00:01
Hello and welcome to
Parasites: Trypanosoma.
00:06
This is another in depth look
at parasites of human importance
that happened to be protozoans.
00:12
And after listening
to this lecture,
you'll understand the life cycles
of trypanosoma parasites,
and how they're
transmitted to humans.
00:21
You'll be familiar with the
pathogenesis of the different forms
of American and African
trypanosomes.
00:28
And you will know how to
prevent and treat trypanosomiasis.
00:36
So, again, these are all
eukaryotic protozoan parasites
of medical importance.
00:43
We've looked at
Plasmodia, Toxoplasma, Leishmania.
00:47
And now let's examine
the trypanosomes.
00:52
Here's a photograph
of the blood form
of trypanosoma species.
00:57
These are very typical
flagellated parasites
with a lovely undulating membrane
on one side.
01:04
And you can see in this photo,
this is a blood smear,
so there are red blood cells
present.
01:09
First we'll look at the
African trypanosomes.
01:12
There are African and American
trypanosomes
with very different outcomes.
01:18
One of the African trypanosomes,
Trypanosoma brucei gambiense,
let's take a look at the
distribution of this.
01:27
You can see that these parasites
are found largely
in central Western Africa.
01:36
Very specific place,
nowhere else on the planet.
01:40
Trypanosoma brucei gambiense.
01:43
The other Trypanosoma
present in Africa
is Trypanosoma brucei
rhodesiense.
01:51
And let's take a look at the
distribution of this parasite.
01:54
Look, it's very different.
01:56
It's on the Eastern Coast
of South Africa.
02:00
Very different from gambiense.
02:04
These infections, these parasites,
are spread by the Tsetse fly
the species Glossina.
02:12
These are very large flies.
02:14
You can't tell from this photograph
an inch long,
that can give you quite a nasty bite
and deliver quite a serious disease.
02:24
Among the African trypanosomiasis,
the animal reservoirs are different
for the East African
and the West African parasites.
02:34
The reservoirs
for the East African parasites
are game animals
like Impalas.
02:40
Like this animal here is an Impala
and other game animals.
02:43
So these parasites exist
in those reservoirs.
02:46
The West African trypanosomes have
as reservoirs either humans, or
domestic animals, like cattle.
02:56
So the very distinct geographic
distributions
of these two different
African trypanosomes
are a consequence
of the distribution
of the animal reservoir.
03:07
In both cases, they are
spread by the same vector,
the Tsetse fly.
03:14
Let's take a look at the
pathogenesis of trypanosoma brucei.
03:19
Here's an overview.
03:20
Again, these are spread by
Tsetse flies.
03:24
They inoculate the parasites,
elysian forms
at the side of inoculation.
03:29
They become blood parasites
in the host.
03:32
And then the fly takes up parasites
and repeats the cycle again.
03:37
And then there are some consequences
of infection that we'll talk about.
03:42
So the infected tsetse fly
is taking a blood meal, of course,
that's why it's biting you.
It takes a blood meal.
03:50
But in so doing
it first inject saliva.
03:53
This is a common theme among
the mosquitoes, and sandflies,
and tsetse fly vectors
that spread these diseases.
04:01
To take a blood meal,
you first inject saliva,
in order to give chemicals
that will for example,
prevent clotting of blood
and in sound,
injecting that saliva,
there go the parasites with it.
04:14
You develop a primary lesion
at the bite site.
04:18
That second circle there,
which is highlighted,
that is a lesion
on the surface of your skin.
04:23
We call that a chancre.
04:25
The bloodstream then becomes
invaded by tripomastigotes.
04:30
Those are those flagellated forms of
which a photo I showed you earlier,
which I think
are quite beautiful.
04:37
Unfortunately,
not very good for you.
04:39
They invade the bloodstream.
04:40
And of course,
once they're in the blood,
they can take a trip to
wherever they please.
04:46
You develop a systemic illness
as a consequence
of the global distribution
of the parasite within you
with fever and lymphadenopathy.
That is swollen lymph nodes.
05:00
After months, in the case of
the rhodesiense
or years,
in the case of the gambiense,
these parasites may invade
the central nervous system.
05:14
So you see,
here at the bottom of the
right hand part of this slide
you have a brain being invaded
by the parasites.
05:20
An early sign that this is happening
is called Winterbottom's sign,
to swelling
at the back of the head
as you can see in this gentleman.
05:29
If you see this on someone
who you know is infected,
that is a bad sign,
it means
the parasites are likely going into
the central nervous system.
05:38
And there you cannot have
good consequences.
05:42
The right hand circle
at the bottom there
shows you a form of pathology
caused by infection
called perivascular cuffing
when parasites
are present in the blood,
immune cells go around
the blood vessels,
and in a cross section such as this,
you can see the dots are
all around the blood vessel.
06:01
And that's called
perivascular cuffing.
06:04
That can be used to diagnose
infection.
06:07
Now, you may be understanding
already at this point
that this infection has gone on
for months or years.
06:14
And apparently,
it's not being cleared.
06:17
Well, if you thought that
you'd be absolutely right,
because during all these months
and years of chronic infection,
you have waves of parasitaemia.
06:26
So what happens is
the parasites get in your blood.
06:28
We make an immune response
that clears the infection,
but a few are left,
a few parasites are left
that are not stopped
by the antibody.
06:39
They have changed their coat.
06:41
And so those grow and you have
another wave of parasite,
you then make antibodies
against those.
06:50
And of course, at this oldest time,
months are going by
your antibodies clear
most of those are a few left
who have changed their coat again.
06:57
And this goes
on and on and on.
07:00
This is a hallmark of trypanosoma
brucei, antigenic variation.
07:06
We can't get rid of this infection
naturally.
07:09
Quite a brilliant strategy.
07:11
So here is an example of this
antigenic variation.
07:15
And there is on the
X axis, time and weeks.
07:18
And these are blood samples
taken from an individual
at different weeks
after infection.
07:24
And we are measuring the number of
trypanosomes in the blood
on the Y axis.
07:29
You can see
there's a peak and a trough.
07:32
And another peak and a trough,
etc, etc.
07:35
This happens with real regularity
as the parasites grow.
07:39
They're eliminated by
the immune response,
they change, they grow again,
and so on, and so on.
07:44
This is the real problem with trying
to clear this infection.
07:50
Now in the end,
if you are infected,
they fly, the tsetse fly
can bite you and ingest blood.
07:59
And of course, take parasites up
in the process
and start the cycle
all over again.