Page 478 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 55 Trematodes
Paragonimiasis is endemic in Asia and India. In the United
excysting in the small intestine, the parasites attach to the
States, it occurs in immigrants from these areas.
mucosa and differentiate into adults. Eggs are passed in the
feces; on reaching fresh water, they differentiate into mira-
Clinical Findings
cidia. The ciliated miracidia penetrate snails and, after
The main symptom is a chronic cough with bloody sputum.
several stages, develop into cercariae that encyst on aquatic
Dyspnea, pleuritic chest pain, and recurrent attacks of bacte-
vegetation. The cycle is completed when plants carrying the
rial pneumonia occur. The disease can resemble tuberculosis.
cysts are eaten.
Pathologic findings are due to damage of the intestinal
Laboratory Diagnosis
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Diagnosis is made by finding the typical operculated eggs
atic, but ulceration, abscess formation, and hemorrhage
in sputum or feces (see Figure 55–4D). Serologic tests are
can occur. Diagnosis is based on finding typical eggs in the
feces. Praziquantel is the treatment of choice. Prevention
not useful.
consists of proper disposal of human sewage.
Treatment
Heterophyes
Praziquantel is the treatment of choice.
Heterophyes heterophyes is an intestinal parasite of people
Prevention
living in Africa, the Middle East, and Asia who are infected
by eating raw fish containing cysts. Larvae excyst in the
Cooking crabs properly is the best method of prevention.
small intestine, attach to the mucosa, and develop into
adults. Eggs are passed in the feces and, on reaching brack-
TREMATODES OF MINOR
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tal stages, cercariae are produced that encyst under the
IMPORTANCE
scales of certain fish. The cycle is completed when fish
carrying the infectious cysts are eaten.
Fasciola
Pathologic findings are due to inflammation of the
Fasciola hepatica, the sheep liver fluke, causes disease pri-
intestinal epithelium as a result of the presence of the adult
marily in sheep and other domestic animals in Latin
flukes. Most infections are asymptomatic, but abdominal
America, Africa, Europe, and China. Humans are infected
pain and nonbloody diarrhea can occur. Diagnosis is based
by eating watercress (or other aquatic plants) contami-
on finding the typical eggs in the feces. Praziquantel is the
nated by larvae (metacercariae) that excyst in the duode-
treatment of choice. Prevention consists of proper disposal
num, penetrate the gut wall, and reach the liver, where they
mature into adults. Hermaphroditic adults in the bile ducts
produce eggs, which are excreted in the feces. The eggs
hatch in fresh water, and miracidia enter the snails. Mira- of human sewage.
cidia develop into cercariae, which then encyst on aquatic
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mebooksfree.com mebooksfree.com mebooksfree.com (A) The visual appearance of male and female schistosomes is the mebooksfree.com
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SELF-ASSESSMENT QUESTIONS
vegetation. Sheep and humans eat the plants, thus complet-
ing the life cycle.
1. Regarding schistosomes, which one of the following statements is
Symptoms are due primarily to the presence of the adult
the most accurate?
worm in the biliary tract. In early infection, right-upper-
same.
quadrant pain, fever, and hepatomegaly can occur, but
(B) Humans are infected by schistosomes when cercariae penetrate
most infections are asymptomatic. Months or years later,
the skin.
obstructive jaundice can occur. Halzoun is a painful phar-
(C) Infection of freshwater fish is a required part of the life cycle of
yngitis caused by the presence of adult flukes on the poste-
schistosomes.
rior pharyngeal wall. The adult flukes are acquired by
(D) The pathology of schistosomiasis is principally caused by the
eating raw sheep liver.
cercariae entering hepatocytes and killing them.
Diagnosis is made by identification of eggs in the feces.
(E) Infection by nonhuman schistosomes can cause meningitis in
There is no serologic test. The drug of choice is triclaben-
people who swim in certain lakes in the United States.
dazole. Adult flukes in the pharynx and larynx can be 2. Regarding Schistosoma mansoni, which one of the following state-
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removed surgically. Prevention involves not eating wild
ments is the most accurate?
aquatic vegetables or raw sheep liver.
(A) The main site of S. mansoni in the human body is the mesen-
teric venules.
Fasciolopsis
the Western hemisphere.
Fasciolopsis buski is an intestinal parasite of humans and
(C) The laboratory diagnosis of S. mansoni depends on seeing eggs
pigs that is endemic to Asia and India. Humans are infected
with a terminal spine in the stool.
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