Entamoeba histolytica

(Anaerobic parasite)

Causes Amoebic dysentery or Amoebiasis.

Distribution: Worldwide.

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.

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.

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.