LIFE CYCLE DETAILS OF ENTAMOEBA HISTOLYTICA
Entamoeba histolytica is a pathogenic parasite in the intestine of human beings and many other primates. It inhabits the mucous and sub mucous layers of the large intestine. It feeds mainly on the tissues of the intestinal wall and often produces severe ulcers. It causes a serious and often fatal disease known as amoebic dysentery or amoebiasis.
In life cycle, it occurs in three distinct forms:
1. Trophozoite or adult form or magna form or feeding form.
2. Pre-cystic or minuta form and
3. Cystic form.
1. The trophozoite amoeba:
The trophozoite is somewhat elongated measuring 18 to 40 µm, but the shape is not fixed as the animal constantly changes its shape. The adult trophic form of Entamoeba is known as Trophozoite or Magna. It inhibits anterior part of large intestine. It resembles amoeba in structure but differs in parasitic mode of life. Its body is covered by plasmalemma, and cytoplasm is differentiated into clear translucent cytoplasm and a granular endoplasm. There is single large, broad, and blunt pseudopodium formed of ectoplasm. Endoplasm contains single spherical nucleus varies from 4 to 6 µm in size and food vacuoles. Nucleus has peripheral crown of chromatin blocks and centrally located nucleolus. The centre of the nucleus is occupied by a karyosome surrounded by a clear halo. The space between the membrane and the karyosome is traversed by fine threads arranged radially in the form of spokes of a wheel. Ingested blood cells are found in the cytoplasm. Trophozoites are anaerobic parasite. The trophozites multiply by repeated binary fission in the intestinal wall of host. Some of the daughter entamoeba grow into normal adult while others stop growing. These are smaller than the normal trophozoites and are called Precyst or Minuta forms. It is the most active, motile, feeding, and pathogenic stage that lives in the mucosa and sub-mucosa membrane of the large intestine. It moves with the help of a lobopodium which is produced anteriorly. Ribosomes, food vacuoles and vesicular, cartwheel shaped nucleus are present in the endoplasm. The absence of mitochondria indicates the obligate anaerobic nature of Entamoeba histolytica. It produces the proteolytic enzyme called histolysin due to which the species name histolytica was assigned to it. Because of this enzyme the mucosa and sub-mucosa of the gut wall are dissolved releasing some amount of blood, tissue debris which are ingested by the trophozoites. Hence the food vacuoles are with erythrocytes fragments of epithelial cells and bacteria. Presence of RBC in food vacuoles and cartwheel shaped nucleus are the characteristic features of the trophozoites of Entamoeba histlytica.
2. Pre-cystic form:
When the host becomes resistant to harmful actions of trophozoites, the trophozoites are converted into the pre-cystic forms inside the lumen of the colon of the host body. Smaller in size than the trophozoites. It measures 12-15 μm in diameter in the magna form, in the small race it is about half the size. Round or slightly ovoid in form with a broad and blunt pseudopodium projecting constantly from the end. Less active than trophozoites. Endoplasm comprises of a single nucleus identical to that of trophic forms, but endoplasm is devoid of erythrocytes and other food particles. It is the non-feeding and non-pathogenic stage of Entamoeba histolytica that is found in the lumen of large intestine. It is a small, spherical, or oval, non-motile form. The cytoplasm of the precystic stage stores glycogen granules and chromotoid bars (made of ribonucleo protein) which act as reserve food.
3. Cystic forms:
They are produced inside the lumen of the large intestine by the pre-cystic forms and liberated from the host body finally along the faeces. They are infective forms of Entamoeba and can infect a new host. A mature cyst is a spherical body, and it measures about 10 to 12 μm in diameter. In early cystic forms the cytoplasm contains one or two glycogen masses. Endoplasm is devoid of erythrocytes and other food particles. Nucleus may be one, two or four in number in mononucleate, bi-nucleate and tetra-nucleate cystic forms respectively (the cyst is uni-nucleate, but its nucleus divides to form bi-nucleate and finally a tetra-nucleate or Quadri nucleate cyst). Nucleus of the mature cystic forms are smaller in size than trophic and pre-cystic forms. It is round and is surrounded by a thin, delicate and highly resistant cyst wall. The process of development of cyst wall is called encystation. Which is a means of tide over the un- favourable conditions that the parasite is going to encounter while passing to a new host. Soon after the encystation, the nucleus undergoes two successive mitotic divisions to form four daughter nuclei. This type of cystic stage is called tetranucleate cyst or mature cyst which is the stage infective to man.
