Description

• Invasive parasitic infection with both intestinal and extraintestinal manifestations
• Endemic worldwide, especially developing areas with poor sanitation
• Populations at risk:
  • Migrants and citizens from endemic regions (India, Africa, Mexico, tropical Central/South America)
  • Travelers with >1 mo stay in endemic areas
  • Institutionalized persons
  • Practitioners of oral–anal sexual activity
  • Men who have sex with men (MSM)
  • HIV-infected individuals
• Risk factors for increased severity of disease and complications:
  • Immunocompromised: Corticosteroid use, HIV infection, malnutrition, malignancy
  • Pregnancy/postpartum state
  • Extremes of age

Etiology

• Entamoeba histolytica, an anaerobic, nonflagellated protozoa causes most symptomatic infections
  • Entamoeba moshkovskii is an emerging species with increasing evidence of pathogenicity
  • Entamoeba dispar is 10× more common than E. histolytica but nonpathogenic. HIV+ patients often colonized, but no increased risk intestinal/extraintestinal disease
• Fecal–oral transmission:
  • Humans are sole reservoir
• Ingested organisms cause invasive colitis
• Extraintestinal spread is hematogenous

Signs and Symptoms

• Intestinal disease:
  • Onset 1 wk–1 mo postexposure, often gradual
  • Acute diarrhea (nondysenteric colitis):
    • 80% of cases
    • Afebrile
    • Occult blood in stool
    • Benign abdominal exam
  • Classic dysentery:
    • Bloody mucoid diarrhea
    • Abdominal pain/benign abdominal exam
    • Tenesmus
    • Weight loss (50%)
    • Fever (38%)
  • Fulminant colitis:
    • Rare. Only 0.5% of cases
    • Toxic-appearing patient
    • Rigid abdomen (25%)
    • Fever
    • Severe bloody diarrhea
    • Can progress to bowel necrosis/perforation
    • >40% mortality if perforated
  • Toxic megacolon:
    • Toxic-appearing patient
    • Profuse diarrhea (>10 stools per day)
    • Fever
    • Distended, tympanitic abdomen with signs of peritonitis
    • Associated with corticosteroid use
    • High mortality
  • Ameboma:
    • Intraluminal granulation. Mimics colon CA
    • Tender palpable mass on exam
  • Amebic strictures:
    • Secondary to chronic inflammation/scarring
    • Crampy abdominal pain
    • Nausea and vomiting (may be feculent)
    • May cause partial or complete bowel obstruction
  • Chronic amebic colitis:
    • Mild recurrent episodes of diarrhea, abdominal cramping, and tenesmus
    • Weight loss occurs during episodes
    • May persist for years. Mimics IBD
• Extraintestinal disease:
  • Amebic liver abscess:
    • Most frequent extraintestinal manifestation (3–9% of cases)
    • Single abscess in right lobe (50–80%)
    • May develop months to years postexposure (median of 3 mo)
    • Fever
    • Right upper quadrant pain
    • Hepatomegaly with point tenderness
    • Rales at right lung base
    • Concurrent diarrhea unusual (20–33%)
    • Complication: Rupture into pleural cavity (10–20%), peritoneum, or pericardium (rare)
    • Increased risk of rupture if >5 cm in diameter or left lobe location
  • Extrahepatic amebic abscess:
    • Brain
    • Lung
    • Perinephric
    • Splenic
    • Vaginal/cervical/uterine
  • Cutaneous amebiasis:
    • Rare. Presents on perineum and genitalia
    • Painful, irregularly shaped ulcers
    • Purulent exudate
    • May cause rectovaginal fistulae

• Possible sources of exposure
• Membership in high-risk group
• Travel to endemic area for >1 mo

• Identify evidence of peritonitis, sepsis, or shock
• Tender abdominal mass mandates workup for liver abscess or ameboma
• Digital rectal exam shows gross or occult blood in >70% of patients

Diagnostic Tests and Interpretation

• CBC:
  • Leukocytosis common in amebic liver abscess and peritonitis
• Alkaline phosphatase and ALT:
  • Elevated in amebic liver abscess
• Serum electrolytes, BUN/creatinine if prolonged diarrhea or evidence of dehydration
• Stool PCR is diagnostic gold standard:
  • 100% sensitive and specific
• Stool ELISA for E. histolyticaspecific antigen:
  • 74–95% sensitive, 93–100% specific
• Serum for anti–E. histolytica antibodies:
  • Essential if suspecting liver abscess. These patients rarely shed parasites in stool
  • 90–100% sensitive in amebic liver abscess
  • 70–90% sensitive in amebic colitis
• Stool microscopy is <60% sensitive and no longer the test of choice
• Fecal leukocytes and culture:
  • Rule out infection of enteroinvasive bacteria
  • Negative in amebiasis

• Abdominal US:
  • 58–90% sensitive for liver abscess
  • Sensitivity influenced by size and location
  • Allows rapid evaluation of abscess for increased risk of rupture (>5 cm or located in left lobe)
• Abdominal CT or MRI:
  • Equivalent to US for delineating liver abscesses
  • Superior to US for detecting abscesses in other organs
• Head CT or MRI:
  • Suspect amebic brain abscess if patient with known amebiasis has altered mental status or focal neurologic findings
  • Irregular nonenhancing lesions
• CXR:
  • Elevated right hemidiaphragm and/or right pleural effusion in liver abscess

