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Microsporidia are obligate, intracellular,
spore-forming protozoal parasites. Their host
range is extensive and includes most
invertebrates and all classes of vertebrates.
Microsporidia is a common infection in man,
however is self-limited or asymptomatic in normal
hosts. Microsporidia have gained more attention
in their recent association with the AIDS
It is now recognized as a common pathogen in
human immunodeficiency virus (HIV) infected
patients. Reported prevalence rates in AIDS have
varied between 2 and 70 depending on the
population studied and the diagnostic technique
Subkingdom Protozoa Phylum
Microspora Class Microsporea Order
Microsporidia Family Encephalitozoonidae
contains one genus
Encephalitozoon which
includes three species cuniculi,
hellem and intestinalis Family
Enterocytozoonidae contains one
genus Enterocytozoon which
includes one species
Family Nosematidae contains three genera
1. Nosema which includes one species
ocularum 2. Brachiola which
includes two species
vesicularum and connori. 3.
Vittaforma which includes one species
orneae. Family Pleistophoridae contains two
genera 1.Trachipleistophora which
includes two species hominis and
anthropophthera 2. Pleistophora Undete
rmined family Contains genus Microsporidium
which includes two species africanum and
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During germination in a new host, the polar tube
is evaginated and the sporoplasm (cytoplasm and
nucleus) pass through it to be injected to the
cytoplasm of a host cell. No other organisms are
known to have this type of infection mechanism .
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The life cycle
All microsporidia are obligate parasites with a
life-cycle involving repeated proliferation by
merogony, followed by sporogony, in which
sporonts divide into two or more sporoblasts that
mature into spores. Meronts usually have a
simple plasma membrane while sporonts have an
electron-dense surface coat which later becomes
the outer (exospore) layer of the spore wall.
In some genera, sporonts produce sporoblasts by
binary or multiple fission in direct contact with
the host cell cytoplasm, so that the resultant
spores are freely dispersed in the host cell. In
other genera, an envelope separates from the
sporonts surface and division again by binary or
multiple fission occurs within this envelope
(sporophorous vesicle), resulting in packets of
spores rather than free spores.
However, all stages of development of
Encephalitozoon are surrounded by a membrane
derived from the host cell, which ultimately
forms the margin of a parasitophorous vacuole.
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Pathogenesis and Clinical Manifestations
Almost all of the organs of the human body can
be infected by one or more of the spectrum of
microsporidian species. Many tissues and cell
types are involved. Clinical manifestations of
microsporidiosis depend on the site of infection,
which includes intestinal, ocular, muscular and
Intestinal infection
Although alterations to the intestinal structure
are not universal in patients infected with
Enterocytozoon bieneusi or Encephalitozoon
intestinalis, there may be villus stunting,
atrophy and other histological changes.
Immuno-competent patients may suffer an acute
self limited diarrhoea, while HIV and AIDS
patients usually have chronic uncontrolled
diarrhoea. They may pass one to several liters of
liquid faeces per day, which lead to dehydration
and extreme wasting. Malabsorption is common in
these patients
Although enterocytes of the small intestine are
the primary site of infection by Enterocytozoon
bieneusi, it may also spread to other epithelial
cells in the biliary tract and nasopharynx
causing cholangitis and rhinosinusitis. In
contrast, Encephalitozoon intestinalis appears to
have a primary infection site in the small bowel,
but can disseminate to the viscera as colon,
kidney, liver, gallbladder and other sites,
including the lower airways.
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Ocular infection
Two types of ocular infection have been
described the first involves the conjunctiva and
corneal epithelium and occurs in HIV infected
patients, infection caused by Encephalitzoon
hellem and Trachipleistophora hominis and
manifested by distressing bilateral punctate
keratopathy with redness, irritation and
decreased visual acuity.The second type involves
the corneal stroma and leads to ulceration and
suppurative keratitis and occurs in
immunocompetent individuals, infection caused by
Microsporidium ceylonensis, Microsporidium
africanum, Vittaforma corneae and Nosema ocularum.
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Muscular including Myocardial Infection
Infection in myocytes of the heart has been
reported for Brachiola connori, Encephalitozoon
cuniculi and Trachipleistophora anthropophthera.
