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Entamoeba histolytica• cosmopolitan distribution• no animal reservoirs• facultative pathogen
• most clear the infection spontaneous in 6-12 months with mild or no symptoms
• can cause a serious invasive disease• worldwide incidence = 0.2-50%
• estimated that 10% of world’s population may be infected
• 50 million cases invasive amebiasis/yr• 100,000 deaths/yr
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1875 Lösch correlated dysentery with amebic trophozoites
1925 Brumpt proposed two species: E. dysenteriae and E. dispar
1970's biochemical differences noted between invasive and non-invasive isolates
80's/90's several antigenic and DNA differences demonstrated
• rRNA 2.2% sequence difference1993 Diamond and Clark proposed a new species
(E. dispar) to describe non-invasive strains1997 WHO accepted two species
Facultative Pathogenicity of Entamoeba histolytica
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Entamoeba histolyticaLife Cycle
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Excystation• cyst wall disruption• ameba emerges• nuclear division (48)• cytoplasmic division
(8 amebala)• trophozoites colonize
large intestine• feed on bacteria and
debris• replicate by binary
fission
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Excystation• cyst wall disruption• ameba emerges• nuclear division (48)• cytoplasmic division
(8 amebala)• trophozoites colonize
large intestine• feed on bacteria and
debris• replicate by binary
fission
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Encystation• trophozoite rounds up• secretion of cyst wall• aggregation of ribosomes
(= chromatoid bodies)• 2 rounds of nuclear division
(14 nuclei)• survive weeks to months
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trophozoiteimmature
cystmature
cyst
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Pathogenesis of Amebiasis• NON-INVASIVE
• ameba colony on intestinal mucosa• asymptomatic cyst passer• non-dysenteric diarrhea, abdominal
cramps, other GI symptoms
• INVASIVE• necrosis of mucosa ulcers, dysentery• ulcer enlargement dysentery, peritonitis• metastasis extraintestinal amebiasis
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• ulcers with raised borders• little inflammation between lesions
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• ‘flasked-shaped ulcer’• trophozoites at boundary of necrotic
and healthy tissue• trophozoites ingesting host cells• dysentery (blood and mucus in feces)
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‘hematophagous’ trophozoites
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Lateral and Downward Expansion of Ameba into Lamina Propria
• localized sloughing
•perforation of intestinal wall
•ulcers coalesce
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Disease Manifestations•ulcer enlargement severe dysentery
•perforation of intestinal wall peritonitis
• local abscesses•2o bacterial infections•occasional ameboma (=amebic granuloma)
•cessation of cyst production
ameboma = inflammatory thickening of intestinal wall around the abscess (can be confused with tumor)
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Extraintestinal Amebiasis
• metastasis via blood stream• primarily liver (portal vein)
• other sites less frequent• ameba-free stools common• high antibody titers
Amebic Liver Abscess
• chocolate-colored ‘pus’• necrotic material• usually bacteria free
• lesions expand and coalesce
• further metastasis, direct extension or fistula
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Pulmonary Amebiasis
• rarely primary• rupture of liver abscess
through diaphragm• 2o bacterial infections
common• fever, cough, dyspnea,
pain, vomica
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Cutaneous Amebiasis
• intestinal or hepatic fistula• mucosa bathed in fluids
containing trophozoites• perianal ulcers• urogenital (eg, labia,
vagina, penis)
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Cutaneous Amebiasis
• intestinal or hepatic fistula• mucosa bathed in fluids
containing trophozoites• perianal ulcers• urogenital (eg, labia,
vagina, penis)
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Cutaneous Amebiasis
• intestinal or hepatic fistula• mucosa bathed in fluids
containing trophozoites• perianal ulcers• urogenital (eg, labia,
vagina, penis)
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• 85-90% of infected individuals are asymptomatic
• ~10% of the symptomatic will develop severe invasive disease
Facultative Pathogenicity
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• molecular probes used to survey for E. dispar and E. histolytica
• E. dispar ~10-fold > E. histolytica• discrete endemic pockets of E. histolytica
• many asymptomatic E.h. infections• ~10% of the E.h. infections are
associated with invasive amebiasis• ~25% seropositive for E. histolytica in
endemic areas
Molecular Epidemiology
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• a pathogen has an inherent ability to break host cell barriers
• virulence usually correlates with ability to replicate within host
• various degrees of virulence may be exhibited depending on conditions
pathogenecity ability to cause disease
(genetic component)
virulence relative capacity to cause disease (degree of pathology)
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• contact-dependent killing of epithelial cells• breakdown of tissues (extracellular matrix)
• secreted proteases?• contact-dependent killing of neutrophils,
leukocytes, etc.
