Infections by Protozoa and Helminths

by Richard Mitchell, MD

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    00:01 Protozoans, really interesting and they can merit a whole day of just talking about them because they're so cool and so many, and they do so many interesting things.

    00:10 So they can be extracellular, they can be obligate intracellular or they can be facultative intracellular.

    00:16 So depending on the bug, they can be in any location inside or outside of the cell.

    00:21 They tend to be motile, they are single-celled, and they are nucleated.

    00:26 And, they have, all of them, have a very complex life cycle, so there can be a venereal life cycle, a fecal-oral life cycle, an insect vector or opportunistic, and each protozoan has got its own little vectors around in the environment before it gets to its human host and then how it gets from the human host back to that vector, is part of the interesting life cycle.

    00:51 Here is just a good example, so this is the pathogenesis associated with toxoplasmosis.

    00:58 The reason that we're showing you the house cat on the left, is that they are the major vector for toxoplasma and can be a major spreader if you're immunocompromised, that's not a good idea to have house cats.

    01:11 They cause a lytic infection, so shown on the right hand side, is a bunch of bradyzoites of the toxoplasma gondii organism within cardiac myocytes.

    01:25 And as they grow, they will cause a lysis, and we will lose cardiac function as a result of that.

    01:32 We have a number of other organism.

    01:35 So malaria also causes lytic infections, Entamoeba histolytica, which is in the GI tract, will cause lytic infections.

    01:42 Toxoplasmosis we just talked about there and leishmania.

    01:46 They don't have to be lytic, however.

    01:49 And for example, they can cause epithelial injury.

    01:53 The brown staining that shown here over the surface of this columnar epithelium, is Giardia lamblia. Giardia doesn't invade, it doesn't cause a lytic infection, it just simply grazes over the surface of the epithelial cells in the GI tract, but in doing so, it inhibits their normal absorption of nutrition and so you get a relatively profuse diarrhea, because you're not absorbing the normal material, and you are also getting some secondary injury, not lytic, but a secondary injury due to the microorganism going over the surface and causing a secretory diarrhea on top of that.

    02:34 Cryptosporidium, another example of that is an opportunistic infection of the GI tract that doesn't cause lytic infection, just causes epithelial cell dysfunction.

    02:44 You can also have Trypanosomiasis, for example, that causes a toxin production.

    02:50 Next microorganism we want to describe over all, are helminths or worms.

    02:54 We have three flavors, we have the round worm, which are nematodes such as Trichinella and Ascaris.

    03:01 We have flat worms such as the tapeworm, and we have flukes such as schistosomiasis.

    03:10 These different forms will have different tropisms and different pathologies.

    03:18 So the helminths can be intestinal or they can be blood tissue.

    03:23 In the case of the tapeworm shown here, they have little suckers on their scolex that allow them to attach to the intestinal wall in the mucosa, and then grow along the entire length.

    03:36 Helminths can also have very elaborate and complex life cycles much as we previously describe for the protozoans.

    03:43 What's shown here is a snail, which is one of the non-human host for schistosomiasis.

    03:50 And they will, once they're invading in the tissue, they will generate eggs and larvae and it's often the response to the eggs or the larvae that causes the overall pathology.

    04:04 There can be no response, so sometimes some of these helminths just don't listen inflammatory response, so for example the tapeworm, the cestode, the way we're looking at, schists happily ever after within the GI tract and is kind of unrecognized by the host other than the host not getting all of its own nutrition, it's all being sucked up by the tapeworm. So there may be no response.

    04:29 The diseases that are usually associated with helminths are secondary to the host inflammatory response in most cases, and in most cases, this is a granulomatous response to eggs and/or larvae, hence proportional to burden.

    04:44 So what we're looking at in the top panel is a helminthic infection of Trichinella and it is forming huge granulomas within the skeletal muscle of this new host, so the granulomatous inflammation.

    04:59 And, finally, you can have eosinophilic inflammation.

    05:02 Indeed, the probable reason that we develop eosinophils in the whole IgE-mediated immune response, is to deal with helminthic infections, so it's quite common when there is a worm present, that we have a very pronounced eosinophilic infiltrate.

    05:20 In the lower slide was, which is Schistosoma haematobium, with that snail as its vector, induces a very profound eosinophilic infiltrate within bladder walls.

    About the Lecture

    The lecture Infections by Protozoa and Helminths by Richard Mitchell, MD is from the course Host–pathogen Interaction.

    Included Quiz Questions

    1. Lysis of cells
    2. Toxic injury
    3. Epithelial cell dysfunction
    4. Host-mediated injury by neutrophil inflammation
    5. Host-mediated injury by granulomatous inflammation
    1. Tapeworm
    2. Hookworm
    3. Pinworm
    4. Ascaris
    5. Trichinella

    Author of lecture Infections by Protozoa and Helminths

     Richard Mitchell, MD

    Richard Mitchell, MD

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