- Clinical science
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.
Epidemiological data refers to the US, unless otherwise specified.
- Pathogen: Entamoeba histolytica, a protozoan
- 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.
- 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.
- 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.
- Intestinal amebiasis: 1–4 weeks
- Extraintestinal amebiasis: a few weeks to several years
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.
- 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
Always consider amebiasis when a patient presents with persistent diarrhea after traveling to a tropical or subtropical destination!References:
- Travel history
Microscopic identification of cysts or trophozoites in fresh stool
- Trophozoites often contain ingested erythrocytes
- The following tests confirm the microscopic findings:
- Stool microscopy is not sensitive; , especially in later phases, so at least three stool samples should be examined before reporting a negative result.
- Microscopic identification of cysts or trophozoites in fresh stool
- Stool analysis
- Extraintestinal amebiasis
- Asymptomatic intestinal amebiasis
- Symptomatic intestinal amebiasis and invasive extraintestinal amebiasis
Aspiration: ultrasound or CT-guided puncture of complicated abscesses at risk for perforation
- 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:
- Fulminant or necrotizing colitis
- Toxic megacolon → colon rupture
- Fistula formation (e.g., rectovaginal)
- Secondary infection → pyogenic abscess
- Abscess rupture → peritonitis
- Dissemination, possibly resulting in a brain abscess
- Direct extension to the pericardium or pleura
We list the most important complications. The selection is not exhaustive.
- 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: