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Schistosoma mansoni and Schistosomiasis (Trematodes) – Helminths

by Vincent Racaniello, PhD

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    00:01 Alright, let's move from the round worms into the flat worms.

    00:06 Schistosomiasis, these are helminths, again, of course they are worms, non-segmented flat worms, also known as trematodes.

    00:16 Schistosomiasis is the second most devastating parasitic disease in tropical countries behind malaria.

    00:22 More than 200 million people in 74 countries are infected with the disease with half of them living in Africa.

    00:30 There is a low mortality rate but the morbidity is high since it often presents as a chronic illness that can damage internal organs.

    00:38 It can also impair the growth and cognitive development of children as well as increase the risk of squamous cell carcinoma or bladder cancer in adults.

    00:49 Schistosomiasis is a caused by parasitic blood flukes or flat worms which can infect various parts of the body including the blood vessels, gastrointestinal tract, lungs, or liver depending on the species.

    01:03 There are differences in geographic distribution among the five species which cause human disease.

    01:09 Schistosoma haematobium is found in Africa, parts of the Middle East, Turkey, and India.

    01:15 Schistosoma mansoni is also seen in Africa, the Middle East, South America, and some Caribbean Islands.

    01:23 Schistosoma japonicum is seen in Asia mainly in China, the Philippines, Thailand, and Indonesia.

    01:30 Despite its name, it has been eradicated in Japan.

    01:34 Schistosoma mekongi is seen a Southeast Asia.

    01:38 And, Schistosoma intercalatum is seen in Central and West parts of Africa.

    01:43 All species have similar life cycles involving fresh water snails.

    01:48 Schistosoma haematobium causes urinary tract disease, the other Schistosoma species cause intestinal disease.

    01:56 Humans are the main reservoirs of infection.

    01:59 Dogs, cats, rodents, pigs, horses, and goats are reservoirs for Schistosoma japonicum.

    02:06 Dogs are the primary reservoirs for Schistosoma mekongi.

    02:11 Transmission of these species does not occur within the United States including Puerto Rico or in Canada, but it may be present in travelers and immigrants from endemic areas.

    02:22 In the life cycle of Schistosoma, eggs are eliminated with feces or urine depending on the species.

    02:28 The eggs hatch and release miracidia, which swim and penetrate specific snail intermedia hosts.

    02:35 The stages in the snail includes two generation as sporocysts and the production of cercariae.

    02:42 Upon release from the snail, the infected cercariae swim, penetrate the skin of the human host and shed their forked tails becoming Schistosomulae.

    02:53 The Schistosomulae migrate via venous circulation to the liver and then develop in the liver, exiting the liver via the portal venous system when mature.

    03:03 Male and female adult worms copulate and reside in the mesentery venules, the location which varies by species.

    03:12 Schistosoma haematobium most often inhabits the vesicular and pelvic venous plexuses of the bladder or it can also be found in the rectal venules.

    03:22 The females measures 7 to 28 mm in length depending on the species and deposit eggs on the small venules of portal and the peri-vesicle systems.

    03:33 The eggs are moved progressively toward the lumen of the intestine in the case of Schistosoma mansoni and Schistosoma japonicum; and the bladder and ureters in the case of Schistosoma haematobium.

    03:47 The eggs are then eliminated with feces or urine.

    03:51 Here, in this photograph, is Schistosoma mansoni.

    03:55 There is actually an adult male and a female in this picture, can you see them? Probably not.

    04:04 You might see the head of the female just sticking out, she is located in a groove along the length of the male worm.

    04:13 They are mating, this is how they mate, this is how they mate inside of you and they shed, the female will then shed eggs which go on to spread to others.

    04:23 So the two worms find each other, they come together.

    04:26 The female fits into this groove, they mate, and they produce eggs, that's Schistosoma mansoni.

    04:32 Here's another Schistosoma, Schistosoma haematobium, these are the cyst forms in a tissue.

    04:38 This is Schistosoma japonicum. This is the cercarial form.

    04:44 We'll talk about what this is in a moment.

    04:47 And another cyst form of Schistosoma mekongi, so all of these different species of Schistosomes can cause the same type of Schistosomiasis.

    05:02 So, remember, again, these are worms, trematodes in particular, non-segmented flat worms.

    05:10 These different Schistosome species are located in different parts of the world depending on a snail, the intermediate host for Schistosomiasis, Schistosomes, is the snail of different kinds and in the snail it goes through a certain morphological transformation, it doesn't complete the whole life cycle that's why it's an intermediate host.

    05:36 Globally they are somewhere between 200 and 300 million infections.

    05:42 Now, sometimes the feces containing the eggs can also come from non-human animals such as primates.

    05:51 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.

    06:12 Now these eggs often get trapped in your tissues, they can cause granulomas and this can undergo fibrosis so this can cause damage to you having so many eggs being produced.

    06:23 They can cause liver fibrosis and cause portal hypertension as they clog the liver circulation, the blood backs up, you get hypertension.

    06:33 They can also cause esophageal varices and bleeding.

    06:37 And as a consequence you may also have blood in the urine from damage caused in the bladder.

    06:46 You can treat Schistosome infections.

    06:48 Praziquantel is a drug that increases calcium permeability of the adult worm.

    06:54 This stops eggs production so the pathologies associated with eggs do not occur.

    07:01 And you can prevent infections by properly disposing human feces, snail control maybe 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 and Schistosomiasis (Trematodes) – Helminths by Vincent Racaniello, PhD is from the course Parasites.


    Included Quiz Questions

    1. Trematodes
    2. Tissue nematodes
    3. Intestinal nematodes
    4. Cestodes
    5. Cyclospora
    1. Snails
    2. Monkeys
    3. Cows
    4. Mosquitos
    5. Birds
    1. Miracidium
    2. Trypomastigote
    3. Amastigote
    4. Schizont
    5. Trophozoite
    1. Schistosomula
    2. Trypomastigote
    3. Schizont
    4. Miracidium
    5. Cercaria
    1. Mesenteric veins
    2. Femoral vein
    3. Popliteal vein
    4. Axillary vein
    1. Increased calcium permeability of adult worms
    2. Blocks G6PD enzyme in larvae
    3. Acts as a precursor of glutathione in larvae
    4. Blocks dihydrofolate reductase in adult worms
    5. Blocks heme detoxification in red blood cells

    Author of lecture Schistosoma mansoni and Schistosomiasis (Trematodes) – Helminths

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD


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