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Amebiasis and infections due to Entamoeba species
are discussed in Chapter 341. The ubiquitous, free-living amoebae
of the genera Naegleria, Balamuthia,Acanthamoeba,
and Sappinia are the etiologic agents of rare infections
of the central nervous system and eyes.1Naegleria
fowleri is the agent of primary amebic meningoencephalitis; Acanthamoeba and Balamuthia cause
granulomatous amebic meningoencephalitis; and Acanthamoeba also
can infect the eye, resulting in amebic keratitis.2-4 Recently, Sappinia
diploidea, another free-living amoeba that is normally
found in soil contaminated with herbivore feces, has been identified
as causing encephalitis in a healthy young man.5Naegleria,
Balamuthia,Acanthamoeba, and Sappinia have
trophozoite and cyst stages; in addition, Naegleria has
a flagellate stage. For Naegleria, only trophozoites
are found in tissues. Naegleria trophozoites are
10 to 30 μm in diameter and have a clear nucleus
with a prominent central dense nucleolus and cytoplasmic pseudopodia. Acanthamoeba and Balamuthia trophozoites
are of similar size and appearance to Naegleria, but
the cyst form of these parasites may also be observed in tissue.1,6Sappinia trophozoites
are 40 to 80 μm in diameter and have an ovoid or oblong
shape. They appear to be flattened with occasional wrinkles on the
surface, and food vacuoles are seen in cytoplasm. The mature cysts
are 15 to 30 μm in diameter and are round. They have
two adjoining nuclei.1 Unequivocal identification
of these amoebae is not routine, and specimens should be referred
to a qualified protozoologist for confirmation.
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Almost every example of acute primary amebic meningoencephalitis
reveals a recent patient history of swimming in fresh or brackish water.
The organisms probably gain access to neural tissue via the nasal
mucosa and the cribriform plate. There have been several epidemics
in which the same swimming facility was the focus of infection.
This disease has been reported in England, the Czech Republic, Australia,
Virginia, Texas, and Florida. Naegleria fowleri has
been isolated from soil and the bottom sediment of lakes and pools
from all parts of the world. It also has been reported in thermally polluted
water, where it can reproduce at temperatures up to 46°C (114.8°F).
Seroepidemiologic studies demonstrate that most young adults in
the southern United States have agglutinating antibodies against Naegleria.7,8
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Granulomatous amebic encephalitis caused by Acanthamoeba affects
those who are immunocompromised and debilitated, whereas Balamuthia causes
a subacute to chronic infection in both immunocompetent and immunocompromised
individuals. These infections have incubation periods that exceed
1 week, and the clinical course, which usually is fatal, can last
for weeks to months. The one known case of granulomatous amebic
encephalitis caused by Sappinia diploidea occurred
in a 38-year-old healthy white male. The patient’s prior medical
history showed a recent frontal sinus infection.5
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Keratitis caused by Acanthamoeba is most common
in individuals who wear contact lenses. It is also seen as a complication
of other corneal injuries. Corneal infection is associated with
wearing the lenses while ...