Amebiasis
Amebiasis
Editor: Christina Naula
Section Contents
Background & Epidemiology
Amebiasis is a parasitic infection caused by the intestinal protozoan Entamoeba histolítica. Among all Entamoeba species known to inhabit humans (E. histolytica, E. coli, E. dispar, E. bangladeshi, E. moshkovskii, E. polecki, E. hartmanni & E. gingivalis), all can be found in the intestinal lumen; except E. gingivalis that is found in the oral cavity. None - other than E. histolytica - are clearly pathogenic.
Transmission: amebiasis is transmitted by ingesting food and water that is fecally contaminated with E. histolytica cysts from a human carrier. The human is the only known host for the infection.
E. histolytica has worldwide distribution, and it is more common among those who:
- Live in tropical and subtropical regions.
- Have low socioeconomic status & poor hygiene.
- Travel (especially long-term) to endemic areas.
- Have increased oral-anal contact [e.g., men who have sex with men (MSM)].
Life Cycle & Pathogenesis
Figure 1. Life cycle of Entamoeba histolítica
Clinical Presentation
Incubation period is generally ~2-4 weeks, but symptoms can occur months to years following infection. Clinical spectrum of amebiasis range from asymptomatic colonization to intestinal and extraintestinal disease. These are listed below:
Answer
as you may have realized, amebic and pyogenic liver abscesses look awfully similar. However, ALA is more common among high-risk groups (e.g., immigrants, MSM), symptoms tend to be more insidious and (as we will learn soon), it commonly comes with a POSITIVE ENTAMOEBA SEROLOGY!!!
Answer
ALA can also be confused sometimes with liver hydatid cysts. However, unlike echinococcosis, ALA presents with fever, leukocytosis, and lesions appear like abscesses (not cysts) with perilesional edema (Echinococcus has no perilesional edema).
Diagnosis
As with all diseases, diagnosis starts with a clinical suspicion in the right epidemiological context. Ask yourself:
- What is the presentation?
- Who is the host?
Definitive diagnosis can be supported through different modalities based on the disease stage:
- Asymptomatic carriers and Intestinal disease: diagnosis is mostly based on stool testing.
- Stool smear microscopy: examination of FRESH and runny stool (produced <30 minutes) is preferred to preserve the motility and display of trophozoites in the slide. Sensitivity is low ~50%. Concentration techniques (e.g. Faust method) and stains (e.g., trichrome) are useful but not widely available.
- Presence of cysts alone is NOT diagnostic as they are morphologically identical to E. dispar and E. mashkovskii (non-pathogenic).
- Presence of trophozoites is considered diagnostic.
- For more information on the morphology of these parasitic forms, can visit the amazing Parasites Wonders blog.
- Stool antigen & PCR: unlike smear microscopy (sensitivity ~50%), stool antigen and PCR are highly sensitive (>90%), but not widely available.
- Serology: enzyme immunoassay (EIA) is variably positive (~70%) in intestinal disease.
- Endoscopy: findings include: i) mucosal inflammation +/- ulceration; ii) tumor-like mass (“ameboma”); iii) strictures. Histopathology can reveal flask-shaped ulcers with trophozoites.
- Stool smear microscopy: examination of FRESH and runny stool (produced <30 minutes) is preferred to preserve the motility and display of trophozoites in the slide. Sensitivity is low ~50%. Concentration techniques (e.g. Faust method) and stains (e.g., trichrome) are useful but not widely available.
- Extra-intestinal disease: diagnosis is mostly based on serology and imaging.
- Serology: EIA is the most useful in amebic liver abscess (sensitivity ~80-95%). Can be falsely negative in the first 1-2 weeks of symptoms, so consider repeating if negative!
- Imaging: i) Ultrasound: ALA appears as round hypo-or anechoic lesions; ii) CT/MRI: ALA shows an ill-defined lesion with enhancing wall and perizonal edema.
- Aspiration & drainage: not performed routinely in ALA (only when diagnosis is uncertain or when abscess is at imminent risk of rupture). Aspirated material has a brown-grey (chocolate color), resembling anchovy sauce.
- Stool testing (smear, antigen, PCR): commonly negative in extra-intestinal disease!
Figure 2. Clinical spectrum of amebiasis
Figure 3. Cyst of E. histolytica
Figure 4. Trophozoite of E. histolytica with intra-cellular erythrocytes
Treatment & Management
The most important concept to remember in amebiasis treatment is that antiparasitics (e.g., metronidazole) are effective against the trophozoites, but NOT against the cysts. To achieve radical cure, a luminal agent (e.g., paromomycin) which is a medication used to treat only parasitic forms inside the intestinal lumen, MUST be given after treatment to kill the cysts. Here is a quick review for you:
| Treatment strategies | |
|---|---|
| Asymptomatic colonization ("chronic carriers") | Luminal agent* |
| Intestinal amebiasis | Antiparasitics** Followed by Luminal agent* |
| Extraintestinal amebiasis | Antiparasitics** with/without aspiration or drainage Followed by Luminal agent* |
| * Luminal agents (options): Paromomycin or lodoquinol or Diloxanide ** Antiparasitics (options): Metronidazole or Tinidazole or Nitazoxanide (Alternative) |
|
Table 1. Summary of treatment strategies in amebiasis
Other Intestinal Protozoa
It is not uncommon for other protozoa to appear in the stool microscopy. We will not be making a big emphasis on this, but we recommend you get famliar with the names because you will see this in clinical practice.
| Pathogenicity | Genus/Species | Clinical Implication | Management |
|---|---|---|---|
| Not pathogenic or probably not pathogenic | Entamoeba dispar | Colonizers | No treatment needed |
| Entamoeba moshkovskii | |||
| Entamoeba bangladeshi | |||
| Entamoeba hartmanni | |||
| Entamoeba coli | |||
| Endolimax nana | |||
| Iodamoeba butschlii | |||
| Pathogenicity is still debated | Blastocystis hominis | Asymptomatic, but has been implicated in enteritis (controversial) | No treatment needed except in rare instances |
| Dientamoeba fragilis | Asymptomatic, but has been implicated in acute/chronic enteritis. Associated with peripheral eosinophilia | Treat with metronidazole or paromomycin | |
| Likely pathogenic | Balantidium coli | Pigs are reservoirs. Asymptomatic, but can cause watery diarrhea or dysentery in severe cases | Treat with tetracycline or metronidazole |
Table 2. Summary for intestinal protozoa of minor medical importance
Prevention
- Avoid fecal contamination of the environment (e.g., proper disposal of human waste).
- Prevent the ingestion of cysts (e.g., hand hygiene, boil water before drinking, thoroughly wash vegetables).
- Counsel patients on the increased risk of enteric infections (inc. amebiasis) with oral-anal practices.
Assessment: Did I Get It? (DIG IT)
DIG ITs are online modules designed to reinforce key learning points for you! This assessment includes all topics surrounding adult and larval cestodes. Please choose the best answer, then check all of the answer choices for more learning pearls
Other Media Resources (Optional)
References
This lesson was last updated May 26 2025