234.1 🎓 醫學生版

234.1.0.1 📌 一頁重點

234.1.0.1.1 Pathogen

• Entamoeba histolytica
• Pathogenic only species — must distinguish from morphologically identical:
  • E. dispar (non-pathogenic, more common globally)
  • E. moshkovskyi (non-pathogenic)
  • E. coli (not related to bacterium; commensal)

234.1.0.1.2 Microbiology

• Two forms:
  • Cyst: 4-nucleated, environmental form, infectious
  • Trophozoite: motile, ingested RBCs (pathognomonic), invasive form

234.1.0.1.3 Transmission

• Fecal-oral
• Contaminated food + water (cysts environmentally stable)
• Person-to-person
• MSM (anal-oral)
• Common in poor sanitation: developing countries
• Cysts NOT killed by chlorine at normal water treatment levels

234.1.0.1.4 Epidemiology

• Worldwide; ~ 50 million symptomatic cases + 40,000-100,000 deaths/yr
• High prevalence: tropical, low-income, poor sanitation
• Developed countries: travelers, immigrants, MSM, immunocompromise
• Asia, Africa, Latin America endemic
• Mexico City + India + Bangladesh high

234.1.0.1.5 Clinical

234.1.0.1.5.1 Asymptomatic (90%)

• Cyst carriers
• Self-limited or persistent
• Reservoir for transmission

234.1.0.1.5.2 Symptomatic Intestinal Amebiasis

• Acute Amebic Colitis (Dysentery):
  • Gradual onset over 1-3 weeks
  • Bloody diarrhea + mucus + abdominal pain + cramping
  • Fever in 1/3
  • Tenesmus
  • Weight loss
• Fulminant Amebic Colitis (5%):
  • Severe systemic toxicity
  • Bloody diarrhea + peritonitis + perforation
  • Megacolon
  • High mortality
• Ameboma: chronic granulomatous mass mimicking colon CA

234.1.0.1.5.3 Extra-Intestinal Amebiasis

• Amebic Liver Abscess (#1 extra-intestinal manifestation):
  • Right upper quadrant pain + fever + hepatomegaly
  • Often without concurrent diarrhea
  • 95% solitary (right lobe predominant)
  • “Anchovy paste” abscess fluid (no PMN, just necrotic debris + trophozoites at periphery)
  • Severe complications: rupture into pleural / peritoneal / pericardial space
• Pulmonary Amebiasis: rupture of liver abscess; cough + chest pain + hemoptysis
• Cerebral Amebiasis (rare): severe + often fatal
• Cutaneous Amebiasis: perianal, vulvar, abdominal wall

234.1.0.1.6 Diagnosis

234.1.0.1.6.1 Stool

• Stool antigen test (E. histolytica-specific) — preferred; high sens + spec; distinguishes from E. dispar/moshkovskyi
• Stool PCR — multiplex panels distinguish
• Stool O+P × 3 — visualize cysts + trophozoites (especially with ingested RBCs = E. histolytica pathognomonic)
• WBC + RBC in stool

234.1.0.1.6.2 Liver Abscess

• US / CT / MRI abdomen — characteristic hypoechoic / hypodense mass in liver
• Serology (anti-Entamoeba histolytica antibody) — positive in invasive disease (vs other parasites + bacteria)
• Stool antigen + PCR — can be negative in pure liver abscess (no intestinal stage)
• Aspiration — diagnostic + therapeutic; “anchovy paste”; trophozoites at periphery

234.1.0.1.7 Treatment

234.1.0.1.7.1 Intestinal Disease

• Metronidazole 750 mg PO tid × 7-10 d OR Tinidazole 2 g PO daily × 3-5 d (tissue invasion)
• PLUS:
• Paromomycin 25-35 mg/kg/d PO divided × 7-10 d (lumen — kills cysts)
• OR Iodoquinol 650 mg PO tid × 20 d (lumen)
• OR Diloxanide furoate (limited availability)

