Biomedical Engineering Reference
In-Depth Information
reproduce asexually. The complex life cycles of flukes, and their ability for
larval stages to reproduce themselves, greatly expanding the number of larval
organisms, are further indications of the remarkable generative abilities of
Class Platyhelminthes (see above the planarian platyhelminth, Schmidtea
mediterranea).
Here is the general life cycle of a Trematode:
1.
Eggs are released into the environment (usually water) by adult trema-
todes within the primary host. For example, Schistosoma haematobium
females release their eggs into the bladder lumen, where they are washed
out with the urine stream.
2.
The eggs hatch into swimming larvae known as miracidia, that penetrate
their first intermediate host (a snail or other mollusc).
3.
The miracidia, living in the first intermediate host, develop into a sac-like
structure called a sporocyst.
4.
The sporocyst, unlike cystic stages in tapeworms, is capable of self-
reproduction (in some species), producing a daughter sporocyst. The
sporocyst also produces the next stage of larval development, the redia, a
larval form that has an oral sucker.
5.
A redia, like the sporocyst, is capable of self-reproduction, and can pro-
duce more rediae. In addition,
the redia is capable of producing a
cercaria.
6.
A cercaria is an organism that develops from germinal cells of a sporo-
cyst or a redia. This means that the cercaria is not just a phase of larval
development produced by morphologic transformation of one larval form
into another; it is a new organism that arises from a particular type of cell
within a larval organism. The cercaria is motile.
7.
The motile cercaria may infect a new host (the primary host), where it
becomes an adult fluke. Or the cercaria may transform into one of two
dormant forms that persist in the environment, often attached to edible
vegetation, or in another intermediate host. These two forms are: meso-
cercaria and metacercaria. The mesocercaria is a larval form of the organ-
ism while the metacercaria is an encysted form.
It is obvious that the flukes are a form of life with an incredibly complex
and flexible life cycle. Trematode infections in humans can be simplified by
remembering the following rule: All pathogenic classes of trematodes, with
one exception, produce human infections wherein the human is the primary
host (i.e., humans host the egg-laying adult fluke); and in which humans
become infected by eating infectious metacercariae, that have settled on
vegetation, or that have infected a second intermediate animal host. The
exception to this rule is Class Schistosomatidae, which will be discussed at
the end of the chapter.
Members of Class Fasciolidae produce adult flukes that live in the liver,
gall bladder, and intestines of the primary host (e.g., humans). All pathogenic
