Page 473 - Review of Medical Microbiology and Immunology ( PDFDrive )

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55
C
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Trematodes

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Trematodes of Minor Importance mebooksfree.com

CHAPTER C ONTENT S

Introduction
Summaries of Organisms
Schistosoma
Clonorchis
Paragonimus

Fasciola Practice Questions: USMLE & Course Examinations
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Fasciolopsis
Heterophyes

INTRODUCTION

Disease
Trematoda (flukes) and Cestoda (tapeworms) are the two
large classes of parasites in the phylum Platyhelminthes.
The most important trematodes are Schistosoma species
1
and Schistosoma japonicum affect the gastrointestinal tract,
(blood flukes), Clonorchis sinensis (liver fluke), and
whereas Schistosoma haematobium affects the urinary tract.
Paragonimus westermani (lung fluke). Schistosomes have Schistosoma causes schistosomiasis. Schistosoma mansoni
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by far the greatest impact in terms of the number of people
Important Properties
infected, morbidity, and mortality. Features of the medi-
The life cycle of Schistosoma species is shown in Figure
cally important trematodes are summarized in Table 55–1,
and the medically important stages in the life cycle of these
organisms are described in Table 55–2. Three trematodes
maphrodites, adult schistosomes exist as separate sexes but
live attached to each other. The female resides in a groove
of lesser importance, Fasciola hepatica, Fasciolopsis buski,
in the male, the gynecophoric canal (“schist”), where he
and Heterophyes heterophyes, are described at the end of
continuously fertilizes her eggs (see Figure 55–2A). The
this chapter.
The life cycle of the medically important trematodes
three species can be distinguished by the appearance of
involves a sexual cycle in humans (definitive host) and
inent lateral spine, whereas S. japonicum eggs have a very
asexual reproduction in freshwater snails (intermediate
hosts) (Figure 55–1). Transmission to humans takes place
small lateral spine and S. haematobium eggs have a terminal
spine (Figures 55–4A and B, 55–5, and 55–6). Schistosoma
either via penetration of the skin by the free-swimming their eggs in the microscope: S. mansoni eggs have a prom-
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cercariae of the schistosomes (Figures 55–2D and 55–3) or
mansoni and S. japonicum adults live in the mesenteric
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veins, whereas S. haematobium lives in the veins draining
via ingestion of cysts in undercooked (raw) fish or crabs in
Clonorchis and Paragonimus infection, respectively.
the urinary bladder. Schistosomes are therefore known as
Trematodes that cause human disease are not endemic
in the United States. However, immigrants from tropical
areas, especially Southeast Asia, are frequently infected.
1
As does Schistosoma mekongi.
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