Schistosoma mansoni

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Schistosoma mansoni :

Schistosoma mansoni

Schistosoma mansoni :

Schistosoma mansoni is a significant parasite of humans, a trematode flatworm that is one of the major agents of the disease schistosomiasis . The schistosomiasis caused by Schistosoma mansoni is intestinal schistosomiasis .

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Schistosomes are atypical trematodes in that the adult stages have two sexes ( dioecious ) and are located in blood vessels of the definitive host. Most other trematodes are hermaphroditic and are found in the intestinal tract or in organs, such as the liver. The lifecycle of schistosomes includes two hosts: a definitive host (i.e. human) where the parasite undergoes sexual reproduction, and a single intermediate snail host where there are a number of asexual reproductive stages. S. mansoni is named after Sir Patrick Manson, who first identified it in Formosa (Taiwan).

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Life Cycle

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Eggs are eliminated with feces or urine  . Under optimal conditions the eggs hatch and release miracidia , which swim and penetrate specific snail intermediate hosts  . The stages in the snail include 2 generations of sporocysts and the production of cercariae . Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host , and shed their forked tail, becoming schistosomulae . Life Cycle

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The schistosomulae migrate through several tissues and stages to their residence in the veins, migrate to portal blood in liver anfd mautre into adults. Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species . The females (size 7 to 20 mm; males slightly smaller) deposit eggs in the small venules of the  portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine ( S. mansoni and S. japonicum ) and of the bladder and ureters ( S. haematobium ), and are eliminated with feces or urine, respectively .


Schistosomiasis Schistosomiasis also knownas bilharzia , bilharziosis or snail fever is a parasitic disease caused by several species of trematodes ( platyhelminth infection, or "flukes"), a parasitic worm of the genus Schistosoma . Snails serve as the intermediary agent between mammalian hosts . Individuals within developing countries who cannot afford proper water and sanitation facilities are often exposed to contaminated water containing the infected snails. Although it has a low mortality rate , schistosomiasis often is a chronic illness that can damage internal organs and, in children , impair growth and cognitive development . The urinary form of schistosomiasis is associated with increased risks for bladder cancer in adults. Schistosomiasis is the second most socioeconomically devastating parasitic disease after malaria .

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This disease is most commonly found in Asia , Africa , and South America , especially in areas where the water contains numerous freshwater snails , which may carry the parasite . The disease affects many people in developing countries, particularly children who may acquire the disease by swimming or playing in infected water. When children come into contact with a contaminated water source, the parasitic larvae easily enter through their skin and further mature within organ tissues. As of 2009, 74 developing countries statistically identified epidemics of Schistosomiasis within their respective populations.


Symptoms Many individuals do not experience symptoms. If symptoms do appear, it usually takes four to six weeks from the time of infection. The first symptom of the disease may be a general ill feeling. Within twelve hours of infection, an individual may complain of a tingling sensation or light rash, commonly referred to as “swimmer’s itch” due to irritation at the point of entrance. The rash that may develop can mimic scabies and other types of rashes. Other symptoms can occur two to ten weeks later and can include fever, aching, cough, diarrhea, or gland enlargement. These symptoms can also be related to avian schistosomiasis which does not cause any further symptoms in humans.


Diagnosis Diagnosis of infection is confirmed by the identification of eggs in stools. Eggs of S. mansoni are approximately 140 by 60 µm in size, and have a lateral spine. The diagnosis is improved by the use of the Kato-Katz technique (a semi-quantitative stool examination technique). Other methods which can be used are enzyme linked immunosorbent assay (ELISA), circumoval precipitation test (COPT) and alkaline phosphatase immunoassay (APIA ).


Treatment Currently there are two drugs available , praziquantel and oxamniquine , for the treatment of schistosomiasis . They are considered equivalent in relation to efficacy and safety. Due to its lower cost per treatment, praziquantel is generally considered the first option for treatment. The recommended dose is: praziquantel , 60 mg/kg of body weight for children up to 15 years old, and 50 mg/kg of body weight for adults; oxamniquine , 15 mg/kg for adults, and 20 mg/kg for children up to 15 years old. The treatment objective is to cure the disease and to prevent the evolution of the acute to the chronic form of the disease. All confirmed cases should be treated.


Prevention - Eliminating or avoiding the snails Prevention is best accomplished by eliminating the water-dwelling snails that are the natural reservoir of the disease.

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