Amebiasis

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.

• 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. ^[1]

Epidemiological data refers to the US, unless otherwise specified.

• Pathogen: : Entamoeba histolytica, a protozoan
• Transmission
  • Fecal-oral
    • Amebic cysts are excreted in stool and can 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. ^[2]

• 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. ^[2]
• 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.

Incubation period

• Intestinal amebiasis: : 1–4 weeks ^[3]
• Extraintestinal amebiasis: a few weeks to several years

Clinical courses

Intestinal amebiasis (Amebic 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.

Always consider amebiasis when a patient presents with persistent diarrhea after traveling to a tropical or subtropical destination!

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
  • Chest pain, pleuralgia
  • Diarrhea precedes only a third of all cases of amebic liver abscesses. ^[4]
• In 5% of cases: abscesses in other organs (e.g., especially the lungs; in rare cases, the brain), with accompanying organ-specific symptoms

Intestinal amebiasis ^[2]

• Stool analysis
  • Microscopic identification of cysts or trophozoites in fresh stool
    • Trophozoites often contain ingested erythrocytes
    • Cysts contain up to four nuclei
  • The following tests confirm the microscopic findings (important since E. histolytica and Entamoeba dispar are morphologically identical ):
    • EIA or coproantigen ELISA
    • 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.
• Colonoscopy with biopsy: flask-shaped ulcers

Extraintestinal amebiasis

• Serological antibody detection
• Aspiration of abscesses: shows brown fluid/pus (exudate resembles anchovy paste)
• In amebic hepatic abscess
  • Liver function tests: ALP, AST, ALT, bilirubin slightly elevated
  • Imaging: shows a solitary lesion, typically in the right lobe of the liver
    • Ultrasound: hypoechoic
    • CT/MRI ^[5]
      • Well-defined, rounded lesions with contrast-enhancing wall
      • Can be unilocular or septate

E. histolytica Engulfs Erythrocytes.

Medical therapy

• Asymptomatic intestinal amebiasis
  • No treatment in endemic areas
  • In nonendemic areas
    • Luminal agents such as paromomycin, diloxanide, or iodoquinol
    • Goal: To prevent the development of invasive disease and the shedding of cysts.
• 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, diloxanide, or iodoquinol) to eradicate intestinal cysts and prevent relapse

Invasive procedures

• Aspiration: ultrasound or CT-guided puncture of complicated liver abscesses at risk for perforation
  • Indications: ^[6]
    • 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!

Intestinal amebiasis

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

Extraintestinal amebiasis

• Secondary infection → pyogenic abscess ^[7]
• Abscess rupture → peritonitis
• Dissemination, possibly resulting in a brain abscess
• Direct extension to the pericardium or pleura

References:^[7][8]

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

Food and water hygiene ^[3]

• 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.

The main principles of amebiasis prevention concern consumption of potentially contaminated food and water and can be summarized as follows: "Boil it, cook it, peel it, or forget it."