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e-Medical
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Parasitic infections are common in
Africa, Asia and Central and South America but are relatively rare
elsewhere.
Persons from industrialized countries traveling to
endemic
areas often can reduce risk by following strict rules for eating and
bathing and taking simple measures to minimize exposure. Casual visitors
from endemic countries are not likely to spread parasitic diseases
because the environmental requirements, vectors or intermediary hosts
needed for transmission of many parasitic infections often are not
present in industrialized countries. However, transmission of imported
infections may occur via the fecal-oral route, by blood transfusions or
organ transplants or by a suitable local vector.
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Laboratory Diagnosis of
Parasitic Infections
Many protozoa and the eggs of some
helminths are shed sporadically. Routine detection of intestinal ova and
parasites requires examination of at least three stool specimens,
preferably collected every other day or on 3 consecutive days.
Freshly passed stools uncontaminated with urine,
water, dirt, or disinfectants should be sent to the examining laboratory
within 1 h, particularly if they are unformed or diarrheal (ie, likely
to contain motile trophozoites). Formed stools may be refrigerated (but
not frozen) if not examined immediately and need not be kept warm while
in transit. If possible, portions of fresh stools should be emulsified
in fixative to preserve GI protozoa. Thin fecal smears preserved in
Schaudinn's fixative are also useful. If necessary, stool samples can be
concentrated by the formalin-ether, zinc flotation, sugar-coverslip, or
Baerman techniques. Anal swabs may demonstrate pinworm or tapeworm eggs.
Antibiotics, x-ray contrast material, purgatives, and antacids hinder
detection of ova and parasites for several weeks.
Sigmoidoscopy should be considered when routine
stool examinations are negative in patients suspected of amebiasis or
schistosomiasis. Sigmoidoscopic specimens should be collected with a
curet or a Volkmann's spoon (cotton swabs are not suitable) and
processed immediately for microscopy. If giardiasis or strongyloidiasis
is suspected in patients with a negative stool examination, duodenal
aspirates or string tests may be performed. Small-bowel biopsies may be
necessary for the diagnosis of such infections as cryptosporidiosis.
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