Page 461 - Review of Medical Microbiology and Immunology ( PDFDrive )

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Minor Protozoan Pathogens 53
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CHAPTER C ONTENT S

Acanthamoeba & Naegleria
Self-Assessment Questions
Babesia
Summaries of Organisms
Balantidium
Practice Questions: USMLE & Course Examinations
Cyclospora
Isospora

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The medically important stages in the life cycle of certain
minor protozoa are described in Table 53–1.

ACANTHAMOEBA & NAEGLERIA
Acanthamoeba castellanii and Naegleria fowleri are free-
living amebas that cause meningoencephalitis. The organ-
isms are found in warm freshwater lakes and in soil. Their
life cycle involves trophozoite and cyst stages. Cysts are
quite resistant and are not killed by chlorination.
Naegleria trophozoites usually enter the body through
mucous membranes while an individual is swimming.
They can penetrate the nasal mucosa and cribriform plate
to produce a purulent meningitis and encephalitis that are FIGURE 53–1 Naegleria fowleri—trophozoite. Arrows point to
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usually rapidly fatal (Figure 53–1). Acanthamoeba is car-
ried into the skin or eyes during trauma. Acanthamoeba
infections occur primarily in immunocompromised indi-
two ameba-shaped trophozoites in brain tissue. (Source: Public Health
viduals, whereas Naegleria infections occur in otherwise
healthy persons, usually children. In the United States,
these rare infections occur mainly in the southern states
ketoconazole, or flucytosine may be effective in Acantham-
and California.
oeba infections.
Diagnosis is made by finding amebas in the spinal fluid.
Acanthamoeba also causes keratitis—an inflammation
The prognosis is poor even in treated cases. Amphotericin
of the cornea that occurs primarily in those who
B may be effective in Naegleria infections. Pentamidine,

TABLE 53–1 Medically Important Stages in Life Cycle of Certain Minor Protozoa Important Stage(s)
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Stage(s) in Humans Most
Stage That Infects
Outside of Humans
Organism
Associated With Disease
Humans
Insect Vector
Acanthamoeba and Naegleria
Trophozoites in meninges
None
Tick (Ixodes)
Sporozoite in tick saliva
Trophozoites and merozoites in
Babesia
red blood cells
450

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