234.1.0.1.7.2 Why Two Drugs?

• Metronidazole kills trophozoites in tissue but not cysts in lumen
• Paromomycin / iodoquinol kill lumenal cysts
• Both needed to prevent relapse + transmission

234.1.0.1.7.3 Liver Abscess

• Metronidazole 750 mg PO tid × 7-10 d OR Tinidazole 2 g PO daily × 5 d
• PLUS Paromomycin (lumenal clearance)
• Drainage if:
  • Large (> 5-10 cm)
  • Imminent rupture
  • Failure to improve with antibiotic
  • Refractory cases
• US-guided percutaneous drainage
• Surgical for rupture

234.1.0.1.7.4 Asymptomatic Carrier

• Paromomycin or iodoquinol alone (lumenal clearance)
• Important for transmission prevention

234.1.0.1.8 Prevention

• Sanitation + safe food + water (especially when traveling endemic)
• Boiling water (chlorine not adequate for cysts)
• Hand hygiene
• MSM safer sex practices

234.1.0.1.9 Differential

• E. dispar + E. moshkovskyi — non-pathogenic
• E. histolytica antigen test + PCR distinguish
• Other amebae (Entamoeba coli, Iodamoeba) — commensal, don’t treat

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234.1.0.2 1️⃣ Microbiology + Life Cycle

234.1.0.2.1 Forms

234.1.0.2.1.1 Cyst

• 10-15 µm, spherical, 4 nuclei when mature
• Resistant to environment + chlorine
• Infectious form
• Released in feces

234.1.0.2.1.2 Trophozoite

• Motile, 15-50 µm
• Ingested RBCs (erythrophagocytosis) in cytoplasm — pathognomonic for E. histolytica vs non-pathogenic species
• Pseudopodia
• Pathogenic form (invasion)

234.1.0.2.2 Life Cycle

1. Cyst ingestion (contaminated food/water)
2. Stomach acid → resist cyst wall
3. Excystation in small intestine → trophozoites
4. Trophozoites colonize cecum + colon
5. Most: commensal (asymptomatic)
6. Some: invade colonic mucosa → ulcers + bloody diarrhea
7. Some trophozoites encyst → cysts shed in feces (continue transmission)
8. Invasive disease: trophozoites breach mucosa → portal vein → liver (abscess)
9. Sometimes: distant spread (lung, brain, skin)

234.1.0.2.3 Pathogenesis

• Adherence to mucosa via Gal/GalNAc lectin
• Cytolysis via amebapores + cysteine proteinases
• Phagocytosis of host cells (erythrophagocytosis)
• Tissue destruction → flask-shaped ulcers (narrow neck, broad base)

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234.1.0.3 2️⃣ Clinical — Intestinal Disease

234.1.0.3.1 Asymptomatic Carriage

• 90% of infections
• Cyst shedding without symptoms
• Reservoir for transmission

234.1.0.3.2 Acute Amebic Colitis

• 1-3 weeks gradual onset
• Bloody diarrhea (dysentery — frequent small-volume stools)
• Mucus in stool
• Tenesmus
• Abdominal pain + cramping
• Fever in 1/3
• Weight loss
• Can mimic ulcerative colitis
• Lasts weeks if untreated

234.1.0.3.3 Fulminant Amebic Colitis

• 5% of acute colitis
• Severe systemic toxicity
• Severe bloody diarrhea + abdominal pain
• Peritonitis (perforation)
• Megacolon
• High mortality
• Often immunocompromise / pregnancy / young / severe

234.1.0.3.4 Ameboma

• Chronic granulomatous mass
• Cecal / sigmoid commonly
• Mimics colon CA
• Diagnosis: biopsy + amebic serology + response to therapy

234.1.0.3.5 Endoscopic Findings

• Discrete flask-shaped ulcers (narrow neck, broad base)
• Normal intervening mucosa
• “Volcano” appearance
• Biopsy: trophozoites at edge of ulcer

234.1.0.3.6 Differential Intestinal

• IBD (UC, Crohn’s)
• Bacterial dysentery (Shigella, EHEC, Salmonella)
• C. difficile colitis
• Ischemic colitis
• Other parasitic (Schistosoma)

