NEET Biology Human Health And Disease Notes - Diseases Caused by Protozoa

Notes - Diseases Caused by Protozoa

Category : NEET

Diseases Caused by Protozoa


Diseases Caused by Protozoans : Protozoans cause many diseases in humans. The major ones in our country are amoebiasis, diarrhoea, ciliary dysentery and malaria. Some diseases are given below:

        (a) Amoebiasis (Amoebic Dysentery, Enteritis) : Amoebiasis is widespread in India due to poor sanitary conditions and polluted drinking water. The disease is caused by Entamoeba histolytica all over the world. The parasites live in the large intestine and lower part of the small intestine of humans. Infection occurs by ingesting cysts with food and drinks.

        The parasites secrete a proteolytic enzyme, cytolysin, that erodes the mucous membrane of the intestine. This may form bleeding ulcers that produce dysentery. In this disease, the patient passes out blood and mucus with the stools. He also experiences severe gripping pain in the abdomen, fever, nausea, exhaustion and nervousness. In chronic cases, the intestinal will is punctured. This may prove fatal. The parasites that invade the intestinal mucous membrane may be carried by the blood stream to the liver, lungs and brain. In these organs, the parasites, feed on cells and produce severe lesions and abscesses. The latter may cause death.

        (b) Diarrohea : Diarrohea is caused by a flagellate protozoan named Giardia intestinalis. Giardia was discovered by Leeuwenhoek in his own stools in 1681. It is the first human parasitic protozoan known. It is found all over the world. It inhabits the upper parts (duodenum and jejunum) of human small intestine all over the world. It lives firmly attached to the intestinal mucous membrane by adhesive disc, each perched on a separate cell. Nutrition is saprozoic, i.e., fluid food is absorbed through the body surface. Reproduction occurs by longitudinal binary fission. At intervals the parasites change into cysts which escape with the host?s faeces. Infection occurs by taking cysts with food and drinks. By covering the mucous membrane of the intestine, the parasites check or reduce the absorption of food, particularly fats. This causes diarrhoea or giardiasis (very loose and frequent stools).

        Preventive Measures : Properly washing hands, fruits and vegetables before eating, and protecting the food articles from dust, flies, ants and cockroaches can check human infection.

        (iii) Malaria : Malaria has been for thousands of years a very serious disease of the tropical and temperate regions. It was almost eliminated a few years back with the efforts of World Health Organization (WHO) and our National Malaria Eradication Programme (NMEP), but unfortunately, it has appeared again.

        Symptoms : The attack of malaria is preceded by yawning, tiredness, headache and muscular pain. During the fever, the patient feels chilly and shivers, and has acute headache, nausea and high temperature. After a few hours, the body perspires freely and the temperature becomes normal. The cycle is repeated if no medicine is taken. Blood smear made during fever shows the malarial parasites. No parasites are seen at other times. In chronic cases, there is general weakness and anaemia (paleness) due to large-scale destruction of red blood corpuscles. This is also accompanied by enlargement of spleen and liver.

        Cause : Malaria is caused by the toxins produced in the human body by the malarial parasites, Plasmodium.

        Transmission : The malarial parasites are carried from the infected to the healthy persons by the female Anopheles mosquito. The mosquito picks up the parasites with the blood, when it bites an infected person. When this infected mosquito bites a healthy person, parasites migrate into his blood with the saliva, which the mosquito injects before sucking up blood to prevent its clotting.

        Types : There are four species of Plasmodium, which cause different kinds of human malaria ?

        (1) P. Vivax : It causes benign tertian malaria, which attacks every third day, i.e., after 48 hours. The fever is mild and seldom fatal. This species is wide-spread in the tropical and temperate regions.

        (2) P. ovale : It also causes benign tertian malaria, which recurs every 48 hours. This species is found only in West Africa and South America.

        (3) P. malariae : It causes quartan malaria, which recurs every fourth day, i.e., after 72 hours. This species is found in both tropical and temperate regions, but it is not very common.

        (4) P. falciparum : It alone is capable of causing three types of malaria, viz., quotidian malaria, which attacks almost daily, malignant tertian malaria, which occurs every 48 hours, but is very severe and often fatal; and irregular malaria. This species is found only in the tropical region.

