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(Anaerobic parasite)

Causes Amoebic dysentery or Amoebiasis.

Distribution: Worldwide.

Entamoeba histolytica – Trophic form

Habits & Habitat:  

  • Found in large intestine (colon) of man, in both mucosal & sub-mucosal layers of it.
  • It feeds on tissue and RBCs mainly.
  • Secrete a toxic substance to dissolve mucous lining of the intestine.
  • It gradually recedes deep in the mucosal & sub-mucosal layers.
  • In chronic cases, it may enter venous circulation further reaching liver, lungs, brain etc.

Life Stages & Morphology:

  • Microscopic, naked, amoeboid.
  • Life Cycle has three distinct stages:
    • Trophozoite
    • Precystic
    • Cystic

Trophozoite Stage (Trophic/Magna form)

  • Pathogenic to man.
  • Colorless, irregular mass.
  • Active, motile, growing & feeding form.
  • Structurally resemble amoeba.
  • Ingested RBCs sometimes clearly visible in cytoplasm.
  • Surface covered with plasmalemma (Thin, transparent, flexible & semipermeable membrane).
  • Cytoplasm is differentiated into:
    • Ectoplasm: Outer, clear, hyaline, non-granular.
    • Endoplasm: Inner, more fluid, granular. Contains Nucleus & food vacuoles.
  • Nucleus:
    • Invisible inside living parasite but appears as round & vesicular structure in fixed & stained specimen.
    • A thin, delicate nuclear membrane present with chromatin granules present on its inner surface.
    • A small distinct central endosome is present with a clear area known as halo.
    • Nucleoplasmic striations present, between endosome & nuclear membrane.
  • Food Vacuoles (in endoplasm): Enclose red blood corpuscles, white blood corpuscles, epithelial cells’ fragments, bacteria etc.
  • Contractile Vacuoles are absent. As it already lives in isotonic environment.
  • Pseudopodium: Monopodial.
  • Nutrition: Holozoic. Feeding by phagotrophy, engulfing food through posterior end.

Precystic Stage (Minuta form)

  • Lives in the lumen of large intestine.
  • Nonpathogenic to man.
  • Small, spherical with a blunt pseudopodium projecting out on the surface.
  • Non motile, non-feeding form.
  • Endoplasm: RBCs & Food vacuoles absent.
  • Nucleus same as trophozoite nucleus.
  • Develops into trophozoite stage – by penetrating into mucosa & sub mucosa, ingesting RBCs & growing in size.
  • Also, undergoes encystation – for infecting new host.
Entamoeba histolytica – Minuta form

Cystic Stage

  • Precystic form during encystation becomes round & gets surrounded by a cyst wall (thin, highly resistant, flexible, colorless, transparent)
  • Cytoplasm:
    • Clear, reserve food in form of two glycogen masses. Masses gradually disappear.
    • Deeply staining one or more bar like chromatoid bodies are present.
  • Nucleus: Same as trophozoite nucleus. Later the cyst becomes binucleate & then tetranucleate.
  • Tetranucleate stage cyst passes out in faeces of the infected patient. Appear as shining spheres.
  • If dessicated/dried cysts die.

HOST:

  • Monogenetic (completes its life cycle in a single host).
  • Natural Host – Man
  • Reservoir host – Wild rats, dogs etc.

REPRODUCTION:

  • Through Binary Fission
  • Occur in trophozoites in the wall of large intestine.
  • With nucleus wall intact, nucleus undergoes mitosis and 6 chromosomes are formed.
  • Nuclear division is accompanied by cytokinesis and form two daughter amoebae.
  • The daughter amoebae starts growth in size, feed on bacteria & intestinal tissue and further reproduce by binary fission.
  • Some daughter amoebae transform to precystic form.

ENCYSTMENT, TRANSMISSION & EXCYSTMENT:

Encystment

  • Occurs in lumen of the intestine, not in tissues.
  • Precystic form rounds in shape & secrete cyst wall around itself.
  • During formation of cyst, Entamoeba is uninucleate, later changing to binucleate & further to tetranucleate.
  • Cysts moves out of body through faeces.

Transmission (to new host)

  • Cysts are unable to develop in the host they are produced in.
  • Infection occurs in new host by consuming contaminated food, water etc. with faecal matter containing mature cysts.
  • Flies, cockroaches are also seen as transmission source through their body surface when they sit on infected faecal matter & later on, carrying cysts along to the food.

Excystment

  • When cyst form is swallowed by a person, it passes gastric juice unaffected due to resistant cyst wall.
  • On reaching small intestine, its wall dissolve due to trypsin present there.
  • A single tetranucleate amoeba form – Metacystic/Excysticformis liberated from each cyst.
  • Metacyst emerges out through a pore in cyst wall.
  • The metacyst undergoes nuclear & cytoplasmic division, resulting in 8 uninucleate small metacyst trophozoites.
  • The young trophozoites are actively motile, reach large intestine, invade mucosal tissue and grow as mature trophozoites.
Entamoeba histolytica – Life History

Pathogenicity

Amoebiasis – Results in diarrhoea, dysentery, liver abscesses etc. Most people infected are asymptomatic.

 Amoebic dysentery:

  • Infection is confined to intestine.
  • Blood & mucous in the faeces present.
  • The parasite release proteolytic enzymes etc. to dissolve mucosa & submucosa.
  • Destruction sites form ulcers & bleed.
  • In more complication, trophozoites may have invaded muscularis.

Chronic Intestinal Amoebiasis:

  • The person usually have diarrhea, bowel irregularity, abdominal pain, fatigue etc.
  • Usually occurs in repeated amoebic dysentery.

Abscesses (Extraintestinal amoebiasis):        

  • Occasionally, the parasite enters blood stream & reach various organs like liver, brain, lungs, spleen etc.
  • In the above organs too they cause destruction of tissue resulting in abscesses.
  • Most common is liver abscesses.

Diagnosis

  • By microscopic examination of trophozoites or cysts in stool, tissue specimen etc.
  • Stool antigen detection & PCR test are also available.

Treatment

  • There are number of effective medications available for the infection.
  • Intestinal infection: Usually nitroimidazole derivatives (effective against trophozoite form) etc.
  • Liver Abscess: Usually metronidazole and chloroquine etc.
  • Asymptomatic infected person: Usually paromomycin etc.

Prophylaxis (Prevention)

  • Washing hands with soap & water especially after using toilets & before eating something.
  • Regularly cut finger nails.
  • Washing fruits & vegetables before use.
  • Covering edible & drinkable items properly.
  • Avoid defecation in open.
  • Proper toilets should be made available at home as well as at public places.
  • Sanitization of streets, roads, open drains etc.
  • Hawkers should cover food sold by them.
  • Isolation & treatment of carriers.

Harjeet Kaur

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