Diseases caused by protozoa

1. Amoebiasis

Amoebiasis is caused in man by a protozoan Entamoeba histolytica.

This is also known as amoebic dysentery.

Entamoeba histolytica was first discovered by Lambl (1859).

Friedrick Losch, a Russian zoologist in 1875, rediscovered this protozoan in the faeces and intestinal ulcers of dysentery patients and succeeded in transferring it to puppies.

Entamoeba histolytica is a microscopic endoparasite of man and is commonly found harbouring the lumen of the upper part of the large intestine i.e. the colon.

The life cycle of entamoeba

It invades the mucosa and sub-mucosa of the intestinal wall and causes amoebic dysentery or amoebiasis.

The trophozoites of the parasite make their way deep by eating through the mucosa of the intestinal wall.

Here they multiply by binary fission and spread rapidly outward to form flask-shaped ulcers containing cellular debris, lymphocytes, blood corpuscles and bacteria.

This causes the formation of abscesses in the intestinal wall.

The penetration into the sub-mucosa by the trophozoites is made possible by histolysis as well as cytolysis.

The mechanism involves the dissolution and necrosis of tissues and cells by a proteolytic enzyme of the nature of histolysin secreted by the trophozoites themselves.

As the sub-mucosa is eroded by the trophozoites the ulcers burst and the blood capillaries rupture.

The blood and the ulcer contents pour into the lumen of the intestine and pass to the outside with stool.

This characterises amoebic dysentery or amoebiasis.


The stool of a dysenteric person is usually acidic and consists of swarms of Entamoebae as well.

A person suffering from amoebic dysentery has repeated blood-mixed, slimy and foul-smelling motions.

Sometimes, the trophozoites make their way through the blood circulation, into the brain, liver, spleen, lungs and gonads.

Here also they destroy the tissues and cause the formation of abscesses (cavities containing pus).

Within the liver, the trophozoites cause severe lesions affecting metabolic activities.

The formation of abscesses in the brain usually proves fatal.

It mainly occurs by ingestion of tetranucleated cysts in food or drinks.

Diagnosis consists of microscopical detection of trophozoites or cysts in faecal smears.

The presence of white, stone-shaped ‘Charcot-Leydon’ crystals in faeces suggests the E. histolytica infection.

Diagnosis and Treatment

Treatment of amoebic dysentery is not very difficult but the permanent cure is sometimes hard to achieve as relapses do occur.

For temporary relief, an alkaloid Emetine is effective.

A synthetic derivative called Dehydroemetine is equally effective.

The most significant advance in the treatment of amoebiasis has been the use of Metronidazole and Tinidazole as an amoebicide.

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.

2. African trypanosomiasis

African trypanosomiasis is a protozoan disease, also known as African sleeping sickness, caused by parasitic protozoa of the genus Trypanosoma.

There are two forms of this disease-

  • west African trypanosomiasis
  • East African trypanosomiasis

West African trypanosomiasis is caused by Trypanosoma brucei gambiense and East African trypanosomiasis by Trypanosoma brucei rhodesiense.

T.B. Gambiense is responsible for more than 95% of cases.
T.B. rhodesiense causes acute infections, which can result in central nervous system involvement with parasites crossing the blood-brain barrier.
There is no vaccine available for trypanosomiasis. Pentamidine or suramin drugs are used for treatment.

3. Giardiasis (= Diarrhoea)

Also known as ‘Backpacker disease’ because travellers are some of the most vulnerable to this disease.

It is caused by a zooflagellate protozoan named Giardia intestinalis.

It is the first human parasitic protozoan known.

It lives in the upper parts (duodenum and jejunum) of the human small intestine.

It is found all over the world.

The parasites perch over the living cells of the intestinal wall by means of their adhesive discs.

They absorb nourishment from the food passing through the intestine, grow and multiply through binary fission.


A large number of parasites interfere with the digestion and absorption of food.

This causes epigastric pain, abdominal discomfort, diarrhoea, headache and sometimes fever.

The disease caused by Giardia is popularly known as giardiasis or diarrhoea (watery and frequent stools).

4. Leishmaniasis

Leishmaniasis is a disease caused by various species of the protozoan genus Leishmania. Leishmania parasites enter mammalian hosts through the female phlebotomine sandfly vector.

Amastigotes are obligate intracellular parasites that normally reside in a phagolysosome compartment within macrophages.

There are three different forms of Leishmaniasis-

  • Visceral leishmaniasis (VL),
  • Cutaneous leishmaniasis (CL)
  • Mucocutaneous leishmaniasis (MCL).

5. Toxoplasmosis

Toxoplasmosis is a protozoan disease caused by an obligate intracellular protozoan Toxoplasma gondii.

6. Malaria

It is caused by Plasmodium.

Learn more about Malaria

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