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234.1.0.4 3️⃣ Clinical — Extra-Intestinal Amebiasis

234.1.0.4.1 Amebic Liver Abscess

234.1.0.4.1.1 Background

• Most common extra-intestinal manifestation
• 10× more common in men (mechanism unclear)
• Often without preceding diarrhea (intestinal stage may resolve)
• Months-years after primary infection sometimes

234.1.0.4.1.2 Clinical

• Triad: right upper quadrant pain + fever + tender hepatomegaly
• Anorexia, weight loss
• Cough (right lung irritation)
• Jaundice rare
• Pleuritic chest pain (RLL atelectasis)
• Splenomegaly rare (vs other liver pathology)

234.1.0.4.1.3 Lab

• ↑ Alkaline phosphatase
• Modest ↑ AST/ALT
• Bilirubin usually normal
• Leukocytosis
• Anemia mild

234.1.0.4.1.4 Imaging

• US (initial): hypoechoic mass in right lobe predominant (75%); 95% solitary
• CT / MRI: hypodense, round/oval lesion with peripheral edema
• Differential: pyogenic abscess (often multiple), hepatocellular CA, hemangioma, hydatid cyst, hepatic metastasis

234.1.0.4.1.5 Aspirate

• “Anchovy paste” — brown / red sticky fluid (necrotic liver + blood)
• NO PMN typically (vs pyogenic = thick + PMN-rich)
• Trophozoites at periphery only (center is necrosis)
• Reserved for diagnostic + therapeutic in selective cases

234.1.0.4.1.6 Complications

• Rupture: pleural / peritoneal / pericardial / bronchial
• Pleuropulmonary (most common rupture site): cough + dyspnea + hemoptysis
• Pericardial (severe): tamponade
• Cerebral (rare)

234.1.0.4.2 Pulmonary Amebiasis

• Direct extension from liver abscess (right lower lobe)
• Cough, chest pain, pleural effusion, empyema
• “Anchovy paste” sputum if rupture

234.1.0.4.3 Cerebral Amebiasis

• Rare, severe
• Mass lesion + meningeal involvement
• Fatal without aggressive Tx

234.1.0.4.4 Cutaneous Amebiasis

• Perianal, vulvar, abdominal wall
• Painful ulcerative lesions
• Trophozoites in tissue

234.1.0.4.5 Genitourinary

• Vulvar, penile lesions
• Cervical ulcerations
• Sexual transmission possible

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234.1.0.5 4️⃣ Diagnosis

234.1.0.5.1 Stool Studies

234.1.0.5.1.1 Stool Antigen (Preferred)

• E. histolytica-specific ELISA
• High sensitivity + specificity
• Distinguishes from E. dispar + E. moshkovskyi
• Available in most labs

234.1.0.5.1.2 Stool PCR

• BioFire FilmArray GI Panel includes Entamoeba histolytica
• Highly specific
• Differentiates from non-pathogenic

234.1.0.5.1.3 Stool O+P × 3

• Cysts + trophozoites visible
• Ingested RBCs in trophozoite cytoplasm = pathognomonic for E. histolytica
• Cannot reliably differentiate from morphologically identical species without further testing
• Multiple specimens × 3 days

234.1.0.5.2 Liver Abscess Diagnosis

234.1.0.5.2.1 Imaging

• US (initial), CT, MRI (more detailed)
• Characteristic hypodense / hypoechoic liver mass
• Solitary right lobe predominant

234.1.0.5.2.2 Serology

• Anti-Entamoeba histolytica antibody (ELISA, IFA) — sensitive in invasive disease
• Often positive 7-14 days into illness
• Negative in asymptomatic carriers
• May remain positive for years after infection (limits acute diagnosis in endemic areas — antibodies may be from previous infection)

234.1.0.5.2.3 Stool Testing (in Liver Abscess)

• May be negative (intestinal stage cleared)
• Or may show concurrent intestinal infection (treat both)

