(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.
