III. PROTOZOA

B. MEDICALLY IMPORTANT PROTOZOA

The overall purpose of this Learning Object is to introduce some of the human infections caused by protozoa.

LEARNING OBJECTIVES FOR THIS SECTION

Protozoa are unicellular eukaryotic microorganisms lacking a cell wall and belonging to the Kingdom Protista. The vegetative, reproducing, feeding form of a protozoan is called a trophozoite (def). Under certain conditions, some protozoa produce a protective form called a cyst (def) that enable them to survive harsh environments. Cysts allow some pathogens to survive outside their host.

Medically Important Protozoa

1. The Sarcomastigophora (Amoeboflagellates)

The amoebas (subphylum Sarcodina) move by extending lobelike projections of their cytoplasm called pseudopodia (def).

a. Entamoeba histolytica (see photomicrograph) which causes amoebic dysentery. The organism produces protective cysts which pass out of the intestines of the infected host and are ingested by the next host (fecal-oral route).

b. Acanthamoeba can infect the eye, blood, spinal cord, and brain and is transmitted by waterborne cysts picked up while swimming in contaminated water, crossing the mucous membranes.

The flagellates (subphylum Mastigophora) move by means of flagella. Some also have an undulating membrane (def).

a. Giardia lamblia (see photomicrograph) can cause a gastrointestinal infection called giardiasis. Cysts pass out of the intestines of the infected host and are ingested by the next host (fecal-oral route).

b. Trichomonas vaginalis (see photomicrograph) infects the vagina and the male urinary tract. It does not produce a cysts stage and is usually transmitted by sexual contact.

c. Trypanosoma brucei gambiens (see photomicrograph) causes African sleeping sickness and is transmitted by the bite of an infected Tsetse fly.

2. The Ciliophora

The ciliates move by means of cilia.

a. The only pathogenic ciliate is Balantidium coli (see photomicrograph) which causes a diarrhea-type infection. Cysts pass out of the intestines of the infected host and are ingested by the next host (fecal-oral route).

3. The Apicomplexans

The apicomplexans are not motile in their mature forms, reproduce both asexually and sexually, and often have complex life cycles for transmission from host to host. They possess a complex of organelles called apical complexes (def) at their apex that contain enzymes used in penetrating host tissues.

a. Species of Plasmodium (see photomicrograph) cause malaria and are transmitted by the bite of an infected female Anopheles mosquito. They reproduces asexually by schizogony (def) in human liver cells and red blood cells but also reproduce sexually by gametes (def) in the mosquito (see the life cycle of Plasmodium).

b. Toxoplasma gondii is another intracellular apicomplexan and causes toxoplasmosis (see the AIDS pathology tutorial at the University of Utah). It can infect most mammals and is contracted by inhaling or ingesting cysts from the feces of infected domestic cats, where the protozoa reproduce both asexually and sexually, or by ingesting raw meat of an infected animal. Toxoplasmosis is usually mild in people with normal immune responses but can infect the brain, heart, or lungs of people who are immunosuppressed. It can also be transmitted congenitally (def) and infect the nervous system of the infected child.

c. Cryptosporidium is an intracellular (def) parasite that causes diarrhea, although in people who are immunosuppressed it can also cause respiratory and gallbladder infections. It is transmitted by the fecal-oral route (see the AIDS pathology tutorial at the University of Utah).

4. Virulence Factors that Promote Colonization of Protozoans

Virulence factors that promote protozoal colonization of the host include the ability to:

1. contact host cells;
2. adhere to host cells and resist physical removal;
3. invade host cells;
4. compete for nutrients;
5. resist innate immune defenses such as phagocytosis and complement; and
6. evade adaptive immune defenses.


Examples of virulence factors that promote protozoal colonization include:

a. Some protozoa, such as Entamoeba histolytica,Trichomonas vaginalis, Giardia lamblia, and Balantidium coli use pseudopodia, flagella or cilia to swim through mucus and contact host cells.

b. Protozoa use adhesions (def) associated with their cytoplasmic membrane to adhere to host cells, colonize, and resist flushing.

c. Some protozoa, such as the apicomplexans (Plasmodium (inf), Toxoplasma gondii (inf), and Cryptosporidium (inf)) possess a complex of organelles called apical complexes at their apex that contain enzymes used in penetrating host tissues and cells.

d. Protozoans such asTrypanosoma brucei gambiens (inf) and Plasmodium species (inf) are able to change their surface antigens (def) during their life cycle in the human. As the protozoa change the amino acid sequence and shape of their surface antigens, antibodies (def) and cytotoxic T-lymphocytes (def) made against a previous shape will no longer fit and the body has to start a new round of adaptive immunity against the new antigen shape.

e. Some protozoa, such as Entamoeba histolytica (inf) shed their surface antigens so that antibodies made by the body against these surface antigens are tied up by the shed antigens.

 

To view a Quicktime movie of Cryptosporidium and electron micrographs of Giardia and Entamoeba, see the Parasites section of the CELL'S ALIVE web page.

E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.

 

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