1. Chagas Disease Sarah Anderson BIOL 402
Chagas Disease: A Latin American disease caused by a protozoan parasite that’s spread through insect vectors

2. Overview
-found primarily in Central and South America-is endemic in 21 Latin American countries
-worst in Latin American countries because this is where insect vector is found
-has reached other countries due to immigration
-10 million people are infected worldwide
-killed more than 10,000 people in 2008

3. T. cruzi -a kinetoplastid protozoa -single celled parasite
-several morphological forms for different parts of its lifecycle
-characterized by flagellum and kinetoplast (region of mitochondrial DNA, have a lot of mito DNA)
-kinetoplastids include free-living and parasitic forms; and biflagellates and uniflagellates
Teixeira, A.R. et.a. “Pathogenesis of chagas’ disease: aparasite persistence and autoimmunity” Clin. Microbiol Rev Jul. 24

4. Life Cycle: Transmission
-Triatomine insects AKA “kissing bugs”
-Are found primarily in Latin America, some in the southern US
-Called kissing bugs because they bite people’s faces while they’re sleeping
-will often deposit feces on their victims-these contain parasite
-if you scratch the bite wound and feces get into the wound or a mucus membrane-you can become infected
-since they primarily bite sleeping victims-they have to be found inside houses-will hide in cracks in walls and thatched roofs of sub-standard housing-why the disease mosty affects the POOR.
-can also be transmitted through: blood transfusions, organ donations, from mother to fetus through placenta, and by consumption of contaminated food (i.e. undercooked meat that is infected).

5. Life Cycle: T. cruzi infects cells
-Recent research has shown that T. cruzi wounds the cell membrane and then takes advantage of Ca2+medicated repair to invade cells.
-Mechanisms by which the parasite wounds cells are unclear, although it may wound the cells mechanically by attaching to cell through posterior end, and the using flagellar motility to tear at the cell membrane.
-Showed that inhibiting Ca2+ repair pathway prevented the parasites from entering cells.
-Ca2+ repair pathway involves-lysosomal exocytosis (lysosomes merge with the cell membrane) and rapid endocytosis (could allow parasite to enter through an endosome).
-Showed that the host enzyme ASM is critical for this repair pathway
-Using this mechanism, the parasite can invade various cell types.
-After the trypomastigote invades the cell-they end up in a lysomsomal endosome-but they escape before they can be digested and turn into amastigotes in the cytoplasm
-Picture on the left: shows parasites that have infected the cell (blue thing is nucleus, green squiggles are parasites)-the parasites are in vacuoles that have recently been internalized from the cell membrane.
-Picture on the right: scanning electron micrograph of T. cruzi anchored to a host cell through its posterior end.
Fernandes, M. C. et. al. “Trypanosoma cruzi subverts the sphingomyelinase-mediated plasma membrane repair pathway.” J Exp Med May 2.

6. Life Cycle: Morphological Forms
Epimastigote:
Amastigote:
Trypomastigote:
-T. cruzi adopts different morphological forms inside and outside of the cell. Top is trypomastigote-which is motile and capable of invading cells. Bottom is amastigote-which is non-motile (no flagella) and is found only inside of cytoplasm host cells-this will undergo binary fission to produce more amastigotes-after a few days, these amastigotes turn back into trypomastigotes and leave the cell by secreting Tc-Tox-a pore-forming protein.
-Another morphological type-epimastigotes-found in insects, not infective (bugs don’t get sick), still motile, but also replicative

7. Chagas Disease: Symptoms
Acute Stage
Chronic stage
-Acute Stage: generally mild symptoms (a few severe cases)-mostly fever and swelling around infection (Romana’s sign-when infection occurred through the eye-get characteristic swelling)
-Chronic stage: 70% of patients remain symptomless throughout life.
30% will develop severe (usually fatal) symptoms after years-especially problems with the heart and intestines.
-These problems develop gradually from the parasite and immune system response causing damage to infected organs.
-it is not really understood whether/how the parasite evades the immune system
-autoimmunity is another explanation for this disease-one possibility is molecular mimicry (i.e. a heart protein looks like a protein from T. cruzi so you’re immune system ends up attacking your own stuff)-however, the mechanism by which autoimmunity occurs is not very well understood.
Teixeira, A.R. et.a. “Pathogenesis of chagas’ disease: aparasite persistence and autoimmunity” Clin. Microbiol Rev Jul. 24

8. Chagas Disease: Diagnosis
Diagnosis can be difficult
-most common way to diagnose-look for parasites in a blood smear
-however, parasites are only common in the blood during the acute stage—most people during the acute stage do not realize that they have contracted a serious disease since the symptoms are not unique to Chagas and are usually mild.
-this test will work for about 6-8 weeks.
-Parasites are not found at high frequency in the blood during the chronic stage, so it is hard to diagnose the disease
-Are currently working on diagnostics for the chronic stage, including PCR tests and serological screens (to see if you’ve built up antibodies against the parasite)
Teixeira, A.R. et.a. “Pathogenesis of chagas’ disease: aparasite persistence and autoimmunity” Clin. Microbiol Rev Jul. 24

9. Chagas Disease: Treatment
Nifurtimox:
Benznidazole:
-The function of these drugs are not well-characterized.
-They both must be activated by parasitic nitroreductases
-Heterozygous mutations in nitroreductases (eliminate one copy of the gene) have been shown to cause resistance to both of these drugs—therefor, need more drugs with different targets for effective combo therapy.
-Also, their efficacy is debated: they are usually only used to treat the acute form (because they will decrease, although not eliminate, the number of parasites found inside you), but whether treating the accute form actually prevents development of later severe symptoms is debated, while some scientists suggest they should also be used to treat the chronic form and prevent severe symptoms
-They both cause severe side-effects-problems with patient non-compliance, and is it worth treating with harsh drug when person is not even guaranteed to get really sick?
Wilkinson, S. R. et. al. A mechanism for cross-resistance to nifurtimox and benznidazole in trypanosomes. PNAS Apr
Teixeira, A.R. et.a. “Pathogenesis of chagas’ disease: aparasite persistence and autoimmunity” Clin. Microbiol Rev Jul. 24

10. Chagas Disease: Prevention
-There is no vaccine
-use of insecticides to eliminate insect vectors from where people live.
-The Southern Cone Initiative: implemented in the Southern Cone of S. America (Peru, Brazil, Bolivia, Argentina, Paraguay, Uruguay and Chile)-sprayed 2.5 million homes with long-lasting insecticides, also improved housing in poor rural areas, and started screening blood donations for the disease.
-by 2000-incidences of Chagas Disease in these countries fell by 94%, number of new cases fell from 700,000 in 1983 to <200,000 in 2000, number of deaths also fell dramatically