Study Notes on Entamoeba Histolytica

In this article we will discuss about Entamoeba Histolytica:- 1. Historical Background of Entamoeba Histolytica 2. Geographical Distribution of Entamoeba Histolytica 3. Habit and Habitat 4. Morphology 5. Life Cycle.

Historical Background of Entamoeba Histolytica:

1. Losch (1875) first described Entamoeba histolytica as Amoeba coli.

2. Leidy first established the genus Enta­moeba in 1879.

3. Schaudin established the species En­tamoeba histolytica in 1903 and differ­entiated into pathogenic and non­pathogenic types.

There are 5 species of Entamoeba in hu­man beings of which E. hartmanni and E. coli are non-pathogenic, and E. histolytica, E. gingivalis and E. polecki are pathogenic.

Geographical Distribution of Entamoeba Histolytica:

The parasite is worldwide in distribution and more common in most countries of trop­ics and subtropics rather than temperate zones. E. histolytica is scarcely pathogenic found in human beings of temperate zones.

Habit and Habitat of Entamoeba Histolytica:

Entamoeba histolytica is an endoparasite and the parasite inhabits the mucous and sub-mucous layers of the large intestine of man. It may also occur in the liver, lungs and rarely invades brain, spleen, etc. pro­ducing ulcers, but the cyst is found in the intestinal lumen of man.

Entamoeba histolytica has also recorded in orang-utang, gorilla, chimpanzee, gibbon, baboon, donkey, dog, cat, rat and pig.

Morphology of Entamoeba Histolytica:

(a) Trophic:

The trophozoites vary in size from 15 to 40 micra, the average being 18 to 25 micra. Dobell (1919) and others have shown that the parasite has got two races, one large and the other small. The trophozite of Entamoeba histolytica in living condition shows two distinct portions, ectoplasm and endoplasm. The ectoplasm is clear and trans­lucent while the endoplasm is granular.

The endoplasm often contains injested red blood corpuscles. The pseudopodia may be long, finger-like or short and rounded in shape (Fig. 10.23). In freshly passed stool the para­site is very active and moves rapidly in a straight line with a single clear pseudopod at the anterior end.

This is known as ‘direc­tional movement’. The movement becomes sluggish when the faeces cool down and in this condition the amoeba throws out pseu­dopodia at various directions and remains stationary.

The nucleus is indistinct in living condi­tion but when stained with haematoxylene it shows a small dot-like central karyosome or endosome, a uniform ring of small periph­eral chromatin granules and at times some chromatin granules in between them.

Some­times there may be traces of linin network in the form of fine fibrils in between karyosome and nuclear membrane. The nuclear mem­brane is very delicate. The size of the nucleus is about 4 to 6 micra in diameter.

(b) Cystic:

The cysts of both races of Entamoeba histolytica vary in size from 10 to 20 micra (average 12 micra) in diameter. In haematoxylin stained preparation a matured cyst looks spherical and quadrinucleate. Its cytoplasm is clear and often contains black rod-like chromatoid bars or bodies with rounded ends (Fig. 10.23D).

The young cysts are uninucleate or binucleate and their nuclear structure is just like that of the trophozoites. But it shows a very small central karvosome and a delicate nuclear membrane.

Presence of chromatoid bodies is the char­acteristic of the cysts of Entamoeba histolytica. They occur either singly or in multiples of two or more.

Remark:

About the exact nature of these bodies there is a controversy. Some authorities consider it as nutrient material of the cyst while others believe them as excess of chromatin thrown off during nuclear division.

The chromatoid bodies occur in the early stages of the cysts but they disappear in the mature quadrinucleate cyst.

In young cysts glycogen is present in a diffused state and can be demonstrated in preparation with Lugol’s iodine solution producing a brownish colour.

Life Cycle of Entamoeba Histolytica:

The life cycle of E. histolytica is completed through a single host-man. Hence it is called monogenetic. Trophozoite and cyst stages of the parasite are concerned with the life cycle.

Encystment:

Entamoeba histolytica multi­plies by binary fission in the trophozoite stage. They have the capacity to encyst. Unfavourable conditions in the habitat such as lack of nutrients, temperature deviations from the optimum range, decreased O₂ ten­sions, lowered pH and accumulation of meta­bolic wastes may be the causes for encyst­ment.

