Now first, we're gonna talk about protozoan parasites.
Protozoans as you know are single-celled eukaryotes.
There are a number of protozoan parasites
of medical importance and they're shown here.
And let's go through some of these
and show you what they look like.
Plasmodium species, the causative agents of malaria
are an example of a protozoan parasite.
Here's a photograph of malaria in the blood.
Toxoplasma is another protozoan parasite. It goes from animals
to humans and for toxoplasma cell that's shown on this slide.
Entamoeba is a protozoa and it happens to be amoeba
that can cause diarrheal diseases in humans.
Giardia is a flagellated protozoan.
It has a flagellum that helps it to move.
This is also responsible for gastrointestinal disease in humans.
Cryptosporidium, another protozoan parasite,
causes disease of the intestinal tract.
Leishmania, these are transmitted by vectors, insect vectors.
Trypanosomes, also transmitted by insect vectors
and you can see these are also flagellated protozoans.
And this picture is taken in the bloodstream.
Finally we have trichomonas, a sexually transmitted
protozoan infection of humans. And cyclospora.
All of these are protozoan parasites. They are single cells.
And they are of medical importance.
They cause significant disease in humans.
A protozoan by definition is rather fragile.
It just has a very simple plasma membrane or lipid bilayer
to protect it from the outside world.
So the protozoan parasites that cause disease have evolved
different strategies to overcome this fragile membrane.
And one of them is to be transmitted by arthropod vectors.
So they go from host to host.
Either from human to human or from an animal to human.
The parasite is carried by an arthropod vector
rather than being exposed to the environment.
So that fact alone protects it from degradation in the environment.
An example of parasite protozoans that are transmitted
by arthropod vectors are the malarias.
These are caused by the species known as Plasmodium.
These are transmitted by mosquitos.
The Leishmanias are transmitted by sandflies. And the Trypanosomes
are transmitted by either Tsetse flies or kissing bugs.
So three examples of protozoan parasite infections with the agent
is protected by being transmitted by an arthropod vector.
The other strategy, again to overcome
the fragility of a single membrane
is to actually alternate between two distinct forms of the parasite,
one that is stable and the other that is more labile.
And this is exemplified by a number of parasites,
and we call these two forms trophozoites.
This is the form that grows within the host.
So here's the picture of a malaria trophozoite.
It's very fragile so it's not good at travelling outside the host
but that doesn't matter because
there's another form that's more stable.
And this is typically a dormant, non-replicating,
what we call a cyst form.
So many protozoan parasites,
when they are exquited into the enviroment
it's as a cyst form that is very stable
in either the soil or in water
and then needs to be reintroduced into a host.
It will then develop into the trophozoite form that can replicate.