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So my name is Norma Andrews, I'm a professor at Yale University.
And what I'm going to be doing in the first segment of my lecture is to give an introduction
on this parasite, Trypanosoma cruzi and the disease that it causes in humans,
which is called Chagas' disease.
Trypanosoma cruzi is a protozoan parasite from the order Kinetoplastida,
and inside this order there's the family Trypanosomatidae
which includes several protozoan organisms of which 2 are very important medically
because they cause serious diseases in man.
And one of them is the genus Trypanosoma which includes not only Trypanosoma cruzi
but also the African trypanosome which causes sleeping sickness in Africa
and Leishmania which I'm going to talking to you about in the second part of this lecture.
So, Trypanosoma cruzi is also known as the American trypanosome
and actually should be called the Latin American trypanosome
because the disease caused by this parasite is found only in South and Central America.
And in this region, a large number of people carry currently the parasite--
between 16 and 18 million people are infected at present.
And the history of Chagas' disease is very interesting in the sense that,
unlike other infectious diseases, it was a single individual--Carlos Chagas--
a Brazilian investigator working practically alone in the field that made all the major findings
that led to this realization of not only a completely new infectious agent
but also the vector that was responsible for its transmission to humans,
the animal reservoirs in the region,
and also the living conditions that really favor transmission to people.
So, what Carlos Chagas noticed...
He was in this rural area in Brazil, working on malaria transmission, and he,
being trained as a medical entomologist,
he noticed that insects that he found heavily infesting these mud huts
very common in this area and still seen in many regions of South and Central America...
These houses were very heavily infested with insects like this one shown here,
which are Reduvids, these crawling bugs that, during the day,
they hide in the cracks of the walls of these houses and come out at night
to feed on the blood of people and domestic animals.
So, Carlos Chagas dissected these insects that he found in these huts
and he saw that they carried these very large forms that, it was clear to him,
that this was a new protozoan organism
that had not been identified before, and he observed mainly two forms.
So, there was this longer form and a smaller one with an undulating flagellum
which is what we know now to be the infectious form.
And the way he learned this was when he sent
some of these infected bugs to the laboratory in Rio,
and they allowed these insects to feed on monkeys.
And these monkeys very soon developed an infection with large numbers of these parasites
which have very similar morphology circulating in the blood.
Carlos Chagas at the same time, he was able to show that
the circulating blood of children which showed signs of infection--
high fever and also these swelling regions in the face--
these children, in the blood, had large numbers of these parasites in circulation.
So in this way, he identified the basic elements of the life cycle of Trypanosoma cruzi
which we now know, inside the insect, we have the epimastigotes replicating in the digestive tract
of the insect and then these forms transform into an infective stage, the trypomastigotes
and are released with the feces of the insect, so transmission actually occurs
by contamination of either the wound bite or the mucosal membranes
in the mammalian host.
When these parasites cross into the mammalian host,
they find host cells--large, different types of whole cells can be infected
and that's where they transform into the amastigotes
the form that replicates in the cytosol, and then the parasites at the end of the cycle are released.
They can either continue this cycle in the mammalian host
or they can be taken up by the insect during a blood meal.
So another important finding that was made several years later
is that there is an important difference between epimastigotes
these forms found in the gut of the insect
and the forms responsible for transmission which are the trypomastigotes
So the epimastigotes are lysed by the alternative pathway of complement activation
And this is the cascade of events that happens, initiated by hydrolysis of C3
which is cleaved into fragments, of which C3b has the capacity of cleaving C5,
and then C5b is a component of this membrane attack complex
that forms after association with C6, C7, C8, and C9,
forming a transmembrane pore that punctures the membrane of cells causing their lysis.
So this explains why the epimastigote form is lysed in mammalian serum
and also why trypomastigotes resist, because they have developed mechanisms
to avoid activation of this pathway.
So, here in this scanning electron micrograph, taken by Edith Robbins at NYU,
we can see a closeup of the trypomastigote -- the infective stage,
attached to the surface of a host cell.
And when these parasites enter the cells, just the morphology of this process indicates that
there is something very different and unique going on because this is a very large parasite.