Precystic form:

Prior to encystment the trophozoite of each parasite loses its pseu­dopodium, eliminates food vacuoles and becomes spherical, called a precystic form. The diameter of this stage varies 10-20 µm and the structure of the nucleus is like the trophozoite stage of the parasite.

Mature cyst form:

The precystic form secrets a thin, tough and transparent mem­brane around it, called the cyst wall. The animal having a cyst is called a cyst. The process of enclosing in a cyst is called encystment or encystation. At the early stage the cyst contains a single nucleus. The single nucleus is divided mitotically forming two nuclei. This is called binucleate cystic stage.

Then the two nuclei are divided by mitosis and four nuclei occur. The nuclear divisions take place without cytoplasmic division and this tetra-nucleate cyst is called mature cyst. The whole process of encystment takes a few hours and the mature cyst lives in the lumen of the intestine of host only two days.

Tolerance of the cyst:

The cysts of E. histolytica can survive about one month in water and about 12 days on dry land. They can tolerate the temperature up to 50° Celsius and 4 hours in formaldehyde solution.

Infection:

At the tetranucleate stage the cyst is infective to a new host. The infective cysts pass out through the host’s faeces and are introduced into the gut lumen of a new host through the contaminated drink, food and vegetables.

Excystment:

Then the infective cysts pass into the lower portion of the small intestine (colon) of the new host. Here the process of excystment occurs. The excystment is the process by which the cysts are transformed into the trophozoites.

The cyst wall in the colon becomes permeable by the action of intestinal enzymes, the trypsin of the intes­tine. The cyst wall ruptures and 4-nucleate amoeba emerges out from the cyst.

Factors for excystment:

Temperature, pH, chemical composition of the medium and the flora of the bacteria may be the reasons for excystment.  

Metacystic form:

After the emergence of quadrinucleate amoeba, the division of cyto­plasm immediately ensues and produces four small metacystic trophozoites.

Trophozoites:

Both the nucleus and cytoplasm of each metacystic trophozoite divide and as a result 8 small amoebulae are produced (Fig. 10.24). These are called young uninucleate trophozoites. They are motile and penetrate the mucous membrane.

The young trophozoites feed on host tissues, blood, bacteria and yeast and gradually in­crease in size to attain maturity. Inside the tissues the trophozoites multiply and start the procystic form of the life cycle.

Remark:

The exact nature of the divi­sion of the nucleus is contro­versial but it is believed by many authors that it is pro­bably a modified type of mito­sis. The chromosome number of Entamoeba histolytica is stated to be six.

Transmission:

Cysts of Entamoeba are transmitted from one individual to another in a variety of ways:

1. The cysts are generally transmitted with food or drink.

2. House flies and cockroaches may trans­mit cysts mechanically.

3. Raw vegetable is also another source of infection.

4. In many countries human faeces are used as fertilizer and thus roots and leaves of plants remain contaminated with viable cysts. Food handlers are also sometimes responsible for the spread of infection owing to imperfect personal sanitary measures.

Symptoms:

Amoebiasis or amoebic dysentery is caused by the infection of E. histolytica.

The symptoms are:

(i) Loose stool

(ii) Stool with mucous and blood

(iii) Irregularity of bowl clearance

(iv) Severe abdominal pain, etc.

Prophylaxis (Prevention of the disease):

The infection of E. histolytica can be pre­vented and the disease caused by the infec­tion of the parasite can be controlled by the following ways:

1. Before meal the hands should be with antiseptic soap.

2. Use of boiled drinking water.

3. The raw vegetables and fruits must be thoroughly washed before use.

4. Foods and drinks must be protected from the contamination by house flies and cockroaches.

5. Proper maintenance of the sanitary dis­posal of faeces.

6. Protection of drinking water supply lines from faecal pollution.

Treatment:

Chloroquine, Emetin, Dehydroemetine, Terramycin, Erythromycin, Aureomycin, Thiocarbarsone, Vioform, Diodoquin, Tinidazole Metranidazole (Flagyl) and Niridazole are effective drugs for amoebiasis and should be used as per doctor’s prescription.