Intestinal Protozoa 101

Protozoa are single-cell, eukaryotic organisms. There are over 200,000 species of protozoa, many of which are free-living and are not parasitic to humans.

NOTE: protozoan = singular, protozoa = plural.

1. Protozoa are unicellular while helminths are pluricellular.
2. Compared to helminths, protozoa are more associated with mortality than with morbidity (for example, Plasmodium spp, Naegleria spp. Toxoplasma spp. Trypanosoma brucei).
3. Protozoan parasites are able to replicate within a given host, often resulting in hundreds of thousands of new cells within just a few days of initial infection. Different from helminths in which infection with one egg usually results in a single adult parasite.

We will focus on two groups of protozoa:

1. Sarcomastigophora: characterized by the presence of flagella and/or pseudopodia. Some examples: Entamoeba spp. Acanthamoeba spp., Giardia spp., Trichomonas vaginalis, Trypanosoma spp. and Leishmania spp.
2. Apicomplexa: they are strict intracellular organisms, and are characterized by a unique apical complex that enables them to invade cells (hence, the name apicomplexa). They all have both sexual and asexual reproduction. Examples: Plasmodium spp. Toxoplasma gondii, Cryptosporidium parvum, Babesia spp.

We know this classification is confusing and boring. We mention it because once you go through the material, you will realize that some parasites are closely related to each other and behave similarly. Associating them based on their phylum will help you remember better. However, for clinical purposes we classified it into something simpler:

1. Intestinal protozoa: primarily affect the intestine.
2. Blood and tissue protozoa: primarily affect other organs and systems.

All intestinal protozoa are mostly food- and/or water-borne. Exceptions are noted in each lesson. We will describe three lessons: Giardia spp., Entamoeba histolytica and intestinal coccidia (Cryptosporidium, Cyclospora and Cystoisospora). Microsporidia are fungi, NOT protozoa. However, they’re grouped with intestinal coccidia because they share many clinical similarities.

Giardia and Entamoeba are in the same phylum and have two parasitic stages: a cyst (infective form) and a trophozoite (pathogenic form). Cysts are environmentally resistant enabling them to lie dormant in the environment for long periods of time. Intestinal coccidia are in a different phylum (apicomplexa) and have a more complex biology, which we’ll describe later.

With the exception of the free-living amoebas, Trichomonas vaginalis and Toxoplasma gondii, all blood and tissue protozoa described in this course are vector-borne. They tend to have very complex biology and are the cause of major disease burden worldwide (for example, malaria).

In this course we will describe: Plasmodium spp. (malaria), Leishmania spp. (leishmaniasis), Toxoplasma gondii (toxoplasmosis), Trichomonas vaginalis, Trypanosoma brucei (sleeping sickness) Trypanosoma cruzi (Chagas disease), Babesia spp. and free-living amoebas (Acanthamoeba, Balamuthia, and Naegleria).

While the biology of parasitic protozoa varies widely from group to group, these organisms share many similarities:

1. Unit membrane: works in a similar fashion to all other eukaryotic cells. Nutrients may be actively transported, phagocytized or moved into the cell by pinocytosis. Exceptions: Giardia lamblia and Dientamoeba fragilis.
2. Mechanisms of motility: some protozoa have an organelle that aids with movement (e.g., cilia, flagella, pseudopods).
3. Reproduction: all species reproduce asexually (usually by binary fission - which means that the division of one cell generates two identical new organisms). Some protozoa are capable of sexual reproduction in the definitive host, resulting in the formation of a zygote.
4. Metabolic pathways: varies from group to group, some are anaerobic (e.g., Trichomonas vaginalis) while others are totally aerobic (e.g., Plasmodium spp.).