Some examples of nematodes or round worms,
Caenorhabditis elegans is an example of a round worm.
It is a free-living nematodes. It lives in the soil.
This is a commonly studied nematode in the labarotory
but it causes no disease in humans. And in fact,
most of the nematodes out there are not parasitic.
There are just a few that cause disease in people.
However, many people on the planet are infected with nematodes.
So round worms again.
We estimated about 4 billion people have at least
one nematode species, and some people have even more than one.
So people are always acquiring this
and they stay within them for long periods of time.
The segmented flatworms, the cestodes, all these are
obligate parasites of the small intestinal tract.
And these are the tapeworms. And there are
three different types that are important to humans.
The first is the beef tapeworm, Taenia saginata,
you can see a picture of it there or you've seen it before.
It can be very very long, occupy your entire intestinal tract.
We acquire this by eating undercooked beef.
Then there's the pork tapeworm, Taenia solium
which is acquired by eating undercooked pork.
And finally, the dog tapeworm, Echinococcus granulosus
which is the name says, it's acquired from dogs.
And you can say they all look quite different.
Now we need to talk about two other concepts in parasitology
and that defines the organism that harbors the parasite.
We have two kinds of host that we talk about.
There is the definitive host of the parasite.
That is the host in which the parasite
reaches its maturity and reproduces sexually.
So if your host ingest say a cyst form,
and that cyst develops into an adult
and reproduces, that's the definitive host of that parasite.
We also have intermediate host. This is a host
that harbors a parasite for a transition period
and usually in that period some developmental stage is completed.
Alright, so we have definitive and intermediate host.
Let's illustrate some of these
so you can understand exactly what they mean.
Taenia saginata, the definitive host is the human.
The intermediate host is the cow.
So this is the beef tapeworm we acquired from cows.
It complete its maturation in the humans.
So we're the definitive host.
Taenia solium, the pork tapeworm, the definitive host is the human.
But there can be two intermediate hosts.
It can be other humans or it can be pigs.