Page 20 - Medical Parasitology_ A Textbook ( PDFDrive )

12 3 Amoebae

Trophozoites encyst due to unfavourable conditions like food deprivation, desic-
cation and cold temperature. The cyst is 7–10 μm in diameter and has a smooth
double wall. They are the resting or the dormant form and can resist unfavourable
conditions, such as drying and chlorine up to 50 ppm. Cysts of N. fowleri have never
been found in cerebrospinal fluid (CSF).

Life Cycle (Fig. 3.5)
(1) Cyst. (2) Trophozoite. (3) Flagellated form showing flagella. (4) The trophozoite
replicates by promitosis. (5) The trophozoite penetrates the nasal mucosa. (6) The
trophozoite migrates to the brain via the olfactory nerves.
Infection occurs when humans go swimming or diving in warm freshwater (lakes
and rivers), hotsprings, heated pools, or nasal irrigation using contaminated tap
water.

Pathogenesis and Clinical Features

Primary amoebic meningoencephalitis (PAM) is usually reported in previously
healthy young adults or children. During contact with contaminated water, the
amoebae invade the nasal mucosa and pass through the olfactory nerves in the
cribriform plate into the meninges and brain to initiate an acute meningitis and
encephalitis. The incubation period varies from 2 days to 2 weeks. The disease
progresses rapidly, causing fever, headache, vomiting, stiff neck, ataxia, sei-
zure and coma and is almost always fatal.

Diagnosis

1 Cerebrospinal fluid (CSF) examination
The CSF is cloudy to purulent, with prominent neutrophils, elevated protein
and low glucose, resembling pyogenic meningitis. Wet film examination of CSF
may show motile trophozoites. Fixed smear can be stained with Giemsa or a
modified trichrome stain for identification. Cysts are not found in CSF or brain.
Histological examination of the brain following autopsy may show the presence
of trophozoites.
2 Culture
Naegleria fowleri in CSF can be grown on non-nutrient agar plates coated
with Escherichia coli. Both trophozoites and cysts can be detected in culture.
3 Molecular diagnosis
PCR on CSF specimen.