Entamoeba histolytica

Parasites and Vectors-

  • Parasite- An organism which derives its nourishment at the cost of another organism (host) in or on which it lives is called a parasite.
  • Protozoology-It is a branch of science which deals with the study of protozoa.
  • Helminthology- It is a branch of science which deals with the study of helminthes.
  • Entomology- It is a branch of science which deals with the study of insects.
  • Host-The organism which harbours the parasite is called a host.
  • Parasitism- The relation which exists between the parasite and the host is known as parasitism. Parasitism may also be defined as an association between two living organisms in which one benefits and the other is harmed.
  • Parasitology- It is a branch of science which deals with the study of parasites (protozoa and helminthes).

Types of Parasites-

  • Parmanent Parasites- These are the parasites which remain inside the host for the whole life until either the age or some immunizing process of treatment removes them. All the pathogenic protozoa and helminthes are permanent parasites.
  • Temporary Parasites- These are the parasites which visit the host for a short period. Only one stage of the parasite development is passed in the host.
  • Pathogenic Parasite- It is a parasite which lives at the expense of the tissues and fluids of the host and inflicts injuries on the host tissues.
  • Obligatory Parasite- It is a parasite which entirely depends on the host for its existence.
  • Facultative Parasite- It is a parasite which leads parasitic life but is also capable of independent existence.
  • Ectoparasite- It is an organism which lives outside the body of the host.
  • Endoparasite- It is an organism which lives inside the body of the host.
  • Non-pathogenic Parasite-It is a parasite which does not harm the host.

Type of Host-

  • Primary Host- It is an organism which harbours the adult stage of the parasite.
  • Alternate Hosts-When two hosts harbour the same parasite, either is called an alternate host.
  • Accidental or Casual Host- It is an organism wherein parasite is not normally found.
  • Intermediate Host- It is an organism which harbours the larval stages of the parasite. In some cases more than one intermediate host is required for the larval development. They are called first and second intermediate hosts.

Entamoeba histolytica-

 E. histolytica is an internal protozoon parasite found in the mucous and subsmucous layers of the upper part of the large intestine, feeds on tissue cells, red blood corpuscles, bacteria and produces several ulcers and abscesses. Faeces becomes loose and slimy containing mucous and blood leading to amoebic dysentery. In chronic cases Entamoeba histolytica may be carried alongwith the blood stream to the liver, spleen, lung, brain and other organs.

  • Amoeba are structurally simple protozoans which have no fixed shape.
  • Kingdom- Protista
  • Subkingdom-Protozoa
  • Phylum- Sarcomastigophora
  • Subphylum- Sarcodina
  • Order- Amoebida
  • E. histolytica was first reported by Fyodor Lesh in 1875.
  • Entamoeba histolytica is an intestinal amoeba.
  • All free living amoeba are opportunistic pathogens.


 The body of Entamoeba is more or less round and distinguished into outer clear ectoplasm and an inner granular endoplasm with a large round nucleus. It occurs in three stages: Tropozoite, precystic and cystic. It feeds only in the trophozoite stage while the precystic and cystic stages are non-feeding stages.

Trophozoite Stage-

 It is a spherical mature parasite or active adult with one or two blunt finger like pseudopodia. Formation of pseudopodium in any direction leads to locomotion in that direction. It nourishes itself at the expense of the host feeding on red blood cells and the cytolysed tissues. The granular endoplasm contains a spherical nucleus and food vacuoles containing red blood cells, bacteria etc. The red blood cell in food vacuole is an important characteristic feature of the parasite. Outside the human body, the trophozoite lives only for few hours and dies.

Precystic Stage-

 It is smaller in size, round in shape and has only one blunt pseudopodium. The granular endoplasm is free of red blood cells and other ingested food particles.

Cystic Stage-

 It is almost similar to precystic stage with the difference that it has a thick cyst wall and there is no pseudopodium. The cytoplasm of the cyst contains oblong chromatid bars and sometimes in the early stages a distinct glycogen mass is also seen. The glycogen mass and the chromotid bars gradually disappear. The single nucleus multiplies in two successive divisions into two and ultimately four daughter nuclei are formed. Thus, a cyst may be uninucleate, binucleate and tetranucleate.

Reproduction and Life Cycle-


Human Pathogenicity

 E. histolytica causes infection at different areas of the invasion. The clinical term used for all the conditions is amoebiasis. The infection is generally limited to intestine causing intestinal amoebiasis or amoebic dysentery. In chronic intestinal amoebiasis, the symptoms of dysentery are accompanied by multiple ulcers in the caecum, colon, rectum, and each ulcer is covered with green or black slough.

 These ulcers may extend to the muscular and peritoneal coats and lead to peritonitis or haemorrhage. Sometimes there may be formation of tumour like masses amoebic granuloma or amoeboma in the large intestine. In chronic condition there is alternate diarrhoea and constipation.

Hepatic complications may develop at any time during intestinal infection. There may be enlargement of liver and liver abscess. The trophozoites through circulation may also reach the lung, brain and skin.

Latent Amoebasis (Amoebic Carriers)

Sometimes the individuals do not show any symptom of amoebiasis but discharge a large number of cysts in the stool. This may be due to immunity developed by the individual resulting in an adjustment like symbiosis between host and parasite. These carriers sometimes develop serious complications since the trophozoites may be carried by the portal vein to liver and other organs. Hence, treatment in the case of carriers should not be neglected.

Mode of Infection-

 Transmission of E. histolytica from person to person is effected through the ingestion of mature tetranucleate cysts in a variety of ways as under-

  • Human excreta is used as a manure for growing vegetables and fruits in the field.
  • Water fleas may drop cysts in drinking water.
  • Houseflies and cockroaches may feed on human faeces, pick up cysts on their legs and wings and drop them on human food. Even dog feeds on human faeces and is a source of infection.
  • Handling of food by infective individuals is a very common method.

Laboratory Diagnosis-

 Entamoeba histolytica is diagnosed by microscopic examination of stool for trophozoites