Protozoa and the Illnesses They Cause

Protozoa are single-celled organisms classified as eukaryotes (organisms whose cells contain membrane-bound organelles and nuclei). Other eukaryotes include:

• Humans
• Other animals
• Plants
• Algae
• Helminths
• 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 protozoa are parasites. Others live in symbiosis with other organisms, each relying on the other for survival.

Protozoa

Protozoa are broken down into different classes:

• Sporozoa (intracellular parasites)
• Flagellates (with tail-like structures that flap around to move them)
• Amoeba (which move using temporary cell body projections called pseudopods)
• Ciliates (which move by beating multiple hair-like structures called cilia)

Infections caused by protozoa can be spread through ingestion of cysts (the dormant life stage), sexual transmission, or through insect vectors.

Many common—and not so common—infections are caused by protozoa. Some of these infections cause illness in millions of people each year; other infections are rare and may be disappearing.

Protozoan Diseases

Common infectious diseases caused by protozoans include:

• Malaria
• Giardia
• 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.

Other protozoan disease include:

• African trypanosomiasis ("sleeping sickness"): Caused by Trypanosoma brucei gambiense (98% of cases) and Trypanosoma brucei rhodesiense (2%). Both are spread by tsetse fly bites.
• Amoebic dysentery: Due to Entamoeba histolytica), which causes diarrhea and GI upset. 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

Treatment options 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 the type of malaria (Plasmodium falciparum, Plasmodium knowlesi, Plasmodium malariae, Plasmodium ovale, and Plasmodium vivax).

Treatment also depends on whether the protozoa is drug resistant. P. falciparum especially has grown resistant to some important drugs over the last few decades.

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

Toxoplasmosis can be identified in a number of different ways depending on where it's causing an infection, including: 

• Antibody blood tests
• PCR tests
• Special stains of tissue
• Direct isolation of the pathogen

Giardia

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 

E. 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

Human African trypanosomiasis can be diagnosed through blood tests or from fluid or biopsy from a lymph node (or a chancre wound). 

Trypanosoma brucei rhodesiense

T. b. rhodesiense parasites can usually be found in the blood of infected people. 

Trypanosoma brucei gambiense

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.