In all cases, each focus contained myriads of
spores. Reactive cells were absent except for
macrophage activity after release of the spores,
but areas with multiple lesions were associated
with necrosis and fibrosis of adjacent tissue
. Myositis of skeletal muscle caused by
Trachipleistophora hominis, has been described in
patients with severe cellular immunodeficiency.
Symptoms include generalized muscle weakness,
myalgia, fever and weight loss.
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Renal Infection
The kidney is a site of predilection for all
three Encephalitozoon spp. and was involved in
disseminated cases of Vittaforma corneae,
Brachiola connori and Trachipleistophora
anthropophthera . Infection is mainly in the
tubules but glomeruli may be involved. Breakdown
of tubule epithelial cells stimulates an
interstitial nephritis and debris accumulates in
the tubule lumina. Kidney tissue destruction may
be massive and there may be spread to the ureters
and bladder. The spores can be detected in urine .
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Cerebral Infection
The brain has been reported as a site of
infection with Encephalitozoon cuniculi and
Trachipleistophora anthropophthera. Spores were
found in parenchyma, perivascular spaces,
astrocytes and endothelial cells causing central
necrosis with spores engulfed by macrophages
surrounded by infected astrocytes.
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Respiratory Infection
Respiratory disease is a common manifestation of
Encephalitozoon species. Manifestations of
respiratory infection may be chronic cough and
shortness of breath and finally respiratory
failure. Infection of the nasal and nasal sinus
epithelia leads to formation of polypoid tissue
and consequent nasal obstruction and discharge.
Cell-mediated Immunity
It is proved experimentally that cell-mediated
immunity plays the major role in controlling
infection. Thus, transfer of sensitized T-cell
enriched spleen cells to athymic mice just prior
to infection with Encephalitozoon cuniculi gave
protection from lethal disease. Also, SCID mice
were protected if the T cell transfer occurred
before infection.
Macrophage activation by cytokine release from
lymphocytes, to stimulate phagocytosis and
degradation of spores by nitrogen intermediates,
has been demonstrated as one mechanism of
protection, and interferon gamma has been
implicated as the mediator.
Humoral Immunity
The role of antibodies in the control of
microsporidial infection appears to be of
secondary importance to the cell-mediated
response. Nothing is known of the role of IgA in
preventing infection via the intestine. Both IgM
and IgG antibodies are produced in response to
infection but are only likely to have an
opsonising effect on spores for uptake by
macrophages .
Epidemiology and Control
The potential sources and means of transmission
of human microsporidial infections are uncertain.
However, infection most likely is acquired by
ingestion of spores, which are released into the
environment via faeces or urine or by death and
degeneration of the hosts .Inhalation of spores,
ocular exposure, ingestion of infected fish
muscle and sexual intercourse may be possible
routes of transmission .
Spores released from patients or from animal
sources could easily enter the water supplies, as
the spores are small enough for all species
reported in man to pass through the filters
used in water purification. Disinfection of
surfaces contaminated with microsporidia has
received little attention. That spores are highly
resistant has been found to be viable after
storage for up to 10 years in distilled water.
They were, however, killed after 10 minutes of
autoclaving at 120?C, or exposure to 2 lysol,
10 formalin, or 70 ethyl alcohol for 10 minutes
Laboratory Diagnosis
I. Examination of Tissue Biopsies
  • Diagnosis of microsporidian spores in tissue
    specimens required both light microscopic and
    electron microscopic examination or touch
    preparations of biopsies. Stains used include
    haematoxylin and eosin, Giemsa, silver, tissue
    Gram stain and periodic acid-Shiff (PAS). A
    tissue stain which shows great promise is
    Warthin-Starry, which stains some sporogonic
    stages and spores even single spores are easily

  • Electron microscopy (EM) remains the gold
    standard for confirmation of microsporidial
    infection in tissues. Demonstration of coiled
    polar tube within spores is pathognomic of
    microsporidial infection and identification to
    generic and sometimes species level can be made
    from ultrastructural features .
  • Immunofluorescent or immunoperoxidase
    antibody staining of microsporidia in tissue
    sections would be useful if such antibodies were
    commercially available.