It is more than 10 microns long,
but we can see that this happens with no extension of pseudopods of the host cell,
which is the usual mode of ingestion of large particles which is phagocytosis.
So in the last segment of this lecture, I'm going to get into some detail of what we learned
about the mechanism by which Trypanosoma cruzi enters mammalian cells.
So in this movie, made by Mark Rioult in my lab,
the movie starts with a parasite already half inside the cell and half outside,
and we're going to be able to see accelerated (this is going to show at 10x real time),
we're going to see the complete process of the parasite entering the cell.
So we can see here that it's the extracellular part that still has the very active motility,
and the parasite gradually slides into the cell, and we're going to actually see the moment here
in which the parasite enters the host cell and gets completely released into the cytosol.
And actually the parasite appears free in the cytoplasm, but we know that at this point,
it is surrounded by a membrane of host origin.
I'll also talk about this in the third part of this lecture.
We learned that this invasion process is actually quite unique,
and it happens by recruitment of intracellular membranes, mostly from lysosomes of the host cell.
So, after acquiring these membranes, Trypanosoma cruzi resides inside this vacuole
for some time, for a few hours, and then this vacuole is disrupted and it is free in the cytoplasm.
And then the next stage of development happens and the parasites replicate.
So these scanning micrographs here show the remarkable transition in morphology
that these parasites undergo while they are escaping from that initial intracellular vacuole,
and we can see that this involves a reduction in the size of the body
and a dramatic reduction in the size of the flagellum in which this form at the end,
amastigote, which is the one responsible for replication inside the host cell
has only a very short flagellum.
What I'm going to show you in this movie here, made by Hertha Meyer in Italy in the 60s
are the last stages of this transformation
of the intracellular parasite into the replicative amastigote.
So we can see as we play the movie, that this parasite that just entered these chicken retinal cells
is going to reorganize itself into the rounded amastigote form,
and as this parasite enters this replicative stage it starts undergoing binary fissions,
which we can see here in this cell,
that there are already several parasites replicating in the cytosol,
and we can see clearly also that the whole cell
remains quite viable throughout this process
and this is going to become obvious just by the observation of the fact that
these cells are capable of going through mitosis normally.
We can see here the condensed chromosomes aligning themselves
at the center of the mitotic spindle
and we're going to be able to see when
these chromosomes are actually pulled apart by the spindle
and then the cell rapidly enters cytokinesis.
And if we focus on these cytoplasmic parasites, it's possible to see that
one of them was actually delivered to one of the daughter cells
while the majority remained in the other cell.
So, this cycle continues, and this was Jim Dvorak at the NIH in the 70s
who really worked out clearly the details of this intracellular cycle
and what he learned is that they go through nine successive divisions,
so each parasite that enters the cell actually originates around 500 parasites
and then at the end of the cycle which is around four to five days after the original infection
they change back again into this highly motile trypomastigote form
that we can see here completely filling the cell at the end of the cycle
and at this stage, the cell degenerates rapidly, and we're going to be able to see actually
in this cell here that the nucleus is already quite degenerated,
and we're going to be able to see the moment in which the plasma membrane breaks down
and these parasites are released into the medium.
So this is how they reach circulation, and they can then be taken up by the insect during a blood meal.
So, this is exactly what happens during the acute phase of this infection.
This is a picture of another child with the classical swelling around the region of the eye,
which is a very common site of entry for these parasites in humans,
and this is the classical diagnostic picture of finding these highly motile trypomastigotes
in the blood of these patients.
So, the acute phase of the disease is characterized by this localized swelling at the site
of entry of the parasite, also very intense episodes of fever and enlargement of the spleen,
and this is actually very possible that death is an outcome of these acute infections
not only in children, but we have learned recently that even adults that had never been exposed
to the parasite in childhood can also die from the acute phase of Chagas' disease.
So, the largest number of people infected with Trypanosoma cruzi are actually in the chronic stage
of the disease because we know clearly now that immunity does develop against these parasites
and the immune system is capable of clearing the large majority of these parasites,
but they're never completely eliminated.
