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128 CHAPTER 5

TREMATODES (FLATWORMS) adult form. These metacercariae may also become en-
cysted in the skin or tissue of freshwater fish and is the
The class Trematoda is estimated to contain 18,000 or route by which most humans become infected. Fish is a
more species that are divided into two subclasses. These staple in the diet of those living in parts of Africa, Asia,
subclasses include Aspidogastrea and Digenea. Aspido- South America, and the Middle East, where infections
gastrea, the smaller of the two, contains approximately from fish carrying the parasite are contracted. In most
80 species. Digenea is the subclass that contains the species of flukes, the metacercariae will then become
majority of the diverse forms of trematodes. Nearly all enclosed in a cyst and will survive on the surface of the
trematodes are parasites of either mollusks or vertebrates water, except for the blood fluke (genus Schistosoma)
(organisms with a backbone) but in some rare instances which does not undergo this stage.
may infect cartilaginous fish which when poorly cooked These cysts will become attached to vegetation and
or eaten raw may infect humans. All trematodes are para- a mammal eats the vegetation that includes the cysts.
sitic but not all species are found in humans; mammals In the digestive system of the mammal, the larvae will
are the host for infections by these flatworms. leave the ingested cyst and will pass through the intes-
tinal walls and migrate to the specific organ which the
Life Cycle of Trematodes particular species has an affinity for. While in the organ,
the immature form of the fluke continues to develop into
Trematodes have characteristically complex life cycles an adult capable of laying eggs. Blood flukes differ from
and the larval form may inhabit different life forms than the path where mammals eat infected water vegetation, as
those where the adult trematode will parasitize as an they become free-swimming cercariae and penetrate the
adult. The most often recovered parasites from the class skin of a host mammal, upon which they will reach the
Trematoda are those of the liver fluke and the blood circulatory system of their hosts.
fluke, both of which commonly parasitize humans. The Flukes are often named by the environment in
flukes, regardless of species, follow a similar path toward which they are found or by the primary host infected
development into adults and the manner in which they by the parasites. Some flukes are called “pond flukes,”
reproduce. as they infect fish found in ponds. Tissue flukes infect
When the adult fluke is living in the organs of a the bile ducts, lungs, and many other specific organs.
mammal for which it is adapted, eggs are produced as Paragonimus westermani is known as the lung fluke,
an immature stage. A female fluke may produce as many and both Fasciola hepatica and Clonorchis sinensis are
as three hundred eggs per day. These eggs pass from the known as liver flukes. Blood flukes inhabit the circula-
digestive tract of their mammalian host into the water tory system of mammals during some stages of their life
where they further develop. Following hatching of the respective cycles. The most significant trematodes from
eggs, a larval form called a miracidium, a free-swimming a clinical point of view may be the blood flukes Schisto-
stage, will search for a snail as a suitable host. Almost soma mansoni, S. japonicum, and S. hematobium.
all trematodes infect mollusks such as the aquatic snail
in the first life cycle of infecting an intermediate host Schistosomiasis (Bilharziasis)
(Figure 5-10). The snail is the intermediate host and the
larvae live in the blood vessels of the intermediate host The three species of Schistosoma have different geo-
until they are released from the intermediate host’s body graphic distributions. S. hematobium is prevalent in
in either the urine or feces, dependent upon the species Africa where the disease caused by the organism is called
of the parasite. bilharziasis, and S. mansoni is primarily found in Africa
While inhabiting the intermediate host, the fluke and South America as well as some Caribbean islands.
goes through a number of stages in their development S. japonicum is more common in the Far East. It is es-
into an adult. In the sporocyst stage, these cells divide timated that up to 250 million people may be infected
quickly and the resulting sporocysts, called rediae, will with the various species of schistosomes and 600 million
become larvae. These parasitic larvae called cercariae more may be at risk due to contact with contaminated
develop in the mollusk host and will progress through water in several areas of the world but primarily in Asia.
stages called mesocercaria or metacercaria toward the The morphology of the adult worms is characteristic.