Page 439 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com GIARDIA mebooksfree.com mebooksfree.com
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PART VI Parasitology
428
Disease

Giardia lamblia causes giardiasis.

Important Properties
The life cycle of G. lamblia is shown in Figure 51–6. The life
cycle consists of two stages: the trophozoite and the cyst
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(Figures 51–2C and D, and 51–7). The trophozoite is pear-
shaped with two nuclei, four pairs of flagella, and a suction
disk with which it attaches to the intestinal wall. The oval
fibers. Each cyst gives rise to two trophozoites during
excystation in the intestinal tract.
FIGURE 51–4
Entamoeba histolytica—cyst. Arrow points to a
cyst of E. histolytica. Two of the four nuclei are visible just to the left of
Pathogenesis & Epidemiology
the head of the arrow. (Source: Centers for Disease Control and Prevention.)
Transmission occurs by ingestion of cysts in fecally con-
Treatment
duodenum, where the trophozoite attaches to the gut wall
but does not invade the mucosa and does not enter the
The treatment of choice for symptomatic intestinal amebia- taminated food and water. Excystation takes place in the
bloodstream. The trophozoite causes inflammation of the
sis or hepatic abscesses is metronidazole (Flagyl) or tinida-
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duodenal mucosa, leading to malabsorption of protein
zole. Hepatic abscesses need not be drained. Asymptomatic
and fat.
cyst carriers should be treated with iodoquinol or
The organism is found worldwide; about 5% of stool
paromomycin.
Approximately half of those infected are asymptomatic car-
Prevention
riers who continue to excrete the cysts for years. IgA defi-
Prevention involves avoiding fecal contamination of food
ciency greatly predisposes to symptomatic infection.
and water and observing good personal hygiene such as
In addition to being endemic, giardiasis occurs in out-
handwashing. Purification of municipal water supplies is
breaks related to contaminated water supplies. Chlorina-
usually effective, but outbreaks of amebiasis in city dwellers
still occur when contamination is heavy. The use of “night
Hikers who drink untreated stream water are frequently
soil” (human feces) for fertilization of crops should be pro-
infected. Many species of mammals as well as humans act
hibited. In areas of endemic infection, vegetables should be tion does not kill the cysts, but filtration removes them.
as the reservoirs. They pass cysts in the stool, which then
cooked.
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contaminates water sources. Giardiasis is common in male
homosexuals as a result of oral–anal contact. The incidence
is high among children in day care centers and among
Clinical Findings
Watery (nonbloody), foul-smelling diarrhea is accompa-
nied by nausea, anorexia, flatulence, and abdominal cramps
persisting for weeks or months. There is no fever.
Laboratory Diagnosis
Diagnosis is made by finding trophozoites or cysts or both
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mebooksfree.com mebooksfree.com mebooksfree.com Giardia antigen in the stool is also very useful. Tests for mebooksfree.com
in diarrheal stools (See Figure 51–7). In formed stools (e.g.,
in asymptomatic carriers), only cysts are seen. An enzyme-
linked immunosorbent assay (ELISA) test that detects
antibody in the serum are not routinely available.
FIGURE 51–5
Entamoeba histolytica—flask-shaped ulcer forms
If those tests are negative and symptoms persist, the
in colonic mucosa resulting in bloody diarrhea. (Source: Dr. Mae Melvin,
string test, which consists of swallowing a weighted piece
Centers for Disease Control and Prevention.)

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