American Trypanosomiasis – Protozoa (Visceral Infection)

by Vincent Racaniello, PhD

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    00:01 Alright, those are the African trypanosomes. Let's take a look at the American trypanosomes.

    00:06 Trypanosoma cruzi. Here's a picture of the blood form, the flagellated form. Slightly different morphology, doesn't have a nice undulating membrane, at least to my eye. These are passed by what I consider to be one of the most horrid bugs on the planet, the reduviid bug, better known as the kissing bug. You'll see why in a moment it's called a kissing bug. This bug by the way feeds on many mammals besides humans. And it's quite impressive, isn’t it? It is not only large, but look at that proboscis at the very end. It is very long and very sharp. I would not like to meet a reduviid, but unfortunately many people in South America do. These bugs feed at night, typically they live in the straw that covers the roof of your home. Many homes in South America are not fortunate enough to have a synthetic roof of some kind, they have straw and reduviid bugs like to live in the straw, and at night while you're sleeping, what is the part of you that's exposed? Your face.

    01:23 If you sleep on your back, your face is up, the rest of your body is covered with sheets, the bugs fall out of the thatched roof onto your face, they take a blood meal. When these bugs take a blood meal, they defecate. They want to make room for the blood that they have just put into their intestines, so they defecate. And then they fly away. And guess what's in the feces? Right, trypanosomes. So, so far those trypanosomes are sitting on your skin and they are not getting into you yet, because the reduviid bug, when it takes a blood meal is not putting parasites into you. They come out in its feces. But what happens when you get bitten by some kind of a bug? What do you do? You either slap it or you scratch it. And there you go, you scratch the bite of the reduviid. You put the feces into the wound that the bug has made and you introduce the parasites into your body. So you’re infecting yourself, lying in bed at night being bitten on the face. I think all of this is a horrible scenario to be honest, but it can happen. Reduviids by the way, not only will live in the thatched roof of homes. If you happen to have a regular roof made of other materials they can still get in, they live behind pictures that you have hanging on your walls. Am I scaring you? You should be! Go check behind the pictures and then come back and listen. Alright, so scratching introduces parasites into the bite wound. Very different from the other parasite transmissions we've talked about, where the vector is injecting the parasites; this is very different, whatever way works I suppose.

    03:19 Okay disease caused by American trypanosomes is called Chagas disease in honor of the man who discovered the agent. So here we have an outline of the lifecycle of the American trypanosomes. Again, they are deposited on your skin by the reduviid bugs and then you scratch and introduce the Trypanosoma into the wound. And then the parasites will eventually get into your bloodstream and spread elsewhere. Now two to four days after you have this bite, swelling develops at the bite site. This is called a Chagoma. Again in honor of Chagas, who discovered the parasite, Chagoma. So if you've gotten a bug bite at night and a few days later you get swelling, you can bet that you've gotten an American trypanosome. Now sometimes the bugs bite very close to your eye, and you rub and you rub bug feces into your eye and then the parasites can get into the mucous membranes of your eye, it will swell shut. So this little child in the picture you see, his or her left eye is swollen shut. When the chagoma or swelling occurs in the eye, it's called Romaña's sign.

    04:44 Again, a telltale sign that you're getting American trypanosomiasis.

    04:51 As this infection proceeds, you develop a mild disease with fever and typically you will recover. But some people remain infected for their whole lives, perhaps 10 to 20 years after which they develop complications that will kill them. So, you can be cleared of infection early on, after a relatively mild swelling, a chagoma, or you can be persistently infected and die many years later of complications. So what happens is that the parasites spread from that initial bite site that get into your blood as shown in this illustration, in most people they’re cleared, but in some they persist for a long time and they can spread to different organs as shown in the bottom. And they can cause damage to nerves in your gastrointestinal tract. They can cause megacolon, a swelling of the colon, or megaesophagus, a swelling of the esophagus, and these cause gastrointestinal problems which are quite specific and you can detect on a careful questioning of the patient. The trypanosomes can also spread to the heart and cause damage to heart muscle. So they can be there for many years, replicating, causing immune reactions that damage the tissue, and a lot of these individuals, 10 to 20 years after the initial bite on the face, have a heart attack caused by destruction of the heart muscle by parasite replication.

    06:22 How do you diagnose American trypanosomiasis? One of the more interesting diagnostic assays that I've ever heard of is, you take a reduviid bug which is free of the disease, and you allow it to bite the patient and a week later you look at the feces of the bug for metacyclic trypomastigotes which are shown in this picture. So you are basically using the bug as an incubator to see if the trypanosomes are present. Now if you don’t want to be bitten by a bug, you can take a little bit of blood into a blood smear and look for those blood forms in the microscope, relatively easy to find. You can stain the trypanosomes with antibodies and do immunofluorescence, the green color that is shown here. Or you can use ELISA-based test to look for antibodies against the parasite, which we develop in the course of an infection.

    About the Lecture

    The lecture American Trypanosomiasis – Protozoa (Visceral Infection) by Vincent Racaniello, PhD is from the course Parasites.

    Included Quiz Questions

    1. Reduviid bug
    2. Tsetse fly
    3. Female Anopheles mosquito
    4. Ixodes scapularis
    5. Sandfly
    1. By defecating on the skin
    2. By injecting parasites
    3. By injecting saliva
    4. By living in superficial layers of the skin
    5. By destroying epidermis and laying eggs
    1. Chagas
    2. Malaria
    3. Spongiform encephalopathy
    4. Kala azar
    5. Athlete’s foot
    1. Eye
    2. Back of head
    3. Big toe
    4. Genitals
    5. Liver

    Author of lecture American Trypanosomiasis – Protozoa (Visceral Infection)

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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