Schistosoma Mansoni

by Vincent Racaniello, PhD

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    00:00 Let's look at the lifecycle of Schistosoma mansoni. We will start with the snail.

    00:07 The snail of course lives in water, wet areas; it gets penetrated by something called the miracidium. This is produced from hatching in an egg that is shed in our feces. The snail is infected with the miracidium, it matures to a first stage in the snail called the cercaria. That's that elongated structure you see next to the right of the snail and we show the picture of that in an earlier slide. The cercaria is modal, it swims about in the water, snails live in water, or in wet places, the cercaria swim around and they look for a host. They can sense you and they will come and infect your skin in a hair follicle much like the hookworms do. Once the cercaria enters you, they then migrate to the lungs and then they eventually go via the bloodstream to the liver. And there, if a male and a female are present, so you have to be infected with more than one sex of cercaria, the adults will mate in your liver and remain in the mesenteric circulation for the rest of their lives. So all of the vessels that supply the liver, that's called the mesenteric circulation and they live in the venules, they spend their lives mating there, and the female lays eggs which then pass into the intestine and eventually are shed in the feces, which in bad times get deposited in freshwater. So this is another case where fecal contamination of the environment with human feces leads to infection. The eggs are deposited in the water, they hatch and the cycle begins again. Now sometimes the feces containing the eggs can also come from non-human animals such as primates. Schistosoma mansoni, these two worms, the male and female in your mesenteric circulation, they can stay there for 10 years or more, constantly laying eggs and shedding them. So you can see the high probability of spread of this infection from an infected individual. Now these eggs often get trapped in your tissues, they can cause granulomas and these can undergo fibrosis, so this can cause damage to you having so many eggs being produced. They can cause liver fibrosis and cause portal hypertension as they clog the liver circulation, the blood backs up, you get hypertension. They can also cause esophageal varices and bleeding. And as a consequence you may also have blood in the urine from damage caused in the bladder. You can treat Schistosome infections. Praziquantel is a drug that increases calcium permeability of the adult worm, this stops egg production, so the pathologies associated with eggs do not occur and you can prevent infections by properly disposing human feces. Snail control may be another option, depending on the environment or community-based drug programs where you treat entire villages to get rid of the egg production and then you don't have any more contamination of the waters and no more infections.

    About the Lecture

    The lecture Schistosoma Mansoni by Vincent Racaniello, PhD is from the course Parasites.

    Included Quiz Questions

    1. Miracidium
    2. Trypomastigote
    3. Amastigote
    4. Schizont
    5. Trophozoite
    1. Schistosomule
    2. Trypomastigote
    3. Schizont
    4. Miracidium
    5. Cercaria
    1. Mesenteric vessel
    2. Femoral vessel
    3. Popliteal vessel
    4. Axillary vessel
    5. Lateral ractus vessel
    1. Increased calcium permeability
    2. Blocks G6PD enzyme
    3. Acts as a precursor of glutathione
    4. Blocks dihydrofolate reductase
    5. Blocks heme detoxification in red blood cells

    Author of lecture Schistosoma Mansoni

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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