الفهرس 
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Abstract
Sarcocystis spp. is a cyst-forming coccidian parasite which usually needs two obligatory hosts during its life cycle, including a carnivorous final definitive host and an herbivorous or omnivorous intermediate host. Sarcocystis spp. is intracellular protozoan parasites infecting a wide range of livestocks. Some of Sarcocystis genus is pathogenic for animals such as sheep and cattle which cause enormous economic losses. Humans serve as definitive host for Sarcocystis fusiformis (S. bovihominis) whose intermediate hosts are cattle and water buffaloes and S. suihominis whose intermediate hosts are domesticated pigs. These coccidian parasites cause intestinal and muscular sarcocystosis in humans due to consumption of raw or undercooked meat or ingesting infective sporocysts, respectively. Consumption of raw or undercooked meat infected with the intracellular cystic stage of the parasite leads to intestinal, non-invasive sarcocystosis with watery diarrhoea, nausea, and other gastrointestinal symptoms in humans. Intramuscular sarcocystosis would be suspected based on combinations of various criteria including persistent myalgia, episodic weakness, subcutaneous nodules and dermatomyositis and confirmed by the presence of intramuscular cysts, mostly without any symptoms or inflammatory response. Definitive diagnosis requires identification of oocyst and sporocysts in feces and this requires several stool examinations beginning several days after ingesting the meat. In muscular sarcocystosis, there is peripheral eosinophilia, and elevated serum creatinine phosphokinase levels. Rats survive and reproduce in close association with humans in households, in agricultural and in commercial places. Wild rodents act as definitive and/or intermediate hosts of many endoparasites including parasites with zoonotic potential such as Sarcocystis spp. So the aim of the present work is to study the role of rats as a definitive host for Sarcocystis spp. In the present research, sarcocysts were collected from 80 slaughtered old buffaloes in slaughters houses in Sirs Ellian, Menoufia Governate, Egypt. It was found that 77.5% (62/80) of selected buffaloes in this study were infected by Sarcocystis. Esophagus was the most frequently organ found to be infected by Sarcocystis in an infection rate of 72.5% (58/80), where the detection of sarcocysts in muscles was 5% (4/80). The sarcocysts were confirmed to be S. fusiformis by H& E examination that revealed that, sarcocysts had thick cyst wall about 1.9 um- 3.5 um with long and dense villar protrusions and also by electron microscopic examination which revealed that, the cyst wall had cauliflower villar protrusions and electron dense granules in the ground substance, in addition the entire merozoites consisted of basic structures i.e. one conoid, micronemes, rhobtries, polysaccharide granules and a nucleus. Sixty male albino laboratory bred rats parasitologically free weighting about 180-200 grams were undergoing in this study. They were divided into 6 groups (10 rats each) as follows group I (none infected), group II (sacrificed after 24hrs. p.i), group III (sacrificed after 48hrs. p.i) group IV (sacrificed after one week p.i), group V (sacrificed after two weeks p.i) and group VI (sacrificed after one month p.i). Every rat was infected by 10 grams of Sarcocystis containing about 20 visible sarcocysts isolated from the infected buffalo muscles as single oral dose. The stool from rats in all groups was collected all over the study for all groups and examined by light microscope. Observation of the infected rats’ behavior revealed disruption of their normal activities, physiological states, weight reduction and thirsty starting on the 5th day p.i in 40%. Stool`s consistency was watery and filled with mucous on the 7th day p.i in 20%. No changes were observed in stool of infected rats after 24hrs and 48hrs in groups II and III respectively, but, in the other groups, sporocysts and oocysts were detected. Histopathological examination of the infected rats’ intestinal tissues revealed no changes after 24hrs and 48hrs, but the other groups, after one week p.i, showed dilated congested capillaries and extravasated RBCs. In addition, a marked increase in the number of goblet cells, intense eosinophilic infiltrate, moderate lymphocytic infiltrate and interstitial edema were observed. Broadening and destruction of intestinal villi were also reported in this study. Histopathological examination of the infected rat intestinal tissue sacrificed after 2 weeks, showed loss of surface epithelial lining and inflammatory edema and subepithelial dense inflammatory infiltrate in the form of lymphocytes and eosinophil’s, in addition to abundant plasma cells. Also, sporulated oocyst was observed which has thin cyst wall and contains two sporocysts with sporozoites. While histopathological examination of rats’ intestinal tissue after one month (group VI) showed, extravasated RBCs with eosinophilic and lymphocytic infiltrate, interstitial edema and goblet cells, in addition to dividing oval oocyst and spoulated oocyst in lamina propria were observed. Rat intestinal tissue was taken and prepared for electron microscopic study, semi thin section was done and oocyst in lamina propria was detected in group IV. In addition, the same tissue showed loss of surface epithelial lining by TEM.