eosin staines. At the humoral level, quantitative changes in the total proteins of plasma and different tissue extracts; head foot, mantle and digestive gland extracts, of infected and uninfected snails was evaluated by Biwet reaction. Humoral defense factors with precipitating and haemagglutination properties were also detennined in the plasma and diSferent tissue extracts against antigens extracted from adult and larval, miracidial and redial, forms of F. gigantica. Ths was performed by using indirect haelnagglutination and gel double diffusion teclmques. The obtained results can be summarized as follow : 1- Reared snails of both category were observed to be readily penetrated by the miracidia and the snails of both categories were too susceptible to infection with the parasite. There is no signtfzicant differences between the susceptibility of both categories. On the other hand, the unsigruficant differences between the mortality of exposed and unexposed snails of small or large ones revealed that the exposure to infection may be not responsible for the mortality of the exposed snails . 2- Haemocyte monolayers prepared from the haemolymph of infected snails at different intervals post infection showed morphological, numerical as well as behavioural changes in haemocytes than those of uninfected controls. After 1,2,4,8,10 and 20 days post infection these liaemocytes gained in volume and iiu~iiber over time and they have many cytoplasmic inclusions, vesicles and vacuoles. They also produce many long pseudopods and spread greatly over the glass surfaces. In addition, they showed an enhanced tendency to aggregate in clustres. 3- Four types of haemocytes were recogpized in haemolymph of L.cailliaudi snail namely, fully spread granulocytes (FSG); partially spread granulocytes (PSG); hyalinocytes and undifferentiated cells. The percentage of each type changes as a consequence of snail exposure to the trematode parasite. At the 8th DPE percentage of FSG, PSG and hyalinocytes were generally similar in infected and control snails. However, the percentage of undifferentiated cells of infected snails was si&icantly hgher. This reveale that the Infection inay stimulate the haemopoietic organ to produce a number of relatively new undrfferentiated haemocytes. At the 10th DPE the infected snails exhibited higlzly. significant increase in the percentage of FSG. On contrary, these snails have sign~ficantly lower percentage of PSG, liyalinocytes and undifferentiated cells in relation to controls. Thls support the suggestion that immature undifferentiated cells eventually are differentiated into spreading granulocytes. The significant decrease in PSG at the 10th DPE reveale that the spread size of some, haemocytes increase inay due to infection like that of FSG. At the 20th DPE the relative abundance of FSG is still significantly elevated whereas, the percentage of PSG is not significantly different from controls. Otherwise hyalinocytes and undifferentiated cells are significantly lower percentages in relatiol to controls. Although, the exact role of hyalinocytes was not detected, but they may be involved in different haemocytes activities.