Entamoeba histolytica – Basic microbiology, parasitology, and immunology; nature, reproduction, growth, and transmission of common microorganisms and parasites in Bangladesh; prevention including universal precaution and immunization, control, sterilization, and disinfection; and specimen collections and examination. Students will have an understanding of common organisms and parasites caused human diseases and acquire knowledge about the prevention and control of those organisms.
Entamoeba histolytica
It is an anaerobic parasitic amoebozoan, part of the genus Entamoeba. Predominantly infecting humans and other primates causing amoebiasis, E. histolytica is estimated to infect about 50 million people worldwide. E. histolytica infection is estimated to kill more than 55,000 people each year. Previously, it was thought that 10% of the world population was infected, but these figures predate the recognition that at least 90% of these infections were due to a second species, E. dispar.
Morphology of E. histolytica:
- Has two stages – Trophozoite and Cyst
- Trphozoites are 20-60 µm, motile with pseudopodia.
- Cytoplasm of trophozoite is divided into clear ectoplasm and granular endoplasm often 0.0 contains food granules and RBC.
- Cysts are non-motile, round, 10-20 µm with thin transparent cyst wall.
- 4 nuclei in mature cyst (immature cyst contains variable numbers but less than 4)
- Chromatoidal bars are characteristic but presence is variable.
Important features of Entamoeba histolytica:
- Host: Man is the only host (Definitive)
- Mode of transmission: Faeco-oral route.
- Infective form: Mature quadrinucleated cyst (Metacyst).
- Incubation period: 4 to 5 days.
- Diagnostic stage: Cyst & trophozoite.
- Pathogenic stage: Trophozoite
Lesions Produced by E. histolytica:
➤ Intestinal Lesions: Involve the large gut only.
- In acute amoebic dysentery; Multiple ulcers, deep and extensive
- In chronic intestinal amoebiasis:
✓ Single latent ulcer in the caecum.
✓ Multiple, small, superficial ulcers scattered throughout the large gut. Amoeboma in the ✓ caecum and other parts of the large gut.
✓ Pigmented or non-pigmented scars.
➤ Extra intestinal or metastatic lesions:
- Liver: In amoebic liver abscess. Multiple small abscesses, usually in them com postero-superior surface of the right lobe.
- Lungs:
✓ Primary: Small, multiple abscesses, may be in one lung or both the lungs.
✓ Secondary: A single abscess of varying size, situated in the lower lobe of right lung. - Brain: A small abscess in one of the cerebral hemispheres
- Spleen: Splenic abscess.
- Skin: Granulomatous ulceration of the skin adjoining a visceral lesion.
- Uro-genital tract: Amoeba gaining entrance either through a recto-vesical fistula or a recto-vaginal fistula.
Life Cycle of Entamoeba histolytica:
➤ Infective form – Mature quadrinucleated cyst.
➤ Habitat – Mucosa and submucosa of large intestine.
➤ Host-Man (only)
➤ Mode of transmission – Faeco-oral route.
➤ Incubation period – 4 to 5 days.
Cycle:
Mature quadrinucleated cyst swallowed with contaminated food or drink
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Passes unaltered through stomach.
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Excystation occur due to digestion of cyst wall by trypsin when it reaches the small intestine.
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Tetranucleated amoeba, which eventually forms eight amoebulae (young trophozoites)
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Invade the tissue and lodges into the submucosa of large gut
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Grow and multiply by binary fissi
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Secretes proteolytic enzyme (histolysin in nature)
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Destruction and necrosis of tissue.
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Ulcerative colitis.
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Trophozoites may enter into deeper layer and passes through portal vein into liver
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Effect of parasites gradually toned down and increase in host tolerance. Lesions commence to heal.
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Trophozoites discharged into lumen.
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Re-encystation (life span of cyst within lumen of bowel is only 2 days) occurs under certain unfavorable
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conditions (e.g. dehydration within the lumen of colon)
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Cyst passes through faeces and contamination of food or drink.
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Cycle is repeated.
Pathogenesis of Entamoeba histolytica:
Pathogenesis of intestinal amoebiasis (Amoebic dysentery):
Young trophozoites released from cyst binds with colonic mucosa by lectin (Gal/GalNAc)
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Colonization follows binding and then destruction by contact dependent cytolysis.
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Invasion is facilitated by many factors like – spore forming proteins, proteases, motility and phagocytosis.
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Reaches submucous coat.
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Here amoeba multiply rapidly, form colony destroy surrounding tissues & utilize cytolysed materials as food.
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The amoeba then spread in different direction especially laterally & then to deep tissues, continuously lysis the adjacent tissues
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Causes coagulative necrosis and formation of abscess by liquefaction due to histolytic property of the amoeba.
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Abscess finally broken down.
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Development of large, irregular, flask shaped ulcer with undermined edge.
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Passage of mucus and blood with stool (amoebic dysentery).
Pathogenesis of hepatic amoebiasis (Amoebic liver abscess):
The trophozoites of E. histolytica are carried as emboli by the radicles of the portal vein from the base of an
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amoebic ulcer in the large gut usually from the caecum and the ascending colon
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The capillary system of the liver acts as an efficient filter and holds these parasites
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Multiply in large numbers and proceed to carry on their cytolytic actions
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Focal accumulation of amoebule
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Obstruction of circulation
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Thrombosis of portal venules resulting in ischaemic necrosis of surrounding liver cells.
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Focal coagulative necrosis of liver cell which forms starting point of liver abscess.
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Necrotic material consists of solid slough.
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Centre liquifies by cytolytic action of amoeba.
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Liquefaction extends radially
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Formation of liver abscess
Laboratory Diagnosis of Intestinal Amoebiasis:
Principle:
Laboratory diagnosis of intestinal amoebiasis is based on microscopic demonstration of hematophagous trophozoite or cyst of E. histolytica and antigen detection from stool. Nucleic acids techniques are also helpful. Culture followed by isoenzyme analysis is the ‘gold standard’ though it is time consuming and highly technical.
Steps:
Specimen collection:
➤ Stool (Fresh, warm, liquid).
➤ Sigmoidoscopic swabs.
➤ Scrapings from large bowel ulcers.
➤ Biopsies from rectal mucosa
Stool examination:
➤ Macroscopic:
- An offensive dark brown semi-solid stool
- Acidic in reaction
- Admixed with blood, mucus and much faecal matter.
➤ Microscopic:
- Trophozoite – in acute infection.
- Cyst – in chronic infection.
- RBC in roulaeux formation and also within the trophozoites.
- Characteristic cellular exudates consisting of pus cell, macrophage and epithelial
cell. - Presence of charcot-leyden crystals.
➤ Antigen detection in stool: By ELISA (the most sensitive and specific method).
➤ Blood Examination:
- Moderate leucocytosis [eosinophilia].
- Serological examination – Sero-negative in early cases but those cases where tissue- invasion without any symptom has existed long enough to stimulate the antibody formation, the serological test may be positive.
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