234.1.0.5.2.4 Aspirate (Selective)

• Diagnostic + therapeutic
• “Anchovy paste” appearance + trophozoites at periphery + no PMN
• Indications:
  • Large abscess (> 5-10 cm)
  • Imminent rupture
  • Failure to improve with antibiotic
  • Refractory
• US-guided percutaneous needle aspiration

234.1.0.5.3 Differential

• Pyogenic liver abscess (more often multiple, polymicrobial, leukocytosis higher, no serology)
• Echinococcal liver cyst (Ch 237)
• Hepatocellular carcinoma
• Hepatic metastases
• Cavernous hemangioma

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234.1.0.6 5️⃣ Treatment

234.1.0.6.1 Intestinal Disease — Two-Drug Approach

234.1.0.6.1.1 Step 1: Tissue Amebicide

• Metronidazole 750 mg PO tid × 7-10 d (most common)
• Tinidazole 2 g PO daily × 3-5 d (alternative; better tolerated)
• Kills trophozoites in tissue + lumen
• Doesn’t eliminate cysts effectively — need second drug

234.1.0.6.1.2 Step 2: Luminal Amebicide

• Paromomycin 25-35 mg/kg/d PO divided × 7-10 d (first-line)
• Iodoquinol 650 mg PO tid × 20 d (alternative)
• Diloxanide furoate (limited availability)
• Kills lumenal cysts → prevents relapse + transmission

234.1.0.6.2 Liver Abscess

234.1.0.6.2.1 Antibiotic

• Metronidazole 750 mg PO/IV tid × 7-10 d
• Tinidazole 2 g daily × 5 d alternative
• PLUS paromomycin (lumenal clearance, prevent relapse)
• Clinical improvement typically 3-5 days

234.1.0.6.2.2 Drainage

• Indications:
  • Large abscess > 5-10 cm
  • Imminent rupture
  • Failure to improve with antibiotic in 5-7 days
  • Pleural / pericardial extension
  • Diagnosis uncertainty
• US-guided percutaneous aspiration / catheter drainage
• Surgical drainage for rupture or refractory

234.1.0.6.2.3 Adjunctive

• Hospitalization for severe / abscess
• Supportive care
• Pain control

234.1.0.6.3 Asymptomatic Carrier

• Paromomycin alone (eliminates cysts, prevents transmission)
• 25-35 mg/kg/d × 7-10 d
• Or iodoquinol 650 mg tid × 20 d
• Treat all confirmed carriers (transmission risk)

234.1.0.6.4 Fulminant Colitis

• ICU + hospitalization
• IV metronidazole
• • Paromomycin (oral once tolerated)
• Surgery for perforation, megacolon, peritonitis
• Empirical broad-spectrum antibiotics for bacterial co-infection
• High mortality

234.1.0.6.5 Pregnancy

• Metronidazole avoided 1st trimester (relative)
• Paromomycin safe pregnancy (non-absorbed) — preferred lumenal agent
• Severe symptomatic 1st trimester: metronidazole if benefit > risk
• 2nd-3rd trimester: standard regimens generally safe

234.1.0.6.6 Pediatric

• Same regimens weight-based

234.1.0.6.7 Test of Cure

• Stool antigen / PCR 1 month post-treatment
• Repeat if still positive (treat carrier)

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234.1.0.7 6️⃣ Prevention

234.1.0.7.1 Personal

• Boil water in endemic areas (chlorine insufficient for cysts)
• Wash + cook produce thoroughly
• Hand hygiene
• Avoid raw vegetables in endemic settings if possible

234.1.0.7.2 Public Health

• Sanitation + clean water + sewage treatment
• Outbreak investigation
• Travel medicine awareness

234.1.0.7.3 MSM

• Safer sex practices
• Education

234.1.0.7.4 Immunocompromise

• Avoid endemic travel if possible
• Increased severity

234.1.0.7.5 No Vaccine

• Various candidates in research (Gal/GalNAc lectin-based) — not near clinical

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