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

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

Entamoeba histolyticaLife Cycle

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

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

Encystation• trophozoite rounds up• secretion of cyst wall• aggregation of ribosomes

(= chromatoid bodies)• 2 rounds of nuclear division

(14 nuclei)• survive weeks to months

trophozoiteimmature

cystmature

cyst

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

• ulcers with raised borders• little inflammation between lesions

• ‘flasked-shaped ulcer’• trophozoites at boundary of necrotic

and healthy tissue• trophozoites ingesting host cells• dysentery (blood and mucus in feces)

‘hematophagous’ trophozoites

Lateral and Downward Expansion of Ameba into Lamina Propria

• localized sloughing

•perforation of intestinal wall

•ulcers coalesce

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)

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

Pulmonary Amebiasis

• rarely primary• rupture of liver abscess

through diaphragm• 2o bacterial infections

common• fever, cough, dyspnea,

pain, vomica

Cutaneous Amebiasis

• intestinal or hepatic fistula• mucosa bathed in fluids

containing trophozoites• perianal ulcers• urogenital (eg, labia,

vagina, penis)

Cutaneous Amebiasis

• intestinal or hepatic fistula• mucosa bathed in fluids

containing trophozoites• perianal ulcers• urogenital (eg, labia,

vagina, penis)

Cutaneous Amebiasis

• intestinal or hepatic fistula• mucosa bathed in fluids

containing trophozoites• perianal ulcers• urogenital (eg, labia,

vagina, penis)

• 85-90% of infected individuals are asymptomatic

• ~10% of the symptomatic will develop severe invasive disease

Facultative Pathogenicity

• 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

• 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)

• contact-dependent killing of epithelial cells• breakdown of tissues (extracellular matrix)

• secreted proteases?• contact-dependent killing of neutrophils,

leukocytes, etc.

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

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

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

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

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

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

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

-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

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

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!