• Colonoscopy with biopsy provides definitive diagnosis of amebic dysentery, colitis, ameboma, and amebic stricture
• Percutaneous fine-needle aspiration of liver abscess to exclude bacterial abscess if nondiagnostic serology or antiamebic therapy fails
  • Not for primary treatment of liver abscesses

Differential Diagnosis

• Intestinal amebiasis:
  • Enteroinvasive bacterial infection (Staphylococcus, E. coli, Shigella, Salmonella, Yersinia, Campylobacter)
  • Inflammatory bowel disease
  • Ischemic colitis
  • Arteriovenous malformation
  • Abdominal aortic aneurysm
  • Perforated duodenal ulcer
  • Intussusception, diverticulitis
  • Pancreatitis
  • Colorectal carcinoma
  • Appendicitis
• Amebic abscess:
  • Bacterial abscess
  • Tuberculous cavity
  • Echinococcal cyst
  • Malignancy
  • Cholecystitis
• Cutaneous amebiasis:
  • Carcinoma
  • STDs (condyloma acuminata, chancroid, syphilis)

Initial Stabilization/Therapy

• Airway, breathing, circulation (ABCs)
• IV 0.9% NS if signs of significant shock

Ed Treatment/Procedures

• Oral fluids if mild; IV if moderate/severe dehydration
• Avoid antidiarrheal agents
• Correct serum electrolyte imbalances
• Stool sample for E. histolytica PCR or ELISA, plus serology for anti–E. histolytica antibodies
• If stool or serum is positive for E. histolytica:
  • Metronidazole or tinidazole is first-line drug for systemic amebiasis (90% cure rate)
  • Chloroquine is an alternative systemic agent
  • Always follow systemic therapy with a luminal agent to eradicate intestinal colonization (erythromycin, iodoquinol, nitazoxanide, paromomycin, or tetracycline)
  • Do not use the luminal agents alone
• If stool or serum is negative for E. histolytica:
  • Refer to gastroenterologist for colonoscopy with biopsy
  • Repeat serology in 7 days
  • Consider empiric course of metronidazole if high suspicion for amebiasis and patient is critically ill
• If evidence of peritonitis or sepsis:
  • Add IV antibiotic directed against anaerobic and gram-negative bacteria
  • Surgical consult if toxic megacolon or perforation
• If liver abscess is suspected:
  • US or CT of hepatobiliary system with concurrent amebic serology
  • If imaging demonstrates an abscess but serology is negative, treat with amebicides and repeat serology in 7 days
  • If symptoms do not improve after 5–7 days of empiric amebicidal therapy, consider fine-needle aspiration to rule out bacterial abscess or hepatoma
  • Consider abscess drainage by surgeon or interventional radiologist in conjunction with amebicidal therapy

• Use metronidazole with caution in first-trimester pregnancy, but do not withhold if patient has fulminant colitis or amebic abscess
• Use erythromycin or nitazoxanide as intestinal amebicides along with metronidazole
• Erythromycin or nitazoxanide may be used alone for mild dysentery in first-trimester pregnancy
• Chloroquine, iodoquinol, paromomycin, tetracycline, and tinidazole are contraindicated

Medication

• Metronidazole: 500–750 mg (peds: 35–50 mg/kg/24 hr) PO/IV q8h for 7–10 d
• Tinidazole: 2 g daily (peds: 50 mg/kg/24 hr) PO for 3–5 d. For children older than 3 yr

• Chloroquine: 1 g PO daily for 2 d, then 500 mg PO daily for 14–21 d
• Erythromycin: 250–500 mg (peds: 30–50 mg/kg/24 hr) PO q6h for 10–14 d
• Iodoquinol: 650 mg (peds: 30–40 mg/kg/24 hr) PO q8h for 20 d
• Nitazoxanide: 500 mg PO q12h for 3 d (10 d if liver abscess) for adults and children >12 yr
• Paromomycin: 25–35 mg/kg/24 hr in 3 divided doses PO for 5–10 d
• Tetracycline: 250–500 mg (peds >8 yr: 25–50 mg/kg/24 hr) PO q6h for 10 d

• Tetracycline is avoided in children <8 yr given alternatives
• Iodoquinol may cause more serious adverse effects when used in children at high doses for prolonged periods

• Use metronidazole with caution in first trimester
• Erythromycin or nitazoxanide preferred

Disposition

• Shock, sepsis, or peritonitis
• Hypotension or tachycardia unresponsive to IV fluids
• Children with >10% dehydration
• Severe electrolyte imbalance
• Patients unable to maintain adequate oral hydration:
  • Extremes of age, cognitive impairment, significant comorbid illness
• Fulminant colitis or toxic megacolon
• Bowel obstruction
• Extraintestinal abscesses
• Failure of outpatient regimen

• Nontoxic presentation of acute or chronic dysentery
• Able to maintain adequate oral hydration and medication compliance
• Dehydration responsive to IV fluids

Follow-Up Recommendations

• Gastroenterology and infectious disease follow-up in 7 d for repeat serology and possible endoscopic evaluation
• Physical exam in 14 d to assess for treatment effectiveness and for development of complications or extraintestinal disease

See Also

• Diarrhea
• Gastroenteritis