4. Transmission:
Infection depends upon intake of food or water contaminated with tetra- nucleate cysts. Cysts are destroyed by drying. The mature cystic forms of E. histolytica after coming to the exterior along with host’s faces enter alimentary canal of new host with contaminated food and drink. Untreated human faeces voided by children and adults on open ground or in crop field are a common source of infection. It is known that houseflies and cockroaches are caprophagous in habit (feeding upon faecal matter), carry viable cysts on their legs and transfer them to unprotected food stuffs.
5. Excystation:
After its entry into the alimentary canal of a host the tetra-nucleate cystic forms pass through the stomach in an unchanged condition as the gastric juice of the host has no action on the cystic forms. Through the contami¬nated food and drink when the infective cysts pass into the lower portion of the small intestine, the cyst wall becomes very delicate (dissolved) inside the small intestine by the action of the pancreatic juice. After five or six hours excystation occurs as the cyst wall is digested by trypsin in small intestine and a metacystic amoeba with four nuclei is liberated within the lumen of the ileum. This is called excystment.
6. Metacystic development:
Each metacystic amoeba or excystic amoeba undergoes binary fission. Its nuclei divide in a specific pattern, accompanied by simultaneous cytoplasmic division to form 8 smaller uninucleate amoebulae or metacystic trophozoites. The metacystic trophozoites feed on the contents of the intestine and grow to form the trophozoites of the next generation. The trophozoites stay in the lumen of the intestine for a period when they may attack the wall of the intestine and start the life cycle again. Entamoeba histolytica causes amoebic dysentery, abscesses in liver, lungs, and brain.
Extraintestinal spread: In some cases, Entamoeba histolytica trophozoites can penetrate the intestinal wall, enter the blood stream, and travel to other organs, most commonly the liver. This can lead to a severe condition called amoebic liver abscess. With in the liver, trophozoites may transform into cysts, allowing the parasite in this extraintestinal location. Liver abscesses can cause symptoms like fever, abdominal pain, and tenderness. If left untreated, amoebic liver abscesses may repture, releasing pus and amoebic material into the peritoneal cavity. This can lead to a life-threatening condition called peritonitis.
Modes of infection in Entamoeba Histolytica:
The life cycle of Entamoeba histolytica passes only in one host, the man. Transmission from man to man is affected through faecal contamination of drinking water and vegetables directly or through the agency of flies or cockroaches. The mature Quadri nucleate cysts are infective forms. Eating of uncooked vegetables and fruits and drinking of water con-taminated with infective forms of parasites lead to the infection of a new host. Cysts can pass through the intestine of the fly or cockroach in a viable condition and infest new hosts through contaminated food. Cooks, mess boys and food handlers are important transmitters of infection in tropical countries. The trophozoites undergo binary fissions in the wall of the large intestine and produce several daughters entamoeba. They feed upon the bacteria and the host's tissue elements, grow and again multiply. After repeated binary fission some of the young ones enter of the lumen of the large intestine and transform into precystic stages. Here, the precystic stages transform into cystic stages which in turn develop into tetranucleate cysts. The entire process is completed only in a few hours. These tetranucleate cysts come out along with the faecal matter and can remain alive for about 10 days. The cyst reaches new host through contaiminated food and water. In the small intestine of a new human host the cyst wall gets reptured releasing the tetranucleate amoebae. Such tetranucleate excystic amoebae are called metacysts. The four nuclei of the metacyst undergo mitotic divisions and produce eight nuclei. Each nucleus gets a bit of the cytoplasm and thus eight daughter entamoebae or metacystic trophozoites are produced. These young ones develop into feeding stages called trophozoites. They invade the mucous membrane of the large intestine and grow into mature trophozoites.