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Intestinal Trematodes 197

Laboratory Diagnosis Treatment and Prevention
Proposed serological methods of diagnosis are limited Praziquantel is an effective drug for treating the three spe-
due to numerous instances of cross-reactions with infec- cies causing schistosomiasis, but oxamniquine is an eco-
tions by other helminthes. When travel history suggests nomic drug for treating infections by S. mansoni only.
visits to an endemic area and symptoms are present, the Control of snail species that are capable of transmitting the
likelihood of schistosomiasis is probable. The recovery of organisms for schistosomiasis and the practice of good san-
eggs from feces and urine will enable diagnosis of the ap- itation practices prevent infections effectively. Avoiding ex-
propriate species based on the characteristics of the three posure to bodies of water in endemic areas are perhaps the
species’ eggs. most efficient way to control the infection rates as well as
Differentiation is through differing morphology of the application of a protective barrier on the skin that poses
ova for the three species affecting humans. a barrier to penetration by the schistosome cercariae.

SUMMARY

The life cycles of the major trematodes, of which some to immunological responses to eggs and to the embedded
are commonly known as flukes, are complex. The class of larvae.
the phylum Nemathelminthes includes the parasites that Liver and lung fluke diseases are the next group in
are commonly known as flatworms. Many of these are the number of worldwide cases, following schistosomia-
parasitic, and either cylindrically shaped or shaped as a sis. Paragonimus westermani is the lung fluke that causes
spindle. The most common genus found as the causative paragonimiasis, where eggs are coughed up into the spu-
agent in nematodal infections is that of the schistosomes. tum and swallowed to start the reproductive cycle in the
Similar stages of a typical fluke occur in the life cycle of intestines. Clonorchis sinensis, also called the Chinese
all trematodes. These stages include: the adult, the ova, a liver fluke, causes clonorchiasis. This fluke is also respon-
miracidium, a sporocyst, a redia, a cercaria, and finally a sible for numerous cases of biliary disease, particularly
metacercaria. in Asia. Opisthorchis spp., and in particular Opisthrochis
Cercarial dermatitis, also known by various names, felineus and O. sinensis, are found in raw and partially
including swimmer’s itch, occurs upon penetration of cooked fish. These organisms are also commonly found
the skin by the metacercaria. This order, Digenetica, in cats and other mammals, humans included.
includes all four groups of flukes that are parasitic in Many important discoveries about flukes were
humans. These four groups are blood flukes, intestinal made during the period of the late 1800s to the early
flukes, liver flukes, and lung flukes, of which only a few 1900s. Observations from other parasitic flukes such as
organisms that are pathogenic to humans are included in Fasciola hepatica in sheep and several others that infect
human parasitology, although some of these also infect mammals other than humans have provided much of the
other mammals. information regarding the life cycles. The life cycles of
Intermediate hosts most often include the copecod these other flukes are essentially the same as those de-
and the snail, a gastropod. Archaeological evidence for scribed for each of the Schistosoma spp. But in a few spe-
many of these organisms exists, providing evidence that cies an additional intermediate host is present between
humans were infected thousands of years ago, particu- the snail and the human in or upon which the cercariae
larly with those of the schistosomes. Schistosomes are encyst. Humans may become infected when they eat the
also notorious for causing cancer of various organs due infected second intermediate host.