AMOEBA

 Amoeba

Amoebae are structurally simple protozoans which have no fi xed shape.The word “amoeba” is derived from the Greek word “amoibe” meaning “change”. Th ey constantly changetheir shape due to presence of an organ of locomotion called as “ pseudopodium”

Classification Based On Habitat 

Amoebae are classified as intestinal amoebae and free living amoebae. 

1.Intestinal amoebae: They inhabitat in the large intestine of humans and animals.

Entamoeba histolytica is the only pathogenic species. 

Others are nonpathogenic such as

E. dispar, E. moshkovskii, E. coli, E. polecki, E. hartmanni, E. gingivalis, Endolimax nana, and Iodamoeba butschlii

2. Free-living amoebae: They are small free living and opportunistic pathogens.

Examples are Acanthamoeba species, Naegleria fowleri, Balamuthia mandrillaris and Sappinia diploidea. All freeliving amoebae are opportunistic pathogens.

Taxonomical Classification

According to the traditional 1980s classification—amoeba belongs to the

 Phylum Sarcomastigophora, Subphylum Sarcodina, Superclass Rhizopoda, Class Lobosea, Subclass Gymnamoebia, Order Amoebida and Family Endamoebidae.

PATHOGENIC INTESTINAL AMOEBA

Entamoeba Histolytica

History and Distribution

E. histolytica was discovered by Lösch in 1875, who demonstrated the parasite in the dysenteric feces of a patient in St. Petersburg in Russia.

In 1890, William Osler reported the case of a young man with dysentery, who later died of liver abscess. Councilman and Lafl eur in 1891 established the pathogenesis of intestinal and hepatic amoebiasis and introduced the terms 'amoebic dysentery' and 'amoebic liver abscess'.

E. histolytica is worldwide in distribution but more common in tropical and subtropical countries.

E. histolytica has three subspecies E. histolytica subspecies ( histolytica, dispar and moshkovskii.)

Cysts and trophozoites of all the three subspecies are morphologically indistinguishable However, on the basis of extensive genetic, immunological, and biochemical analysis, currently all the three subspecies are formally accepted as diff erent yet closely related species

E. histolytica is the pathogenic species causing amoebic dysentery and a wide range of other invasive diseases, including amoebic liver abscess, where as other two are considered as nonpathogens that colonize the large intestine.

Epidemiology

Amoebiasis is a major health problem worldwide. The largest burden of the disease occurs in tropics of China, Central and South America,and Indian subcontinents affecting 10% of the world’s population. (500 million).

It is the third most common parasitic cause of death in the world (after malariaand schistosomiasis). Approximately 50 million cases and 110, 000 deaths are reported annually by WHO (World health Oranization) In India, the prevalence rate is around 15% (ranges from 3.6% to 47.4%) with a higher prevalene reported from Maharashtra, Tamilnadu and Chandigarh.

Morphology

E. histolytica occurs in 3 forms 

Trophozoite

Precyst

Cyst.