a. A fluke is an exclusive parasite. This means that it cannot sustain itself and
must live on a host to receive nourishment and oxygen for survival. Flukes may be
found in the intestine, liver, lungs, blood, and other body parts. People sometimes
become infected with flukes by eating uncooked fish, crabs, or vegetables.
b. There are classes or subgroups of flukes: Monogenea, Aspidogastrea, and
Digenea. The Monogenea are external parasites which attach themselves to fish,
amphibians, and reptiles. The Aspidogastrea, either internal or external parasites, are
usually found on mollusks. Digenea, internal parasites found in humans and other
warm-blooded animals, is the only group of major importance to humans. Parasites of
this group cause a variety of disease in humans throughout the world.
c. The life cycle of digenea flukes requires at least one intermediate host,
sometimes two depending on the species. Sexually mature flukes lay hundreds of
thousands of eggs which are passed into or near fresh water in feces from an infected
person or animal. Either the eggs hatch immediately after they are expelled, or they are
injected by an intermediate host. Mollusks--usually snails, mussels, or crabs--are the
intermediate host in which a complex development cycle takes place, the development
of the larvae. Some flukes are able to infect the final host from this stage. Other flukes,
still in the larval stage, leave the first host and go to another host--a fish, crustacean, or
another mollusk. Finally, the larvae escape from the second intermediate host, encyst
on vegetation, or penetrate directly into the skin of the final host.
d. Digenea are further classified according to the area of the host's body in
which fluke development into adults is completed and their eggs are deposited. From
this standpoint, there are flukes that live in the intestine, liver, lungs, and the portal
venus system. These are important to humans:
(1) Blood fluke--developing and laying eggs in the portal venous system and
Liver fluke--makes its home in the liver.
Intestinal fluke--its habitat is the intestine.
Lung fluke--residing in the lungs.