        Incubation Period : The incubation period for malaria caused by Plasmodium vivax is about 10 days.

        History : The name malaria was given by Mucculoch in 1872 on the belief that it was caused by the foul air of the marshy localities (Italian mala = bad, aria = air). In 1880 Laveran, a French army medical officer, discovered the malarial parasites in the blood of a malarial patient. Sir Ronald Ross of the Indian Medical Service established the ?mosquito-malaria relationship? on August, 29 1897, ever since called the ?Mosquito Day?.

        Life-history : Plasmodium completes its life cycle in two phases and two hosts : asexual phase in the human host and sexual phase in the female Anopheles mosquito host.

        r Ciliary Dysentery : Ciliary dysentery is caused by a ciliate protozoan named Balantidium coli. The latter inhabits the human large intestine (colon) all over the world. It feeds on tissue fragments, red blood corpuscles, bacteria and faecal matter. It reproduces asexually by transverse binary fission and sexually by conjugation. The latter is followed by cyst formation. Cysts pass out in the host?s faeces. Infection occurs by ingesting cysts with food and drinks. Balantidium coli causes ulcers in the colon and invades mucous membrane by secreting cytolysin. This generally results in diarrhoea, but may lead to severe or fatal dysentery.


Pathogenic Protozoa

S. No. and name of parasite

Host and site of parasite in its body

Diseases caused

Method of transmission

Class Rhizopoda




(1)  Entamoeba histolytica

In the colon of man, sometimes in dogs and cats also. It may reach liver, spleen, lungs and brain etc.

Amoebic dysentry. It also causes ulcers in the Intestine.

By contaminated food and water.

(2)  Entamoeba coli

In the colon of man.

Gastro-intestinal disturbances.

By contaminated food and water.

(3)  Entamoeba gingivalis

In the buccal cavity of man.

Bleeding gums.

By mouth contact.

Class Mastigophora




(4)  Trypanosoma gambiense

In the blood of Africans.

African sleeping sickness.

By the bite of the fly, Glossina palpalis.

(5)  Trypanosoma rhodesiense

In the blood of Africans.

Rhodesian sleeping sickness.

By  the bite of the fly, Glossina morsitans.

(6)  Trypanosoma cruzi

In early stages, it is found in the muscles, heart,  brain, spinal cord and gonads of children but in later stages in the blood

Chaga?s disease.

By a bug.

(10) Leishmania donovani

In the liver, lymph glands and leucocytes of man, dog and cat.

Kala-azar fever.

By sand fly, Phlebotomus supp.

(11) Leishmania infantum

In the spleen of children

Enlargement of spleen.

By sand fly, Phlebotomus supp.

(12) Leishmania tropica

In endothelium of blood capillaries of skin of man

Oriental sore.

By sand fly, Phlebotomus supp.

(13) Leishmania brasiliensis

In the infected man, dog and cat.

Skin disease (Espundia in man).

By sand fly, Phlebotomus and contact.

(14) Trichomonas buccalis

In the infected gums of man.

Associated with pyorrhoea.

By infected food.

(15) Trichomonas hominis

In colon of man and other vertebrates.

Associated with dysentry.

By contaminated eatables and water.

(16) Trichomonas vaginalis

In urinogenital tract of women.


During sexual intercourse.

(17) Giardia intestinalis

In small intestine of man


By contaminated food.

Class Sporozoa




(18) Plasmodium vivax

      In erythrocytes and liver of man.

Different types of malaria fever.

By the bite of female Anopheles mosquito.

(19) Plasmodium falciparum

(20) Plasmodium malariae

(21) Plasmodium ovale

(22) Babesia bigemina

In erythrocytes of cattle.

Taxas fever and diarrhoea.

By the bite of fleas.

(23) Isospora hominis

In small intestine of man.

Diarrhoea and other gastric troubles.

By contaminated food.

(24) Eimeria stiedae

In cells of mucous membrane of hepatic ducts and liver of rabbit.

Diarrhoea and liver disorders.

By their oocysts.

Class Ciliata




(25) Balantidium coli

In colon of human beings.

Ulcers in colon and diarrhoea.

By spores.


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