Amebic meningoencephalitis , caused by Naegleria
fowleri , is characterized by acute suppurative inflammation of the
brain and meninges.
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Amoebiasis (Entamoeba histolytica)
N. fowleri is a nonparasitic, free-living
soil ameba that causes rhinitis when introduced into the nasal cavity.
The amebae may subsequently invade the olfactory nerves, migrate
through the cribriform plate, and proliferate in the meninges and
brain. The infections are fulminating and uniformly fatal.
The disease has been recognized in many parts of the
world including the United States, Europe, Australia, New Zealand,
South America, and Africa.
It has been diagnosed principally on histologic
evidence, with only a few cases confirmed by cultures.
Almost all patients give a history
swimming in freshwater lakes or pools shortly before the onset of
symptoms.
In tissue sections, trophozoites of Naegleria
measure from 8 to 15 micro meter across.
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The cytoplasm is not distinctive,
but the nuclei are sharply outlined and deep stained with hematoxylin.
Some amebae have perinuclear vacuoles that tend to obscure the nuclear
margin.
Cysts of the Naegleria have not been seen in tissue sections.
Clinically, acute and fulminating rhinitis and meningoencephalitis lead to death, usually within 1 to 2 weeks.
Most
victims have been in their teens or younger.
The cerebrospinal fluid
contains numerous neutrophils, blood, and amebae.
Grossly, the brain is swollen and soft, with
vascular congestion and a thin purulent exudates on the meningeal
surface, most prominent over the lateral and basal areas.
There is
massive destruction of the brain by amebae, which invade the brain
along the Virchow-Robin spaces.
Thrombosis and destruction of blood
vessels are associated with extensive hemorrhage in the affected
areas.
The olfactory tract and bulbs are
enveloped and destroyed, and there is an exudate between the bulb and
the inferior surface of the temporal lobe.
Histologic sections show severe damage to the
olfactory tract and bulb and invasion by amebae of nerve fibres in the
passages through the cribriform plate.
The exudate expands the
subarachnoid space.
Extensive proliferation of Naegleria in the brain
often leads to formation of solid masses of amebae (amebomas).
Meningitis can extend the full length of the cord.
The cerebellum is usually heavily involved, with amebae
surrounding and damaging Purkinje cells.
There may be large areas of necrosis in
the gray matter of the cerebellum and cerebrum.
During life, diagnosis is made by
finding organisms in the cerebral spinal fluid or in biopsy specimens.
The fluid may be placed directly in the counting
chamber, where motile amebae are seen.
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