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II. Examination of Stool, Urine and Other body
  • Several staining techniques have proved for
    detection of microsporidian spores in faeces,
    urine and other body fluids. The original
    chromotrope-based stain and its modifications
    stain spores pinkish red and can be used with
    light background counterstains to advantage on
    faecal and urine smears. Other useful stains are
    Gram's, which stains spores blackish purple, and
    Ziehl-Neelsen, which stains them red.

  • Another approach involves the use of
    chemofluorescent agents such as calcofluor and
    Uvitex 2B. These reagents are sensitive but
    nonspecific objects other than microsporidial
    spores will also fluoresce.
  • The newest approach for identification of spores
    in clinical specimens uses antisera in an
    indirect immunofluorescent-antibody procedure.
    This technique offers a more sensitive approach
    than routine staining methods.

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III. Culture of Microsporidia from Clinical
Microsporidial spores from clinical samples such
as urine, nasal polyps, nasopharyngeal aspirates,
faeces and muscle biopsies can sometimes be
cultured in various cell lines . Once established
in culture, enough spores can be produced from
the original sample to allow further tests as
PCR, to be carried out which can help to
establish the species involved .
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IV. Polymerase Chain Reaction (PCR)
Recently it has become possible to generate
sequence data rapidly by direct sequencing of
amplified ribosomal DNA using PCR. This technique
has identified sequence variation and shows
considerable promise for the specific
identification of microsporidian species.
Techniques based on PCR still largely depend on
cultured organisms from clinical sources, which
limits their usefulness in rapid diagnosis, but
amplification directly from clinical specimens
should improve the speed of diagnosis.
V. Detection of antibodies In-Patient's Sera
Several serological tests as ELISA, counter
-immuno-electrophoresis and indirect
immunofluorescent-antibody test have been
designed to detect antibodies in human sera and
thus determine the extent to which microsporidian
infections occur in immunocompetent healthy
people, as well as in those suffering AIDS or
other diseases.
Fumagillin drug has shown potent activity when
applied topically in cases of keratoconjunctivitis
. Fumagillin when applied alone as drops, brought
about relief of symptoms and reduction of
epithelial damage of ocular lesions but did not
eliminate the spores. So, when it is used as
drops of 0.03 solution coupled with oral
albendazole (400 mg) twice daily achieved
complete resolution of conjunctival infection .
Oral albendazole, 400 mg twice a day for 4 weeks
has been effective in treating intestinal and
disseminated infection caused by Encephalitozoon
spp.. Albendazole, 400 mg twice a day for 4
weeks, reduced the number of bowel movements in
intestinal disease by Enterocytozoon bieneusi but
did not clear the infection.
A new analogue of fumagillin TNP-470 has been
shown to inhibit tumour growth by preventing
neovascularisation. Significant inhibition of
proliferation of Encephalitozoon spp. in the
presence of TNP-470 was observed at
concentrations of 10 ng/ml. Also, TNP-470 shows
promise as a drug for amelioration of
microsporidiosis due to Enterocytozoon bieneusi.
Much remains to be learned about microsporidia,
especially in relation to human infection and
almost nothing is known about epidemiology or
pathogenesis of these parasites in humans .
Whether there are animal sources of human
infection will become clearer as epidemiological
and experimental studies become better
established. Molecular techniques are becoming
more commonly available and provide considerable
evidence of microsporidial infection and latent
infection and also establish the phylogenetic and
taxonomic relationships of the species involved.
1. Microsporidia must be put into consideration
as a differential diagnosis in any case suffering
from chronic diarrhoea. 2. Microsporidia
infection must be excluded before administration
of any immuno-suppressive drug. 3. Patients under
immuno-suppressive therapy or with pathologically
induced immuno-suppression should be routinely
investigated for microsporidia.
4. It is recommended that more than one procedure
be used when working with stool specimens , this
is because of the large number of artifacts
present in stool. 5. The presence of infective
spores in human clinical specimens suggests that
precautions when handling body fluids and
personal hygiene measures may be important in
preventing primary infections in the health care
setting. 6. Comprehensive guidelines for disease
prevention will require more definitive
information regarding sources of infection and
modes of transmission. 7. Further developments in
treatment are needed, especially in relation to
Enterocytozoon bieneusi infection.
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