Infectious colitis


Topic Completed: 1 April 2015

Minor changes: 20 September 2021

Copyright: 2003-2021,, Inc.

PubMed Search: Amebic colitis [title]

Elliot Weisenberg, M.D.
Page views in 2020: 12,202
Page views in 2021 to date: 10,221
Cite this page: Weisenberg E. Amebic. website. Accessed October 24th, 2021.
Definition / general
  • Infection by pathogenic species of amebae, overwhelmingly Entamoeba histolytica
  • May be called amebiasis
  • Estimated that 10% of the world's population is infected with E. histolytica
  • It is endemic in tropical and subtropical regions
  • In temperate developed countries affected patients usually are immigrants, travelers, men who have sex with men, and residents of institutions
  • Other sources state that many, if not most, individuals believed to carry E. histolytica may be carrying non-pathogenic E. dispar or E. moshkovskii that are morphologically identical to E. histolytica
  • It is estimated that there are fifty million cases of amebic diarrhea on a worldwide basis annually with 100,000 deaths
  • Disease involves the colon, most commonly the cecum, followed by the right colon, rectum, sigmoid and appendix
  • Involvement of the terminal ileum may occur
  • Metastasis may occur, overwhelmingly to the liver
  • Disease may spread from the liver to the thorax or rarely the brain
  • Rectovesical fistula and fistulous involvement of the skin have been reported
  • Cysts are ingested from fecally contaminated food or water; sexual transmission also occurs
  • Excystation to 8 motile trophozoites occurs in the small intestine
  • The cysts are resistant to gastric acid (and chlorine in water supplies)
  • Trophozoites are potentially invasive and multiply by binary fission
  • In an estimated 20% of infections invasion into the wall of the colon with tissue destruction occurs
  • Adherence to colonic mucosa is mediated by a lectin on E. histolytica's surface
  • The parasite then induces apoptosis of epithelial cells through a channel forming pore protein
  • E. histolytica ingests remnant cells
  • Some trophozoites undergo encystation through signalling pathways completing the cycle
Diagrams / tables

Images hosted on other servers:
Missing Image

Model of mechanism for cytotoxicity

Clinical features
  • The majority of individuals with ameba infection are asymptomatic, but do pass cysts (asymptomatic intraluminal amebiasis)
  • This is true for all cases of E. moshkovskii and (most) E. dispar and up to 80% of E. histolytica infections
  • While E. dispar is generally believed to be non-pathogenic, recent reports suggest it may sometimes cause symptoms (Trop Parasitol 2015;5:9)
  • The most common symptom from amebiasis is diarrhea without dysentery (no mucus or blood in stool)
  • Amebic dysentery or colitis: mucus or blood is grossly visible or microscopic
    • 15% to 33% of cases with diarrhea
    • Usually symptoms develop gradually over three weeks to a month with worsening diarrhea and abdominal pain
    • Symptoms may also develop acutely and these cases may mimic acute abdomen
    • Cases where symptoms develop months after infection may occur
    • Young children may develop intussusception or necrotizing colitis that may lead to perforation
    • Rare complications are toxic megacolon or colonic amebomas
    • Amebic liver abscesses are more common in men (M:F ~ 10:1)
      • The abscess is usually solitary
      • Symptoms include fever, cough, and dull, achy right upper quadrant pain that may also include referred pain in the right chest or shoulder
      • Only 1/3 have gastrointestinal symptoms
      • Symptoms usually develop over two to four weeks
      • The liver is usually enlarged and tender
      • Leukocytosis and elevated liver enzymes are present
  • Usually detected in stool ova and parasite examination
  • Many authorities recommend using antigen detection or PCR based assays to distinguish E. histolytica from non-pathogenic ameba (Trop Parasitol 2014;4:90)
  • May be seen in tissue biopsy material
  • Patients with amebic liver abscesses usually have antiamebic antibodies and amebic antigen in serum
  • Cyst aspiration is sometimes performed, and although it is unusual to see parasites, the absence of other microorganisms is supporting evidence of amebic liver abscess
Case reports
  • Metronidazole
    • E. histolytica lacks mitochondria and is an obligate fermenter of glucose
    • Metronidazole targets ferridoxin-dependent pyruvate oxidoreductase necessary for fermenting glucose
Gross description
  • Discrete ulcers with normal intervening mucosa
  • May show areas of colitis or inflammatory polyps
Gross images

Images hosted on other servers:
Missing Image Missing Image


Microscopic (histologic) description
  • Ameba burrow into lamina propria and cause tissue necrosis with relatively little inflammation
  • Early lesions show scattered neutrophils
  • More developed cases generally show broad based "flask" shaped ulcers 1 to 2 mm in diameter
  • The trophozoites of Entamoeba histolytica are 6 to 40 nm and resemble macrophages
    • They are round to oval and may be surrounded by a halo
    • The cytoplasm is abundant and vacuolated and may contain ingested red blood cells that indicate tissue invasion
    • The nuclei are small and round with prominent nuclear membranes and a central karyosome (chromocenter)
    • They are often found clustered at the luminal surface or within debris
Microscopic (histologic) images

Case #402

46 year old man (anal Pap smear) - Iodamoeba bütschlii

Case #257
Missing Image Missing Image Missing Image Missing Image

50 year old man - intestinal spirochetosis and amebiasis

Contributed by Hanni Gulwani, M.D.
Missing Image Missing Image Missing Image Missing Image

Amoebic colitis

Images hosted on other servers:
Missing Image

Amoebic dysentery

Missing Image Missing Image


Missing Image Missing Image

Low power

Missing Image

Medium power

Missing Image Missing Image

Amoeba in stool (trichrome)

Missing Image


Missing Image

Leptin receptor

Positive stains
Negative stains
Back to top
Image 01 Image 02