• Clinical science

Amebiasis

Abstract

Amebiasis is an infectious disease caused by the anaerobic protozoan Entamoeba histolytica. Transmission usually occurs via the fecal-oral route (e.g., via contaminated drinking water) when traveling in an endemic region. Depending on its manifestation, amebiasis is termed either intestinal or extraintestinal. After an incubation period of one to four weeks, symptoms such as loose stools with mucus and fresh blood in combination with painful defecation develop. In extraintestinal amebiasis, amebic abscesses (mostly a single liver abscess in the right lobe of the liver) may form, resulting in pain as well as a feeling of pressure in the right upper quadrant (RUQ). Important diagnostic steps include stool analysis and liver ultrasound to assess extraintestinal amebiasis. Treatment consists of paromomycin to destroy intestinal amebae and metronidazole for invasive disease. Image-guided needle aspiration may be indicated in cases involving complicated liver abscesses.

Epidemiology

  • Occurence: E. histolytica is very common in tropical and subtropical regions (e.g., Mexico, Southeast Asia, India) and affects more than 50 million people worldwide. Amebic infection is relatively rare in the US.
  • Men and especially immunocompromised individuals have a higher risk of developing liver abscesses.

References:[1][2]

Epidemiological data refers to the US, unless otherwise specified.

Etiology

  • Pathogen: Entamoeba histolytica, a protozoan
  • Transmission
    • Fecal-oral
      • Amebic cysts are excreted in stool → contaminate drinking water or food
      • Transmission may also occur through sexual contact.
    • Infection typically occurs following travel to endemic regions such as the tropics and subtropics.

References:[1][3]

Pathophysiology

  • Life cycle: ingestion of mature cysts → excystation in the small intestine → cysts divide into 4 and then 8 → noninvasive colonization of the colon by trophozoites (may lead to intestinal and extraintestinal disease) → trophozoites encyst → the cysts are excreted (along with trophozoites) → cysts are reingested by the same patient or spread to another individual.
  • Stages
    • Cyst stage: Cysts are very resilient (even against gastric acid) and are able to survive outside the host for months.
    • Vegetative stage: trophozoite formation
      • Trophozoites can produce proteolytic enzymes that allow them to invade the intestinal submucosa. They can then enter the bloodstream where they consume erythrocytes and disseminate to target tissues like the liver via the portal system.
      • Trophozoites formed during gastrointestinal passage are classified into minuta and magna forms
        • Minuta form: remain in the gastrointestinal tract and are considered harmless
        • Magna form: minuta form amebae can transform into the pathogenic magna form (for reasons still unknown), which causes extraintestinal disease.

References:[4][1]

Clinical features

Incubation period

  • Intestinal amebiasis: 1–4 weeks
  • Extraintestinal amebiasis: a few weeks to several years

Clinical courses

  • Intestinal amebiasis (dysentery)
    • Loose stools with mucus and bright red blood
    • Painful defecation, tenesmus, abdominal pain, cramps, weight loss, and anorexia
    • Fever in 10–30% of cases and possible systemic symptoms (e.g., fatigue)
    • High risk of recurrence, e.g., through self-inoculation (hand to mouth)
    • A chronic form is also possible, which is clinically similar to inflammatory bowel disease.
  • Extraintestinal amebiasis
    • Mostly acute onset of symptoms; subacute courses are rare
    • In 95% of cases: amebic liver abscess, usually a solitary abscess in the right lobe
      • Fever in 85-90% of cases (compared to amebic dysentery)
      • RUQ pain or pressure sensation, possibly aggravated by inspiration
      • Chest pain, pleuralgia
      • Diarrhea precedes only a third of all cases of amebic liver abscesses. However, a recent study found that the colon, particularly the right side, was involved in ⅔ of cases
    • In 5% of cases: abscesses in other organs (e.g., especially the lungs; in rare cases, the brain), with accompanying organ-specific symptoms

Diarrhea precedes only a third of all cases of amebic liver abscess!

Always consider amebiasis when a patient presents with persistent diarrhea after traveling to a tropical or subtropical destination!References:[5][1][6]

Diagnostics

  • Travel history
  • Intestinal amebiasis
    • Stool analysis
      • Microscopic identification of cysts or trophozoites in fresh stool
      • The following tests confirm the microscopic findings:
        • Isoenzymatic or immunologic analysis: e.g., EIA or copro-antigen ELISA (antigens found in feces)
        • Molecular methods: e.g., PCR
      • Stool microscopy is not sensitive; , especially in later phases, so at least three stool samples should be examined before reporting a negative result.
  • Extraintestinal amebiasis

References:[3][1]

Treatment

Medical therapy

  • Asymptomatic intestinal amebiasis
    • No treatment in endemic areas
    • In nonendemic areas: a luminal agent such as paromomycin or diloxanide to eradicate the infection
  • Symptomatic intestinal amebiasis and invasive extraintestinal amebiasis
    • Initial treatment with a nitroimidazole derivative such as metronidazole or tinidazole to eradicate invasive trophozoites
    • Followed by a luminal agent (e.g., paromomycin or diloxanide) to eradicate intestinal cysts and prevent relapse

Invasive procedures

  • Aspiration: ultrasound or CT-guided puncture of complicated abscesses at risk for perforation
    • Indications:
      • Localized in the left lobe
      • Pyogenic abscess
      • Multiple abscesses
      • Failure to respond to pharmacotherapy
  • Surgical drainage: should generally be avoided, but may be indicated for inaccessible abscesses or ruptured abscesses in combination with peritonitis

To ensure successful treatment, the patient's stool must be analyzed regularly!References:[7][1]

Complications

Intestinal amebiasis

  • Fulminant or necrotizing colitis
  • Toxic megacolon → colon rupture
  • Ameboma
  • Fistula formation (e.g., rectovaginal)

Extraintestinal amebiasis

References:[1][8]

We list the most important complications. The selection is not exhaustive.

Prevention

Food and water hygiene

  • Unpeeled fruits or vegetables should not be consumed if there is a potential risk of contamination by Entamoeba histolytica cysts (e.g., endemic region with low hygiene standards).
  • Even chlorinated water can contain high concentrations of amebae; therefore, water should be boiled before use.

"Boil it, cook it, peel it, or forget it."References:[1][5]