Amebiasis

Basics

Description

  • Invasive parasitic infection with both intestinal and extraintestinal manifestations
  • Endemic worldwide, especially resource-limited 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
    • HIV-infected individuals
  • Risk factors for increased severity of disease and complications:
    • Immunocompromised: Corticosteroid use, HIV infection, malnutrition, malignancy, alcoholism
    • Pregnancy/postpartum state
    • Extremes of age

Etiology

  • Entamoeba histolytica, an anaerobic, nonflagellated protozoan parasite causes most symptomatic infections
    • Entamoeba moshkovskii is an emerging species with increasing evidence of pathogenicity
    • Entamoeba dispar considered nonpathogenic
    • Entamoeba Bangladeshi pathogenicity is under investigation
  • Fecal–oral transmission:
    • Humans are sole reservoir
  • Ingested organisms cause invasive colitis
  • Extraintestinal spread is hematogenous

Diagnosis

Signs And Symptoms

  • Majority of infections are asymptomatic
  • 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)
      • 10× more common in men
      • 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
      • Complication: hepatic vein and IVC thrombosis
    • Extrahepatic amebic disease
      • Brain
      • Lung
      • Heart
      • Perinephric Splenic
      • Vaginal/cervical/uterine
    • Cutaneous amebiasis:
      • Rare. Presents on perineum and genitalia
      • Painful, irregularly shaped ulcers
      • Purulent exudate
      • May cause rectovaginal fistulae

Pediatric Considerations

Fulminant colitis is more likely

Pregnancy Considerations

Fulminant colitis is more likely

History

  • Acute to subacute diarrhea with risk factors
  • Possible sources of exposure
  • High-risk group
  • Travel to endemic area for >1 mo

Physical Exam

  • 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

Lab

  • 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. histolytica–specific 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

Imaging

  • 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

Diagnostic Procedures/Surgery

  • 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, or if cyst >10 cm, at imminent risk of rupturing, or left lobe abscess with risk of rupture into pericardium
    • Not for primary treatment of liver abscesses

Differential Diagnosis

  • Intestinal amebiasis:
    • Enteroinvasive bacterial infection (Staphylococcus, E. coli, Shigella, Salmonella, Yersinia, Campylobacter, Clostridium difficile, some vibrio species)
    • 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)

Treatment

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 (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 d
    • 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 d
    • If symptoms do not improve after 5–7 d 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

Pregnancy Considerations

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

Medication

First Line

  • 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

Second Line

  • 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

Pediatric Considerations

  • Iodoquinol may cause more serious adverse effects when used in children at high doses for prolonged periods

Pregnancy Considerations

  • Use metronidazole with caution in first trimester
  • Nitazoxanide preferred

Follow-Up

Disposition

Admission Criteria

  • 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

Discharge Criteria

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

Issues For Referral

Consult surgery if evidence of peritonitis, toxic megacolon, bowel necrosis, colonic perforation, or liver abscess

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

Pearls And Pitfalls

  • Avoid antidiarrheal medications
  • Always give double therapy with both a systemic amebicidal (metronidazole, tinidazole, or chloroquine) PLUS an intestinal amebicidal (iodoquinol, nitazoxanide, paromomycin, or tetracycline) unless contraindicated
  • Always be vigilant for high-mortality complications such as fulminant colitis or extraintestinal disease

Additional Readings

  1. American Academy of Pediatrics. Red Book 2024–2027. Report of the Committee on Infectious Diseases. 33rd ed. American Academy of Pediatrics; 2024.
  2. Carrero JC, Reyes-López M, Serrano-Luna J, et al. Intestinal amoebiasis: 160 years of its first detection and still remains as a health problem in developing countries. Int J Med Microbiol. 2020;310(1):151358.  [PMID:31587966]
  3. Kantor M, Abrantes A, Estevez A, et al. Entamoeba histolytica: updates in clinical manifestation, pathogenesis, and vaccine development. Can J Gastroenterol Hepatol. 2018;2018(1):4601420.  [PMID:30631758]
  4. Shirley Debbie-Ann T, et al. A review of the global burden, new diagnostics, and current therapeutics for amebiasis. Open forum infectious diseases. Vol. 5. No. 7. Oxford University Press, 2018.

See Also (Topic, Algorithm, Electronic Media Element)

Authors

Victoria Gonzalez