Amoeba

Amoeba:

•        Locomotion- pseudopodia.

•        According to pathogenicity and habitat:

-         Pathogenic:

 1. intestinal amoeba- Entamoeba histolytica.

-         Non-pathogenic:

1.    mouth amoeba- Entamoeba gingivalis

2.    intestinal amoeba- Entamoeba coli,

                                       Enolimax nana,

                                       Iodamoeba butschii and

                                       Dientamoeba fragilis.

Entamoeba histolytica

•        Cause: diarrhoea, dysentry and liver abscess in man

•        Distribution: Worldwide, more common; tropics and sub tropics than in the temprate zone.

•        Lives in mucous and submucous layer of large intestine of man.

•       Although pigs and primates may be infected, these infections are rare and unimportant.  This parasite is transmitted from human to human.

2. Pre-cystic stage:

-         smaller in size, 10-20µm

-          Round or oval with a blunt pseudopodium projecting from the pheriphery.

-          Endoplasm free of RBCs and food particles.

-         Nucleus retains characteristic of trophozoites.

3. Cystic stage:

-         Shape and size: spherical, 12-15µm

-         Cyst wall: During encystment, parasite becomes rounded and is surrounded by highly refractile membrane called cyst wall.

-         Nucleus: initially there is one nucleus which multiplies by sucessive division into two and then four daughter nuclei. Mature cyst contains four nuclei.

-         Cytoplasm: contains 1-4 chromatoid bodies, look refractile oblong bars with rounded ends in iodine solution. Glycogen mass stains brown with iodine.

•        Encystation:

-         Transformation of trophozoites to cyst.

-         Occurs inside lumen of the intestine of infected individual.

-         Whole process takes place within a few hours and the life span of mature cyst inside the lumen of the bowel of original host is only two days.

•        Multiplication: 

-         Occurs by simple binary fission

•        Infection by Entamoeba histolytica occurs by ingestion of mature cysts (1) in fecally contaminated food, water, or hands.

•         Excystation (2) occurs in the small intestine and trophozoites (3) are released, which migrate to the large intestine.

•         The trophozoites multiply by binary fission and produce cysts (4) , which are passed in the feces. Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment and are responsible for transmission. (Trophozoites can also be passed in diarrheal stools, but are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment.)

•        In many cases, the trophozoites remain confined to the intestinal lumen (A: non-invasive infection) of individuals who are thus asymptomatic carriers and cysts passers.

•         In some patients the trophozoites invade the intestinal mucosa (B: intestinal disease), or, through the bloodstream, extraintestinal sites such as the liver, brain, and lungs (C: extra-intestinal disease), with resultant pathologic manifestations. It has been established that the invasive and noninvasive forms represent separate species, respectively E. histolytica and E. dispar, which are morphologically indistinguishable.

•        Transmission can also occur through fecal exposure during sexual contact (in which case not only cysts, but also trophozoites could prove infective).

•        Entamoeba histolytica infection can lead to amebiasis or amebic dysentery. Symptoms include dysentery, diarrhea, weight loss, fatigue, and abdominal pain.

•        Virulence factors:

1. Amoebic lectin- mediates in the adherence of amoebae to the intestinal mucosa.

2. ionophore like protein- cause leakage of ions i.e Na⁺ , K⁺ , Ca⁺ from the target cells.

3. Hydrolytic enzymes- cause proteolytic destruction of the tissue.

4. Toxins and haemolysins-

•        After adherence, trophozoites lyse the target cells by its ionophore like protein. Cause a leakage of ions from the cytoplasm of the target cells. Lysis of target cells is completed extracellularly.

•        Enzyme cause destruction by:

-         Digesting the extracellular matrix and

-         Breaking down IgA molecules and minimizing certain components of the complement.

•        Tissue destruction caused by amoebae gives rise to typical flask shaped amoebic ulcers

Extra-intestinal amoebiasis

•        Liver, lung or brain biopsy samples are subjected to routine histology and giemsa-stained touch preparation which will reveal trophozoites in extra-intestinal lesions. Trophozoites can be detected in the scraping material from the wall of amoebic abscess, and rarely from the aspirated pus or expectorated sputum.

•        Serological test;

-         Indirect haemagglutination assay

-         ELISA

-         Latex agglutination test

-         Gel diffusion precipitation

-         Counter current immunoelectrophoresis etc