Nematodes (Roundworms) and Cestodes (Flatworms) - Parasites

by Vincent Racaniello, PhD

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    00:01 Some examples of nematodes or round worms, Caenorhabditis elegans is an example of a round worm.

    00:08 It is a free-living nematodes. It lives in the soil. This is a commonly studied nematode in the labarotory but it causes no disease in humans. And in fact, most of the nematodes out there are not parasitic.

    00:22 There are just a few that cause disease in people.

    00:25 However, many people on the planet are infected with nematodes. So round worms again.

    00:31 We estimated about 4 billion people have at least one nematode species, and some people have even more than one.

    00:40 So people are always acquiring this and they stay within them for long periods of time.

    00:45 The segmented flatworms, the cestodes, all these are obligate parasites of the small intestinal tract.

    00:54 And these are the tapeworms. And there are three different types that are important to humans.

    01:00 The first is the beef tapeworm, Taenia saginata, you can see a picture of it there or you've seen it before.

    01:07 It can be very very long, occupy your entire intestinal tract. We acquire this by eating undercooked beef.

    01:15 Then there's the pork tapeworm, Taenia solium which is acquired by eating undercooked pork.

    01:22 And finally, the dog tapeworm, Echinococcus granulosus which is the name says, it's acquired from dogs.

    01:30 And you can say they all look quite different.

    01:34 Now we need to talk about two other concepts in parasitology and that defines the organism that harbors the parasite.

    01:44 We have two kinds of host that we talk about. There is the definitive host of the parasite.

    01:49 That is the host in which the parasite reaches its maturity and reproduces sexually.

    01:57 So if your host ingest say a cyst form, and that cyst develops into an adult and reproduces, that's the definitive host of that parasite.

    02:08 We also have intermediate host. This is a host that harbors a parasite for a transition period and usually in that period some developmental stage is completed. Alright, so we have definitive and intermediate host.

    02:23 Let's illustrate some of these so you can understand exactly what they mean.

    02:29 Taenia saginata, the definitive host is the human. The intermediate host is the cow.

    02:36 So this is the beef tapeworm we acquired from cows. It complete its maturation in the humans.

    02:42 So we're the definitive host.

    02:45 Taenia solium, the pork tapeworm, the definitive host is the human. But there can be two intermediate hosts.

    02:52 It can be other humans or it can be pigs.

    About the Lecture

    The lecture Nematodes (Roundworms) and Cestodes (Flatworms) - Parasites by Vincent Racaniello, PhD is from the course Microbiology: Introduction.

    Included Quiz Questions

    1. Caenorhabditis elegans
    2. Taenia saginata
    3. Taenia solium
    4. Echinococcus granulosus
    5. Pork tapeworm
    1. ...reaches maturity.
    2. ...reproduces asexually.
    3. ...reaches a developmental stage.
    4. ...reaches an intermediate stage.
    5. ...definitively infects the host.

    Author of lecture Nematodes (Roundworms) and Cestodes (Flatworms) - Parasites

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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