- Clinical science
Amebiasis
Summary
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.
-
Fecal-oral
References:[1][3]
Pathophysiology
-
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.
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
- Chest pain, pleuralgia
- Diarrhea precedes only a third of all cases of amebic liver abscesses.
- 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
- Trophozoites often contain ingested erythrocytes
- The following tests confirm the microscopic findings (important since E. histolytica and Entamoeba dispar are morphologically identical ):
- 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
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
- 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
- Secondary infection → pyogenic abscess
- Abscess rupture → peritonitis
- Dissemination, possibly resulting in a brain abscess
- Direct extension to the pericardium or pleura
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]