Colon non tumor
Infectious colitis (specific microorganisms)
Amebic colitis

Authors: Elliot Weisenberg, M.D. (see Authors page)

Revised: 29 September 2016, last major update April 2015

Copyright: (c) 2003-2016, PathologyOutlines.com, Inc.

PubMed Search: Amebic colitis [title]
Cite this page: Amebic colitis. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/colonamebic.html. Accessed December 9th, 2016.
Definition / General
  • Infection by pathogenic species of amebae, overwhelmingly Entamoeba histolytica
Terminology
  • May be called amebiasis
Epidemiology
  • 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
Sites
  • 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
Pathophysiology
  • 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

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Life cycle

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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
Diagnosis
  • 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
Treatment
  • Metronidazole
    • E. histolytica lacks mitochondria and is an obligate fermenter of glucose
    • Metronidazole targets ferridoxin-dependent pyruvate oxidoreductase necessary for fermenting glucose
Gross Description
  • Discreet ulcers with normal intervening mucosa
  • May show areas of colitis or inflammatory polyps
Gross Images

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Ulcers

Micro 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
Micro Images

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Case of the Week #402:

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



Case of the Week #257:

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50 year old man-Intestinal spirochetosis and amebiasis



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Amoebic dysetery

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Trophozoites

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Medium power

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Courtesy of Dr. Hanni Gulwani, Bhopal Memorial Hospital and Research Centre (India)

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Amoeba in stool (trichrome)

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PAS

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Leptin receptor

Positive Stains
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