Hello, and welcome to parasites, plasmodium.
We're going to be going a bit deeper into diseases caused by several individual types of parasites.
Just to remind you, protozoa are single celled eukaryotes
and we're going to be talking about protozoan parasites in the next two lectures
and these include the plasmodium species, the causative agents of malaria.
There are a variety of different plasmodium that cause human malaria,
plasmodium falciparum for example
and this blood smear stain to visualize two parasite containing red blood cells.
The pictures are slightly different for each organism.
Plasmodium vivax is another malaria-causing protozoan parasite,
plasmodium malariae, and finally, plasmodium ovale,
the disease is caused by these different plasmodium species.
Have overall features in similar fashion but they do have some differences
and there's slight geographic differences as to the location of the parasites.
This global map shows you the regions of the world
in which infection with the plasmodium species occur.
The malaria, as you can see most of Africa is affected in red, a good part of Asia, Central and South America.
So the regions essentially boarder the equator,
these are where the mosquitos are found that transmit the infection.
Malarias are responsible for two billion infections every year,
just think of that in the context of the population of Earth, this is huge
and three million deaths mostly in Africa and mostly in children less than five years of age,
and this is the real tragedy of malaria, that it targets children
and makes them unable to learn and have productive lives.
As a consequence of multiple infections with these parasites
and multiple bouts of malaria in the lifetime of these individuals,
the effect is a reduction in both economic and social development.
If we could rid the globe of malaria we'd make so many more people
able to have productive lives and contribute to the human experience.
Humans are the only reservoir for those four species of plasmodium that I showed you at the beginning.
There are animal variants of these malarias and as far as we know, they don't cause human disease.
There's some suggestion that these maybe changing but for the most part,
the four species I showed you, humans are the only reservoir.
They are transmitted among humans
by the bite of an infected female anopheline mosquito, alright, female mosquito.
Now can you tell me, do male mosquitos bite you?
I'll give you a few seconds, the answer is no.
Whenever you're bitten by a mosquito, it's a female mosquito doing that
and that's because she needs the blood in order to lay eggs,
interesting little observation, so all the diseases transmitted by mosquitos,
whether they be parasitic or viral, it's all spread by female mosquitos.
Let's take a look at the life cycle of plasmodium falciparum,
both in the mosquito and in the human host.
There are slight differences between falciparum and the other species
but this will suffice to illustrate the general pattern.
We start at the top of the slide with the mosquito injecting parasites,
we're gonna continue through the liver stage, the blood stages,
and eventually we're gonna get back into the mosquito.
So this is a cycle that is maintained among people by the mosquito vector.
So let's look at step one, the mosquito, again, it's a female mosquito is biting someone
and delivering the form of the parasite called a sporozoite.
Sporozoites come out of the salivary gland of the mosquito
and the mosquito of course is taking a blood meal so the mosquito inserts its proboscis into the skin,
it actually injects some saliva because that saliva contains a variety of compounds
to make it easier for the mosquito to draw a blood meal,
like anti-coagulants and other things as well, and of course,
if the mosquito is infected, it will deliver sporozoites as well,
so the simple fact that the mosquito has to take a blood meal injects those sporozoites
and of course the mosquito will withdraw a blood from that host.
Those sporozoites are then entering into the circulatory system of a human.
Within 30 minutes, those sporozoites go to the liver,
it's gonna be their main stop on their trip to the human body
and they become a form called cryptozoites.
Now, many parasite go through multiple stages in their life cycles with various names,
sporozoites, what's in the mosquito; cryptozoite is what the sporozoite becomes in the liver.
The parasite replicates in the liver and it does so inside an infected cell
and that cell eventually produces many, many merozoites,
the cell ruptures within 8 to 14 days after the initially mosquito bite
and those new merozoites are released into the blood stream.
In the blood, the merozoites then infect its second target which is the red blood cell.
They enter the red blood cell in which they multiply.
Now, here, we're showing you several of the stages
that occur within the red blood cell starting from the top,
the free moving red blood cell that's infected with what we call a ring forms, you can see those two in the upper left;
and then below it are the different forms of the infected stages within the red blood cell.
If you do smears of infected individuals, you'll see all these forms in the blood,
and in the blood the merozoites multiply and at some point the blood cell ruptures
and releases more merozoites into the bloodstream.
This kind of multiplication that we've been talking about both in the red blood cells
and in the liver is asexual, its simply binary fusion.
But, at some point, in the reproduction, the sexual stages are made
and this is an organism that can reproduce in both ways,
sexually and asexually, so at some point during the replication,
both male and female gametocytes, that's the name for the sexual stage of the organism, are made,
and now these will be picked-up by a new mosquito if this person who's now full of malaria parasite is bitten
and of course, if you're in an area endemic for malaria,
this is likely that you'll be bitten thousands of times a day,
so mosquitos will deliver malaria parasites and they'll pick up new ones from infected people.
Now when a mosquito takes a blood meal,
the blood goes to the gut of the mosquito, of course,
cuz that's where it is digested and so go the male and female gametocytes,
those will eventually mate and produce the asexual stages.
And so from the gut, the two gametocytes will mate,
they all migrate back to the salivary gland, of course,
that's where they need to be to be delivered to the next victim,
so sexual mating in the mosquito and the production of sporozoites
which will then going to be delivered to the next host.