Primary
Amebic Meningoencephalitis (PAM) in order to
distinguish this central nervous system (CNS) invasion from other secondary
invasions other amoebas such as E. histolytica can cause (Butt, 1966). An interesting retrospective study
found the likely first recorded case of PAM occurred in Ireland in 1909 (St. Symmers, 1969).
Morphology (Martinez, 1985)
There
are three distinct morphological stages in the life cycle of N. fowleri: trophozoite, flagellate, and cyst (Figure 1A-C). The trophozoite
is the infective stage of the amoeba. They are ~10-20mm long and contain a nucleus with a large karyosome surrounded by a halo. Trophozoites
reproduce by binary fission and are motile due to round processes filled with
granular cytoplasm called lobopodia. N.
fowleri is a thermophilic
organism and can tolerate temperatures up to 45ˇC; the ideal growth temperature for trophozoites is 42ˇC.
When free-living, trophozoites use a structure
called a food-cup (Figure 1D) to ingest bacteria and yeast – in a human
host this same structure is used to ingest red blood cells, white blood cells,
and tissue. Another important
structure is the contractile vacuole.
This vacuole ruptures, empties, and reforms in a rapid process and is
valuable in recognizing amebic trophozoites among
other tissue cells.
The
flagellate stage is entered as a response to a change in pH or ion
concentration of the amoebaŐs environment. In just minutes to a few hours trophozoites differentiate into bi-flagellated cells. This change can be induced by placement
of trophozoites from culture into distilled water.
Additionally,
in unfavorable conditions (low nutrient, crowding, cold temperatures,
desiccation), N. fowleri
can form cysts. These cysts are
~8-15mm long and if they are introduced to the
favorable environment of the human nasal passages can revert to the trophozoite stage and become infective.
Figure 1: Stages of N. fowleri.
(A) trophozoite (B) cyst
(C) Flagellate (D) EM of food cup
Life Cycle (CDC, 2009)
The
life cycle of N. fowleri
can occur in a human host, or freely in an aquatic or soil environment (Figure
2). In a warm, high nutrient, aquatic environment the trophozoite
stage predominates. This is the reproductive stage and a trophozoite
that undergoes promitosis results in two trophozoites. If pH or ionic changes occur surrounding the
organism, the trophozoite can transition to the more
mobile flagellated form. If the
environment becomes depleted of nutrients, cold, or dry the trophozoite
can encyst to survive the unfavorable
conditions. Cysts and trophozoites can enter the human through nasal passages,
usually related to water activities.
Trophozoites are infective, and their
penetration of the nasal mucosa and subsequent migration to the brain results
in PAM. Visualization of trophozoites in a personŐs CSF or brain tissue is
considered the diagnostic stage.
Transmission/Reservoir
Naegleria fowleri have
been isolated from soil, swimming pools, cooling towers, hospital hydrothermal
pools, and sewage sludge (Visvesvara et al., 1990). Most reported infections occur after swimming in warm bodies
of water. Introduction of
trophozoites to the nasal passages of humans is the first step in Naegleria fowleri
infection. There are no animal
reservoirs of N. fowleri. The bodies of water and soil
contaminated with N. fowleri
may be considered physical reservoirs – as a free-living amoeba they can
survive out of human hosts as long as the conditions remain favorable.
Incubation Period
The
period between initial contact with the pathogenic N. fowleri and the onset of clinical signs and symptoms varies from 2-3
days to as long as 7-15 days. Once
symptomatic, however, progression of PAM is rapid and often fatal (Ma et al.,
1990).
Pathogenesis (Ma et al., 1990)
The
portal of entry of N. fowleri
into the human host is the nasal cavity.
After entry, the trophozoite penetrates the
nasal mucosa and migrates along mesaxonal spaces of unmyelinated olfactory nerves terminating at the olfactory
bulb in the subarachniod space. This space is quite vascularized
and is a route of dissemination of trophozoites to
other areas of the CNS. There are histopathological characteristics of the invaded tissues
– for example the olfactory mucosa and olfactory bulb have hemorrhagic
necrosis. An important note is
that trophozoites only are found in PAM lesions.
Clinical Presentation in Humans (Ma et al., 1990)
After
the initial incubation period, N. fowleri infection is characterized by abrupt onset of bifrontal or bitemporal headache,
fever, nausea, vomiting (often projective), and encephalitis. Sometimes early in the progression of
disease changes in smell (parosmia) and taste (ageusia)
occur as trophozoites damage the olfactory
system. After early signs
described above, progression to coma and seizures is rapid – over a
period of 3-7 days. PAM often
resembles purulent bacterial meningitis and early in its course differences
cannot be distinguished. The vast
majority of PAM cases end in death (98%), on average only one week after
appearance of the first symptoms.
