Types of parasitism in protozoa and human diseases

Types of parasitism in protozoa

Before discussing parasitism in protozoa we have to understand the parasite and parasitism. We know that parasites can be defined as organism exists on another organism, parasites are two types one is ectoparasites, living on external body organism and another is endo-parasites, living inside the body of another organism. There are several parasitic species of protozoans.

a. Ectoparasites

There are only a few protozoans that have ectoparasitic nature such as Hydramoeba hydroxena, which is ectoparasites of hydra and feed on epidermal cells. And Ichthypthirius multifilis, parasites of the epidermis of freshwater fishes.

b. Endoparasites

There are several categories of endoparasites protozoan –

Parasites of the gastrointestinal tract

These are endo-parasites living in the lumen of the gastrointestinal tract and feed on the mucus of the intestinal wall and digested food present in the intestine of hosts. Some endo-parasites of the gastrointestinal tract are as follow- Giardia lambliaEntamoeba histolyticaBalantidium coli, etc.

Parasites of the buccal cavity (mouth)

Some endo-parasites inhabit the mouth of a host, such as Entamoeba gingavalis reside in the human’s mouth particularly in the pocket between teeth and the gum cause pyorrhoea in teeth, and Trichomonas tenax also inhabit the same and cause pyorrhoea.

Parasites of the genital tract

Few protozoans like to inhabit the genital tract of the host, such as Trichomonas vaginalis reside in the vagina of females and cause leucorrhoea.

Parasites of body tissue

Some parasites enter into host tissue through the body surface or from the gastrointestinal tract and reside within the tissues of the host. Such as Trypanosome, Leishmania, Plasmodium, among these, plasmodium is parasites of blood, a type of connective tissue.

c. Hyper-parasites

When parasites reside on other parasites it is called hyper-parasitism and residing parasites are called hyper-parasites. Some protozoan parasites also show hyper-parasitism and reside on another protozoan parasite. For example, certain Amoeba parasitized Zelleriella living in the frog’s intestine. And Nosema notabilis parasitize Sphaerospora polymorpha living in toad fish’s urinary bladder.

Pathogenic/disease-causing parasites

Mainly protozoans are parasites without causing disease or produce mild symptoms, but on other hand, certain protozoan parasites cause severe diseases in their host. These disease-causing parasites are called pathogenic parasites. For example, Plasmodium vivax cause malaria in the host, Trypanosoma gambiense causes sleeping sickness in the host, and Leishmania donovani cause kala-azar in their host.

Host specificity among parasitic protozoan

Most parasites are host specific, one particular type of parasite inhibits the particular body part of particular parasites. Some parasites can parasitize a wide range of hosts such as TrypanosomaEntamoeba, etc. Host-specific parasites can’t infect another host such as Plasmodium.


Protozoan parasites are transmitted to their host in several different ways. Some parasites are transmitted by contaminated food and water such as Entamoeba coli, on other hands some parasites are transmitted by the intermediate host such as Trypanosoma is transmitted by a tse-tse fly (Glossina palpalis), Leishmania donovani is transmitted by sand-fly PhlebotomusPlasmodium is transmitted by the female anopheles mosquito.

Life cycle

Many parasites complete their life cycle in a single host they are known as monogenetic such as Entamoeba, on other hand, some parasites complete their life cycle in two different the host, primary host, and secondary host they are called as digenetic such as Plasmodium, Trypanosoma, Leishmania.

Types of parasitism in protozoa and human diseases
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Protozoan and human diseases

There are several types of human diseases present, which are caused by protozoans. Some of the diseases caused by protozoan are as follow –


Amoebiasis is caused by Entamoeba hystolytica, which is a trophozoite, which lives in the gastrointestinal tract. Secrete the histolytic enzyme and feed on intestinal cells causing the lesion/ulcer in the intestinal tract. When these ulcer ruptures and discharges blood and mucus into the intestine which passes out with stools. Hence, Entamoeba hystolytica cause bloody stool also called amoebic dysentery.


Giardiasis is caused by Giardia (Grand old man of the intestine). Parasites live in the small intestine of man and cause diarrhea, others symptom is epigastric pain and abdominal discomfort headache loss of appetite. Parasites forms cysts during the life cycle and these cysts pass out with feces, cyst inter into the new host by contaminated food and water, Giardia infection is more common in children.


Trypanosomiasis is caused by species of Trypanosoma parasite that live in intestine blood of the vertebrate host and within the gut of the invertebrate host. Trypanosoma gambiense causes sleeping sickness, a dangerous disease of the human being in tropical Africa. Trypanosoma is transmitted by the invertebrate host tsetse fly (Glossina palpalis). Trypanosome cruzi causes Chaga’s disease in America.


Leishmaniasis is caused by species of Leishmania. Parasites live in phagocytic cells in the vertebrate host and within the gut of the insect (Invertebrate host). Leishmania donovani cause Kala-azar or visceral Leishmaniasis. Mostly this disease occurs in India, South China, and Mediterranean countries. The main symptom of the disease is an enlargement of the spleen. Leishmania tropica causes skin Leishmaniasis. Species of Leishmania are transmitted by sand-fly (Phlebotomus).


Trichomoniasis is caused by Trichomonas vaginalis. Pathogen inhabits the vagina of women and causes vaginitis. The main symptom of the disease is inflammation and itching in the vagina and frothy vaginal discharge. The disease may be transmitted by sexual intercourse by male members, males act as intermediate. Symptoms do not appear in males but sometimes infection in the urethra and prostate.


Malaria is caused by the species Plasmodium, which is a sporozoan parasite. Plasmodium has four species Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparumPlasmodium vivax causes benign tertian malaria, fever occurs every third day. Plasmodium ovale cause mile tertian malaria, fever occur every third day. Plasmodium malariae cause quartan severe malaria, fever occurs every fourth day. Plasmodium falciparum causes malignant tertian malaria, fever occurs every third day, this is a very dangerous type of malaria with a high death rate. The fifth type of malaria is also present, it is caused when more than one type of malarial-parasites infect a single person simultaneously.


Toxoplasmosis is caused by Toxoplasma gondii a sporozoan parasite, occur all over the world. Parasites live in reticuloendothelial and nerve cells. Parasites remain in the host and do not cause any symptoms, but when infected female conceive pregnancy abortion takes place. If infection occurs during the third month of pregnancy then it affects the nervous system of the developing fetus, and the infant dies after birth.

Balantidial dysentery

Balantidial dysentery is caused by Balantidium coli. The symptom of the disease is diarrhea and ulcer of the large intestine. The disease is caused by contaminated food & water.

References and Sources

  • Text Book of Zoology Invertebrates by R.L. Kotpal Fourth Edition.
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  • link.springer.com PARSITIC PROTOZOA.
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