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182 CHAPTER 8

INTRODUCTION apparently due to water fowl ingesting waste products
and then fouling the water by eating the intermediate
Trematodes were broadly discussed, along with some hosts, or by another similar mechanism. This order,
specifics, in Chapter Five, where the three types of Digenetica, includes all four groups of flukes that are
“worms”—trematodes, nematodes, and cestodes—were parasitic in humans. These four groups are found in the
compared. Flukes, or flatworms, are parasitic members following table (Table 8-1).
of the class Trematoda, and to the phylum Platyhelmin- Flukes are characteristically complex in their life
thes. The order Digenetica, to which the schistosomes cycles and most often require two intermediate hosts,
belong, also have a complex life cycle. The term where snails are most often the first intermediate host.
digenetic refers to two stages in trematodes where an Most trematodes are hermaphroditic, meaning that they
asexual phase of reproduction—a sexual generation liv- contain both male and female organs responsible for
ing in a vertebrate host as the final or definitive host— reproduction. Fluke eggs might or might not contain
occurs in mollusks. Stages of a typical fluke include a miracidium, depending upon the species of fluke
the adult, the ova, a miracidium, a sporocyst, a redia, a involved. The miracidium is a ciliated larval form of the
cercaria, and finally a metacercaria, which is similar in fluke, and some unembryonated eggs require a period of
the trematode life cycle. development in the water before the development of the
miracidium is complete. The miracidium infects a snail
Trematode Life Cycle living in a body of fresh water. The miracidium reproduces
inside the snail and produces a large number of cercariae,
The original symptoms of a condition technically a free-swimming stage of the fluke’s life cycle.
known as cercarial dermatitis occur upon penetration The cercariae from some fluke species infect a
by metacercaria. A number of terms are used to refer host by penetrating the skin when the susceptible or-
to infections by trematodes, including “swimmer’s ganisms, including humans, are exposed to fresh water.
itch,” as commonly used in some developing countries. Other species infect (encyst) a second intermediate
Swimmer’s itch translates into “rice paddy itch” and host, such as vertebrate (possess a backbone) fish and
other descriptive terms in areas where rice is a staple invertebrates, such as crayfish that possess an exoskele-
food, as well as other terms reflected by the widespread ton rather than bones to provide strength and structure
geographic regions in which these organisms are found. to the organism. In addition, some cercariae cling to or
Even in the United States, where intestinal trema- encyst vegetation, which may be eaten by vertebrates,
todes are somewhat common as in coastal New Jersey, whereupon the life cycle continues. The encysted cer-
the malady is commonly called “duckworms” and is cariae in the animals or vegetation evolve into a form

TABLE 8-1 Human Flukes

REPRESENTATIVE ANATOMIC REGION
COMMON NAME ORGANISMS INHABITED
Blood Fluke Schistosoma haematobium, S. mansoni, and Mesenteric and pelvic veins
S. japonicum
Intestinal Fluke Gastroidiscoides hominis, Fasciolopsis buski, Human intestine
Heterophyes heterophyes, and Metagonimus
yokogawai
Liver Fluke Clonorchis sinensis, Fasciola hepatica, Dicrocoelium Liver, biliary, and pancreatic
dendriticum, and Opisthorchis felineus ducts
Lung Fluke Paragonimus westermani Lung tissue exclusively