What is Entamoeba histolytica?
Entamoeba histolytica is an extracellular enteric protozoan parasite that causes amebiasis, transmitted via the fecal-oral route through contaminated food and water, and represents the second leading cause of death from parasitic diseases worldwide. 1, 2
Organism and Transmission
- E. histolytica is a pathogenic protozoan parasite that primarily affects people in developing countries with limited hygiene conditions, where it is endemic 1
- Infective cysts are transmitted by the fecal-oral route, excysting in the terminal ileum and producing invasive trophozoites (amoebae) 1
- Sexual transmission occurs through oral-anal contact, particularly among men who have sex with men (MSM), and can occur even in non-endemic areas 3, 2
- The parasite mainly colonizes the large intestine, where it can remain asymptomatic or invade the mucosa to cause disease 1, 4
Clinical Symptoms and Presentations
Intestinal Amebiasis
Most E. histolytica infections are asymptomatic, but when symptomatic, patients present with acute diarrhea, dysentery, abdominal pain, and amebic colitis. 4
- Acute intestinal amebiasis manifests as bloody diarrhea (dysentery), abdominal pain, and fever 5, 4
- Chronic presentations include chronic diarrhea, abdominal pain, and weight loss 3
- Severe cases can progress to intestinal obstruction, perforation, and acute abdominal syndrome requiring surgical intervention 5
- The parasite breaches the mucosal epithelial barrier, producing tissue lesions that progress to abscesses and acute inflammatory response 1
Extraintestinal Amebiasis
- Amebic liver abscess is the most common extraintestinal manifestation, presenting with right upper quadrant pain, fever, and hepatomegaly 6, 3
- Dissemination to soft organs can cause abscesses in various locations 4
- Diagnosis of extraintestinal disease is confirmed by positive serology (indirect hemagglutination or ELISA) in symptomatic patients with consistent clinical or radiographic findings 6
Diagnostic Approach
Intestinal Amebiasis Diagnosis
Microscopic examination of fresh stool specimens should be performed to identify E. histolytica trophozoites or cysts, with care taken to distinguish trophozoites from large white blood cells. 7, 8
- Microscopy alone cannot distinguish E. histolytica from non-pathogenic Entamoeba species (particularly E. dispar), so specific antigen detection or PCR-based assays should be used when available 8, 2
- Stool culture or histopathology of tissue biopsy/ulcer scrapings can confirm diagnosis 6
- In resource-limited settings without microscopy, patients with bloody diarrhea should initially be treated for shigellosis; only if no improvement occurs after 4 days of appropriate antibiotic therapy should amebiasis be considered 6
Extraintestinal Amebiasis Diagnosis
- Positive serology (indirect hemagglutination or ELISA) confirms diagnosis in symptomatic patients with clinical or radiographic findings consistent with extraintestinal infection 6
- Antibody detection is highly valuable for hepatic abscess, where direct parasitologic confirmation is often impractical 6
- Important caveat: Positive serology in asymptomatic persons does not indicate active disease—antibodies may persist from past infection 6
Treatment Recommendations
Treatment for Invasive Amebiasis (Intestinal or Extraintestinal)
Tinidazole 1.5 g daily for 10 days is the first-line treatment for E. histolytica infection, with a cure rate of 96.5%, superior to metronidazole. 8
Alternative Tissue Amebicide
- Metronidazole 500 mg three times daily for 7-10 days is an alternative option with approximately 88% cure rate 8, 9, 4, 2
- Both metronidazole and tinidazole are FDA-approved for treatment of amebiasis (intestinal amebiasis and amebic liver abscess) 9, 10
Mandatory Luminal Amebicide Follow-up
All patients must receive a luminal amebicide after completing tissue amebicide treatment to eliminate intestinal cysts and prevent relapses, even in patients with negative stool microscopy. 8
- Diloxanide furoate 500 mg three times daily for 10 days, OR 8
- Paromomycin 30 mg/kg/day divided into 3 doses for 10 days 8
- This two-step approach (tissue amebicide followed by luminal amebicide) is essential to prevent relapse and transmission to sexual partners or close contacts 8, 2
Treatment for Asymptomatic Cyst Carriers
- All E. histolytica infections, including asymptomatic cyst carriers, should be treated to prevent progression to invasive disease 8
- However, tinidazole is not indicated for asymptomatic cyst passage 10
- Use luminal amebicides (diloxanide furoate or paromomycin) for asymptomatic carriers 8
Special Considerations for Amebic Liver Abscess
- Metronidazole or tinidazole therapy does not obviate the need for aspiration or drainage of pus in amebic liver abscess 9
- Follow-up ultrasound may be necessary to confirm resolution of hepatic cysts 8
Follow-up and Monitoring
- Follow-up stool examinations are necessary to confirm elimination of the parasite 8
- Ultrasound may be necessary to confirm resolution of hepatic cysts if present 8
Key Clinical Pitfalls to Avoid
Diagnostic Pitfalls
- Do not rely on microscopy alone—it cannot distinguish E. histolytica from non-pathogenic E. dispar, leading to overdiagnosis 8, 2
- Do not mistake large white blood cells for trophozoites on microscopy—amebic dysentery tends to be misdiagnosed 7, 8
- Do not interpret positive serology in asymptomatic persons as active disease—antibodies may reflect past infection 6
- Consider amebiasis in patients with acute abdominal syndrome even without travel history or immunodeficiency, as sexual transmission can occur in non-endemic areas 5, 3
Treatment Pitfalls
- Do not omit the luminal amebicide step—failure to follow tissue amebicide with luminal agent leads to relapse and continued transmission 8, 2
- Do not treat asymptomatic persons with positive serology alone—this does not indicate active extraintestinal disease 6
- In mixed aerobic and anaerobic infections, antimicrobials appropriate for aerobic infection should be used in addition to metronidazole 9