Protozoa and the Illnesses They Cause

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In This Article

Protozoa is a single-celled organism that is a eukaryote (which are organisms whose cells contain membrane-bound organelles and nuclei). Other eukaryotes include us, other animals, and plants. Eukaryotes also include other microorganisms: algae, helminths, and fungi.

Protozoa are found everywhere. They can live on their own as free-living organisms in the environment, often in the soil, water, or moss. They can also be resting cysts, which lets them survive through dry times. Some are parasites. Others live in symbiosis with other organisms; each relies on the other for survival.


Protozoa are broken down into different classes: Sporozoa (intracellular parasites), flagellates (which possess tail-like structures that flap around for movement), amoeba (which move using temporary cell body projections called pseudopods), and ciliates (which move by beating multiple hair-like structures called cilia).

What's in a Name?

The word protozoa comes from the word protos which was Greek for "first" and zoia which meant "animal". It was first coined in the 1800s. Before then, the microscopic protozoa, defined by their organelles, were unable to be fully appreciated.

Infections caused by protozoa can be spread through ingestion of cysts (the dormant life stage), sexual transmission, or through insect vectors. There are many common—and not so common—infections caused by protozoa. Some of these infections cause illness in millions of people each year; other infections are rare and hopefully disappearing.

What Diseases Do They Cause?

Common infectious diseases caused by protozoans include malaria, giardia, and toxoplasmosis. These infections are found in very different parts of the body — malaria infections start in the blood, giardia starts in the gut, and toxoplasmosis can be found in lymph nodes, the eye, and also (worrisomely) the brain. Likewise, sleeping sickness is caused by a protozoan infection, as is dysentery due to Entamoeba histolytica.

Human African trypanosomiasis is caused by Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense. The former causes most cases (about 98 percent) but both are spread by tsetse fly bites.

Entamoeba histolytica can cause diarrhea and GI upset. It can, in fact, cause amoebic dysentery in severe cases, as well as asymptomatic cases for others. It can also travel through the walls of the intestines and go into the bloodstream and on to other organs, like the liver, where it can create liver abscesses.


Treatment options just depend on what protozoa are infecting you. Some are a lot more successful than others. Malaria is a common illness worldwide that has straightforward treatment, though the treatment depends on what type of malaria (Plasmodium falciparum, Plasmodium knowlesi, Plasmodium malariae, Plasmodium ovale, and Plasmodium vivax). Treatment also depends on whether there is resistance (P. falciparum especially has grown resistant over the last few decades to some important drugs).

Detecting Infections

Unlike other pathogens, samples with protozoa cannot be simply identified through a culture. Sometimes they can be seen under a microscope, whether inside of red blood cells (as in malaria) or in stool (as in Giardia and E. histolytica). There are also rapid blood tests for antibodies or antigens, as well as PCR tests that detect their genetic material.

Toxoplasmosis can be identified in a number of different ways depending on where it's causing an infection. It can be identified through antibody blood tests. It can be found through PCR tests. It can also be found through special stains of tissue and through direct isolation of the pathogen.

Giardia can be found through an antigen test of the stool and also by looking at the stool under a microscope. It may take multiple stool samples (maybe three) to diagnose this.

Entamoeba histolytica can also be identified from stool samples like Giardia. It may be identified under a microscope, through a PCR test, antigen test, or through an antibody test of the blood.

Human African trypanosomiasis can be diagnosed through blood tests or from fluid or biopsy from a lymph node (or a chancre wound). T. b. rhodesiense parasites can usually be found in the blood of infected people. T. b. gambiense has a lower burden of protozoa in the blood so blood microscopy is usually unable to identify it, but microscopic examination of a lymph node biopsy (posterior lymph node) is more likely to identify the infection. 

Can Sleeping Sickness Be Eradicated?

There are plans to make this disease and this protozoa history. Most cases are found in the Democratic Republic of the Congo. There are plans to greatly reduce the spread of the disease (currently the flies that spread the disease are found in at least 36 countries) and greatly reduce the burden of illness. The disease can cause serious neurologic effects and the treatment is difficult. As it strikes in the poorer and resource-limited areas, it can be hard to identify and treat. It would be really nice to have this protozoa become extinct.

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