Infected
tissue, upon close inspection has distinct macro and microscopic morphology.
Macroscopically, the cerebral hemispheres are observed to be swollen and
olfactory bulbs are necrotic and hemorrhagic. Trophozoites can be foun in fascicles of unmeylinated
axons of the olfactory nerves and in nasal mucosa. On a smaller scale, the cortical gray matter is observed as
a preferred site of ingestion.
Also, trophozoites can be identified in
purulent exudates by their prominent karyosome.
Diagnosis
Because
of the incredibly quick progression of PAM, rapid diagnostics must be developed
for early detection, as the progression of the disease is so fast. Currently much of the clinical
diagnosis is based on patient history – whether or not the patient has
recently swam in warm bodies of water – along with presenting
symptoms. Final diagnostic
confirmation is not achieved until trophozoites are
isolated and identified from CSF or brain tissue. While N. fowleri do grow easily in culture, this can take
multiple days – time that is precious in an infection that will kill in a
week.
Current
research is, understandably, focused on development of real time PCR diagnostic
methods. One method being
developed involves monitoring the amplification process in real-time with
hybridization of fluorescent labeled probes targeting the MpC15 sequence – which is unique to N. fowleri (Madarova et al., 2009). Another group has multiplexed three
real-time PCR reactions as a diagnostic for N.
fowleri, as well as Acanthamoeba spp. And Balamuthia mandrillaris
(Qvarnstrom et. al, 2006). This
could prove to be an incredibly efficient diagnostic test.
Treatment
Currently,
if N. fowleri
infection is diagnosed or suspected treatment Amphotericin
B is the standard of care. Amphotericin B is a polyene
compound that disrupts selective permeability of plasma membranes. It is administered intravaneously
and is something of a Ôlast resortŐ drug as it has high toxicity. While not particularly effective, every
one of the 4 documented survivors of PAM have been treated with Amphotericin B.
As
there is no effective treatment for PAM, the development of a therapeutic is an
area of great research interest.
Currently, much work is being done to determine what specific to N. fowleri
makes it pathogenic and if these virulence factors can be targeted by drugs.
One potential player in motility of the amoeba is the Nfa1 protein. When Nfa1 is expressed in non-pathogenic N. gruberi and the amoebas are
co-cultured with target tissue cells, it was observed that the protein was
located on the food cup which is responsible for ingestion of cells during
feeding(Song et al., 2006).
Following up that research, Nfaq gene expression knockdown experiments were preformed
using RNAinterference. In this experiment dsRNA targeting
the Nfa1 sequence was introduced and
subsequently expression levels of the gene product dramatically decreased (Jung
et al., 2008). This method could,
potentially be a technique applicable for knockdown of expression of pathogenicity factors in N. fowleri trophozoites.
Epidemiology
PAM
due to Naegleria fowleri has a
worldwide distribution and occurs most frequently in tropical areas and during
hot summer months. The majority of
the reported cases, 121 from 1937-2007, occurred in the United States (Primary Amebic Meningoencephalitis,
2008). A distribution of those
infections that occurred in the U.S between 1937-1990 can be seen in Figure 3
(Visvesvara et al., 1990). This high rate of occurance
in the U.S. is likely due to under-reporting elsewhere, and not a dramatically
higher prevalence in the U.S. Most cases are diagnosed upon autopsy, and in
many countries autopsy is not standard. Globally, there were a total of 144
cases reported through 1990 – the U.S., Australia, and the Czech Republic
reported the majority of these (Visvesvara et al., 1990). Major outbreaks,
including one south of Richmond in Virginia, and one in the Czech Republic,
tend to be the result of swimming in a warm body of water.
Figure 3. Distribution of PAM in the United
States from 1937-1990 (Visvesvara et al., 1990)
Public Health Prevention Strategies
Currently
there are no widespread efforts for prevention because of the low prevalence of
N. fowleri
infections. However, because of
the fatality of the ensuing meningoencephlitis there
are efforts in research and development of both diagnostics and treatment (see
above). Additionally, a case can
be made for increased awareness of N. fowleri and its infection for more accurate reporting.
Useful Websites
Information from the
United States Center for Disease Control:
http://www.cdc.gov/ncidod/dpd/parasites/Naegleria/factsht_naegleria.htm
General information
(wikipedia = user generated):
http://en.wikipedia.org/wiki/Naegleria_fowleri
eMedicine summary of N.
fowleri and PAM:
http://emedicine.medscape.com/article/223910-overview
CDC Morbidity and
Mortality reports from PAM cases:
Works Cited
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JF (2004). Molecular definition and the
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S.Y., et al., Naegleria fowleri: nfa1
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