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E. histolytica vs E. dispar
CRITERIA E. dispar E. histolyticaIn Vitro Culture xenic axenic
ConA Agglutination - +
Complement Resistance - +
Zymodemes (isoenzymes) I & III II
Numerous Antigenic Differences(eg., GIAP Epitopes) 1-2 1-6
Numerous DNA Sequence Differences(eg., rRNA) 2.2% sequence diversity
RFLP/DNA ProbesB133
cEH-NP1P145
cEH-P1
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Galactose Inhibitable Adherence Protein
• trophozoites adhere to mucins, epithelial cells, leukocytes, etc• mediated by galactose-inhibitable lectin activity
• lectin activity due to surface protein (GIAP)• 170 kDa heavy chain mediates binding (multigene
family)• 35 kDa light chain anchor to membrane
• -GIAP Abs abrogate complement resistance• ~85% identity between Eh and Ed
• Are there differences in adherence?• after contact the target cell is lysed and
phagocytosed by the trophozoite
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Host Cell Lysis and Phagocytosis
• Amebapore• pore-forming peptide• potent anti-bacterial
activity• located in vacuoles, not
secreted• Eh and Ed sequences
are 95% identical• GluPro change breaks
-helix• Ed had 80% less
activity than Eh
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Entamoeba Proteases• Eh expresses and secretes higher
levels of cysteine proteases• 6 cys-protease genes (ehcp1-6)
• ehcp1 and 5 are missing in Ed• 90% inhibition of ehcp5 did not affect
trophozoite mediated destruction of host cell monolayers
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Epidemiologic Risk FactorsPrevalence Severity lower socioeconomic status crowding lack of indoor plumbing
endemic area institutionalization communal living promiscuity among male
homosexuals
children, esp. neonates pregnancy and
postpartum states corticosteroid use malignancy malnutrition
Modified from Ravdin (1995) Clin. Inf. Dis. 20:1453
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Clinical SyndromesAssociated with AmebiasisIntestinal Disease asymptomatic cyst passer symptomatic nondysenteric
infection amebic dysentery fulminant colitis
+ perforation (peritonitis) ameboma (amebic granuloma) perianal ulceration
Extraintestinal Disease liver abscess pleuropulmonary amebiasis brain and other organs cutaneous and genital diseases
Intestinal Symptoms
• range • mild to intense• transient to long lasting
• nondysenteric• diarrhea• cramps• flatulence• nausea
• dysenteric• blood/mucus in stools• cramps/pain• tenesmus
• ameboma• palpable mass• obstruction
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DiagnosisIntestinal stool examination
cysts and/or trophozoites sigmoidoscopy
lesions, aspirate, biopsy antigen detection
histolytica/dispar
Extraintestinal (hepatic) symptoms
history of dysentery RUQ pain enlarged liver
serology (current or past?) imaging (CT, MRI, ultrasound) abscess aspiration
only select cases reddish brown liquid trophozoites at abscess wall
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-GIAP MonoclonalAntibodies
mAb E.h. E.d.3F4 + +8A3 + +7F4 + -8C12 + -1G7 + -H85 + -Reactivities of mAbs againstE. histolytica and E. dispar
Possible Outcomesand Interpretations
capture/detection mAbs
3F4/8A3 8C12/1G7 interpretation
+ + E. histolytica
+ - E. dispar
- + inconclusive
- - negative
Antigen Detection Assay
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DiagnosisIntestinal stool examination
cysts and/or trophozoites sigmoidoscopy
lesions, aspirate, biopsy antigen detection
histolytica/dispar
Extraintestinal (hepatic) symptoms
history of dysentery RUQ pain enlarged liver
serology (current or past?) imaging (CT, MRI, ultrasound) abscess aspiration
only select cases reddish brown liquid trophozoites at abscess wall
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Treatment Control and Epidemiology
• avoid fecal-oral transmission• not normally associated with travelers diarrhea• > 1 month stay
• institutions• mass drug treatment little
affect staff and improved housing
conditions lowers prevalence• male homosexuals
• 40-50% in NYC and SF during late 70’s
• lower since AIDS/safe sex
asymptomatic• iodoquinol or
paromomycin• endemic areas?
symptomatic• metronidazole or
tinidazole• followed by lumenal
agents
drain liver abscess• only with high
probability of rupture!