Microbiology's Questions



January 08 2015

Entamoeba Histolytica

Write short notes on Entamoeba Histolytica

  • Last Comment by Neuros
January 08 2015 #

Entamoeba Histolytica is an anaerobic protozan infecting humans and primates. It is estimated to have affected around 10% of the world's population. 

  • Source of Infection: fecal - oral transmission from contaminated food or water
  • Infective form: Cyst (has 4 small nuclei)

Pathogenesis: When the cyst is ingested, excystation occurs in the intestine where the cyst undergoes nuclear and cytoplasmic changes to form 8 trophozoites. These trophozoites then colonize the large intestines. This colonization leads to formation of the classic flask shaped ulcers. When the parasites migrate to the liver or the lung, it causes formation of abscesses.  The disease caused by this parasite is called Amebiasis. 


  • Asymptomatic (most common) presentation
  • Bloody Dysentry
  • Liver Abscess: fever, RUQ (pain), jaundice, weight loss
  • Cutaneous Amebiasis (rare): can lead to significant tissue necrosis. Mostly though to occur because of fistulas arising from the underlying GI abscess.


  • Microscopic Examination of stool: reveals trophozoites, cysts and occult blood
  • Antigen detection: rapid and more sensitive
  • Polymerase Chain Reaction (PCR)
  • Serology: the Indirect Hemagglutination (IHA) test has high sensitivity to detect the parasite. However, its drawback is it remains positive for years after an episode of Amebiasis
  • Colonoscopy: to detect ulcers and take biopsy samples from the edges of ulcers
  • Imaging: Ultrasound or CT scan can be done to detect liver abscess

TREATMENT: Metronidazole followed by Iodoquinol


  1. Upinder Singh, William Petri Jr. Principles and Practice of Clinical Parasitology: Amebas (pg 199-211)
  2. Tao Le, Vikas Bhushan, Jeffrey Hofmann. First Aid for the USMLE Step 1 (2012) Parasitology: Protozoa GI infection (pg 175)
  3. USMLE Step 1 Lecture Notes by Kaplan (2004). Section 1 - Medical Parasitology: Entamoeba Histolytica (pg 129)

Medical Guru
Medical Guru
January 09 2015 #

The spread from the gut to the liver is mostly through a hematogenous or portal circuulation route. From here the parasite can move to other places directly. The most common organ after liver to get infested is the lung as it is very close to the liver.

January 09 2015 #

Also to add one more point - intentinal infections are diagnosed using microscopic exam of stool and antigen detection while extra-intestinal infection is diagnosed using serology

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