Structure life-cycle pathogenicity and control measure of Entamoeba histrlytica.

Habits and Habitat:

Entamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba. Predominantly infecting human and other parasites causing amoebiasis or amoebic dysentery. It is a microscopic endoparasite of man. It is commonly found in the upper part of the intestine (colon) and is very often lodged in the liver, lungs, brain and testes. E.histolytica is infected about 50 million people worldwide. Previously, it as though that 10% of the world population was infected, but these figures predate the recognition that at least 90% these infections were due to a second species, E.dispar. In a person suffering from this disease,the intestinal wall exhibits, in large numbers, a minute ulcers which are formed as a result of the dissolution and destruction of the mucous lining of the intestine. This is accomplished with the aid of a tissue-dissolving substance secreted by a parisite.

Morphology or Structure:

Entamoeba histolytica is a small microscopic parasitic amoeba. It occurs in three distinct form. They are 1.Trophozite or Magna, 2. Precystic (minute form), and 3. Cysts.

1. Trophozoite or Magna:

Trophozoite of E. histolytica is also known as the trophic or magna form. It is the most active, motile, and feeding form which is pathogenic to man. It lives in the mucosa and submucous layer of the large intestine of man. It usually measures 20 to 30 microns in diameter and, more or less, resembles the common amoeba in all structural details. Its body is covered by plasmalemma and cytoplasm is differentiate into ectoplasm and endoplasm. Endoplasm contains a single spherical nucleus and food vacuoles . Nucleus has a peripheral crown of chromatin blocks and a centrally located nucleus.

The trophozoites multiply by repeated binary fission in the intestinal wall of the host. Some of the daughter entamoeba's grow into adults while others stop growing . These are distinctly smaller than the normal trophozoites and are called Minute forms.

2. Precystic or minute:Precyatic or minute form of E.histolytica is small, spherical, non-mortile and non-feeding form. It measures 12 to 15 microns in diameter. In structural details, it resembles the trophozoite except that it is smaller in size and the food vacuoles are absent. Normally it lives in the lumen of the intestine and rarely found in tissue. It undergoes encystations and helps in transmission of parasites from one host to other.

3. Cysts:Under normal condition, minute forms undergo encystation. It becomes rounded and surrounded by a thin, highly resistant and refractile cyst wall. A mature cyst is a spherical body of 10 to 12 microns in diameter. Its cytoplasmic is clear and contain one or two glycogen masses and one or more characteristic, refractile, bar-like chromatoid bodies or chromidial bars with rounded ends. Chromidial bars are made of ribonucleoprotein which disperses throughout cytoplasm with their disappearance. Nuclei retain their characters of uninucleate but its nucleus divides to form a binucleate and finally a tetra-nucleate or quadrinucleate's cyst.

Life cycle of E. histolytica:

Encystation:

In the pre-cystic forms, entamoeba remains only in the lumen of the intestine and not in tissue. They round up and secret a thin, refractile, tough and flexible cyst wall around them. The cysts in this stage are uninucleate. The single nucleus of the cyst soon undergoes a mitotic division to form two nuclei, leading to the formation of binucleated cysts. A second mitotic division the two nuclei into four and thus tetranucleated cysts are formed. The whole process of encystation is complete within few hours.

Tetranucleated cysts consititude the transmission or infective stage. They do not develop further but pass out of the host in the faeces. They can remain alive for 10 days in the moist stool. They are very susceptible to desiccation and die at a temperature of 50 degreeC.

Transfer to new host:

The infective cysts remain viable for a considerable length of time outside a human intestine environmental conditions are favourable. Infection of fresh human host takes place by swallowing the infective cysts with the contaminated food and drinks.

Excystation:

In the new host, the ingested cysts pass down the alimentary canal and reach the small intestine. The cysts wall protects them from the action of host's gastric juice during their passage through the stomach. After 5 or 6 hours excystation takes place as the cyst all is digested by trypsin in small intestine releasing the tetranucleated amoeba, called the ex-cystic amoeba or meta cyst.

Meta cystic development:

Each meta cyst immediately proceeds to divide a specific pattern, accompanied by simultaneous cytoplasmic division, to produce 8 small uninucleate amoebae or megacystis trophozoites. These meta cystic trophozoites pass into the large intestine, invade the mucous lining and grow into mature trophozoites.

Pathogenicity and symptoms:

Infection of E.histrolytica in the intestine from asymptomatic to severe or fatal invasions. Asymptomatic infections are responsible for the spread of the parasite with numerous cysts being passed in normal stools. Diarrheic stools primarily contain trophozoites which cannot persist in the environment.

Invasive forms of the disease lead to amoebic dysentery in which the trophozoites invade the intestinal wall, leading to the formation of amoebic ulcers. This results in severe diarrhoea with blood and mucus present. In such cases, it is important to identify E.histolytica in the stools to differentiate among other cases of dysentery.

If trophozoites penetrate the intestinal wall, serious problems can occur, including liver abscesses, or spread io the lungs and brain, usually resulting in death.

Diagnosis:

Diagnosis, is a simple way, consists in the microscopical detection of trophozoites or cysts in faecal smears. The presence of white , stone shaped 'Charcot-Leyden' crystal in faeces suggests the E. histolytica infection.

Treatment and Prevention:

Treatment of amoebic dysentery is not very difficult but the permanent cure is sometimes hard to achieve.For temporary relief, an alkoxide Emetine is effective. The antimalaria drug, Chloroquine is effective against amoebic abscesses in the liver only. Some of the latest iodine compounds, such as Vioform, Chiniofon, Diodoquin,etc have shown more lasting results. Certain antibiotics such as Fumagillin, Terramycin, Erythromycin and Aureomycin have been used for the eradication of the parasitic.The most advanced in the treatment of amoebiasis has been the use of metronidazole. It is very active against both intestinal and extra- intestinal amoebiasis.

Prevention of infection is entirely a matter of hygiene, both personal as well as municipal. Following measures may be helpful in protection yourself against the disease.

• Sanitary disposal of the faecal matter.
• Proper sanitation of roads, streets, lanes, and open drains.
• Perfect sanitation and protection of water and vegetables from pollution.
• Proper disposal of sewage.
• Washing of hands with antiseptic soap and water before touching the food.
• Cleanliness in preparing the food.
• Covering of the food particles by the traders.
• Perfect sanitation and protection of water and vegetables from pollution.
• Protection of foods and drinks from housefly, cockroaches, etc.
• Raw and improperly washed cooked vegetables should be avoided.
• Chemical treatment of human faeces to be used as fertiliser.
• Periodical examination of food handlers to find out whether they are infected ith E.histrolytica. On positive findings, they should be treated properly.

Reference:

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

https://en.wikipedia.org/wiki/Entamoeba_histolytica

https://msu.edu/course/zol/316/ehisgut.htm

http://www.biologydiscussion.com/parasites/the-structure-and-life-cycle-of-entamoeba-with-diagram/2735