So these patients that carry the parasite...
a large fraction (around 40%) are actually asymptomatic throughout their lives,
but they still have the parasite and then there's a large fraction of around 45%
that have the more serious form of the disease which is the cardiomyopathy
which involves enlargement of the heart
and then a smaller fraction of these patients develop megaesophagus or megacolon
which is this dramatic enlargement of internal organs
that requires correction by surgery.
So, the serious form of the disease which is the cardiomyopathy is actually
responsible for sudden death in around 58% of the patients that have this form of the disease.
So it's the most common form of sudden death in these endemic areas for Chagas' disease,
and we can see here that... here is a picture of cardiomyocytes infected by the parasite,
and this movie that I'm going to play now down here just shows isolated cardiomyocytes
that contain a large number of parasites close to the end of the cycle,
and you can see this cell beating, showing that these were heart cells which are the cells
that are preferred by these parasites for infection in vivo
So another very important point with Chagas' disease is that even this large number of
asymptomatic patients, they carry the parasites
and they can transmit the infection through blood transfusions,
so it's something that is very important in the endemic area.
Usually the blood banks screen the blood for the presence of Trypanosoma cruzi
but this is something that in many developed countries, actually,
there's not enough awareness for the possibility of blood infection with Trypanosoma cruzi
and this is increasingly more important with the high mobility of the human population.
So what is important and what is very good news with Chagas' disease is that it has been clear
almost since the beginning of the 19th century, Carlos Chagas had already pointed out
that transmission of the Trypanosoma cruzi to man can be interrupted
and this can be done by very simple measures
which involve just the control of the insect vectors.
So just simple spraying of the houses with insecticide
which is what is shown here in these images
can have a profound effect on the incidence of the disease
and also another very important factor is adequate finishing of the walls,
so not providing these cracks where these insects like to hide.
What is at the same time very disturbing is that, although this was known to be effective
since the 40s, it was only decades later that these programs
of controlling this vector have been implemented
throughout this region, and this slide here actually illustrates very well the problem
which what is shown here is the distribution of the various species of the insect vector
that are capable of transmitting Chagas' disease, and we can see, for example,
that here in the south of the US, there is the wild cycles, so wild animals are found easily
carrying Trypanosoma cruzi and there are insects
which are responsible for maintaining this cycle
but this does not cause human infections, and the sole reason is because the living conditions
are much superior in the US than they are in these poor areas of Central and South America.
So Chagas' disease is clearly a disease of poverty.
And it's a disease that has already been demonstrated that it could potentially be eliminated
just by a consistent program of surveillance and elimination of the domestic vector.
So a very important initiative in this sense is called the Southern Cone Initiative
that was created by the Pan American Health Organization (PAHO)
and by the World Health Organization (WHO) around 1991.
And you can see here the numbers here really look great in a short period
Chile, Uruguay, and large regions of Brazil and Argentina
have practically eliminated transmission to humans
and many other countries in this area are also reaching excellent results
and they are now in what is called the surveillance phase
which is just maintenance of this measure and to prevent reinfection of the homes.
So, this initiative has been cited as one of the 17
most cost effective international public health interventions
that have been done, and it's actually more than proven to be highly effective.
So it is a matter of political will, so it is pretty disturbing that in large fractions
of Central and South America, this has not been achieved yet.
We can see that these initiatives here in the Andean countries and in Central American countries
have only been initiated much more recently and it was only in 2001
that a Mexican initiative for the control of the domestic vector was put into place.
But this should rapidly progress if these initiatives are maintained
so the important point here is that Trypanosoma cruzi will never be eliminated from nature.
It is known that more than 100 vertebrate species can serve as hosts for this parasite in nature.
However, Chagas' disease can be prevented and this can be done very effectively
by an improvement in the social and economic conditions of the population,
so it is a disease of poverty and it is expected to improve dramatically with development.
The critical issues are, of course, the effective and sustained surveillance of these regions
and to prevent re-infestation of the homes and also treatment -- better drugs and less toxic drugs
for treating the very large chronically infected population that exists presently.
So, thank you for your attention and in the next segment
I'm going to also introduce a related parasite which is also very important medically
and causes serious infections in poor areas of the world which is Leishmaniasis.