Welcome to Parasites: Toxoplasma. This is one
of the series of videos where we explore in
detail individual parasites and the diseases
they cause. And after you've listened to this
video, you will understand the lifecycle of
toxoplasma gondii, including how it's transmitted
to humans. You will be familiar with pathogenesis
of toxoplasmosis and why pregnant mothers
should avoid infection. And finally you will
understand how to prevent and treat toxoplasmosis.
Now remember these latter videos are considering
protozoan parasites, these are single celled
eukaryotes, one cell only not multiple cells
and they include parasites of medical importance.
We've talked about Plasmodium, we are going
to be talking now about Toxoplasma.
Here is a lovely photograph of a toxoplasma
species. There are four cells in this photograph
in sort of pairs, and they’re stained with
a dye that makes them glow in a beautiful
way. But you can see that they are composed
of single cells and each one has nuclei.
The organism that we’re going to focus on today
is Toxoplasma gondii, the causative agent
of toxoplasmosis. Now it turns out that much
of the world is infected with toxoplasma,
between you and I, one of us is probably infected
and we don't even know it, because fewer than
1% of infected individuals develop disease.
This is a very successful parasite. It manages
to cause so many infections without causing
any harm or very little harm. Now I think
a great proportion of the population is infected
with toxoplasmosis, if they like to eat raw
or undercooked meat as you will see, and there
are certain countries in the world that like
to do that more than others. Strains of Toxoplasma
infect all the animals on the planet, so not
only are humans largely infected, but all
the animals around us as well. So as I said
this is an incredibly successful parasite.
Remember the definitive hosts for parasites
that we defined in our beginning lecture on
parasites, well for toxoplasma gondii, the
cat is the definitive host. Your little cat
at home might be harboring a toxoplasma.
How do cats get infected? When they eat another
animal by predation like a mouse that they
catch in the wild. Now if you keep your cat
inside and you never feed it wild animals
and just feed it canned foods, they won’t
develop toxoplasmosis, although depending
what's in the food they might develop a spongiform
encephalopathy, but that's the subject of
a different video of course.
Let's talk a little bit about the cat cycle
of toxoplasmosis. Cats acquire toxoplasma
by eating mice, who in turn are infected.
Mice are often eaten by cats and it is said,
although it's probably not true, that the
presence of toxoplasma in the mouse makes
them fear cats less than they normally would
and therefore they're caught. Well that sounds
like a good story, but the real truth is that
the presence of toxoplasma in the mouse probably
makes them less averse to cat urine, that
is what scares them away in the first place.
So the mouse is eaten by the cat, the toxoplasma
then enters the cat where it stays for the
life of the animal and cats then will shed
toxoplasma in their feces and that's one way
that humans may be infected. Now you may say,
how would I ever be infected by something
in my cat's feces? Well you know you have to
clean the cat's litter box frequently and
in doing so you may contaminate yourself.
Cats also may go outside and they may defecate
in your backyard and then your children will
go out and play in the yard and pick up the
parasites that way. You know in the end the
world is a dangerous place and you just need
to know microbiology to know how to take care
of yourself, that's why we’re teaching you
this. So humans can get infected from cats
or as I said, by ingesting raw or undercooked
meat which harbors the toxoplasma parasite.
The parasite will then enter our circulation
where it will reproduce and cause problems or
perhaps not. So we can acquire the parasite
either from cats or by eating undercooked
meat, including lamb, mutton and beef containing
the cysts of the parasite. These are sort
of the stable form that will persist for a
long time. And as I've said, cats acquire
the infection by eating mice, they shed this
oocyst in their feces and we can also get
it from cats in that manner as well. When
we ingest a parasite, it goes into our gut,
the parasites penetrate through the gut wall,
they drill through your intestine and enter
the bloodstream, where they then of course
go to the liver. One of the first places anything
goes when it gets into your blood is the liver
and there it encounters macrophages and it
will infect them and move about your body
in macrophages and the parasite will even
reproduce in the macrophages, at some point
it will burst out and spread more within your
body. Within 4 to 6 weeks after your initial
ingestion of this parasite by whatever means,
your host immune system is finally kicked
in. It will start to control infection and
in fact that leads to the production of dormant
This is a very large cyst form, a stable form.
This is produced in response to the immunity
that you put up towards the parasite infection
and those cysts can stay with you forever.
So once you're infected with toxoplasma, you
harbor it for the rest of your life. And it
may or may not cause a serious initial illness,
but then of course if you are immunosuppressed,
if you develop AIDS or if you get an organ
transplant and you need to take immunosuppressive
drugs, those dormant cysts will start to come
alive again and replicate, make more toxoplasma
and they can cause serious problems by invading
various organs as shown here,
and they can cause serious brain infections.
So AIDS is one way that you can reactivate
the cysts, any form of immunosuppression.