Microbiology III ; DOMAIN EUKARYA, Protozoans

DOMAIN EUKARYA

B. Protozoans.

Protozoans are unicellular, nonphotosynthetic eukaryotes.

1. Taxonomy and nomenclature. The protozoans are classified into phyla by mode of motility or life cycle, and then named by genus and species based on the degree of relatedness. 

a. Flagellates, which are distributed over four phyla, move by flagella—the arrangement of which can vary in different species (e.g., Giardia, Trichomonas, Trypanosoma, and Leishmania).

b. Amebas (phylum, Amoebozoa) move by extrusion of pseudopodia (e.g., Entamoeba and Acanthamoeba).

c. Ciliates (phylum, Ciliophora) move by coordinated motion of hairlike structures (cilia). (The ciliates contain no major contributors to human disease although Balantidium is considered a minor human pathogen.)

d. Sporozoans (phylum, Sporozoa) have complex life cycles that include sexual and asexual reproduction phases, often involving invertebrate and vertebrate hosts (e.g., Plasmodium, Toxoplasma, and Cryptosporidium).
 

2. The structure of the protozoans varies with the organism and the stage of the life cycle. The life cycle of many protozoan parasites (e.g., amoebae, Giardia, cryptosporidia) consists of two stages:
the motile trophozoite, which is found in the host, and the nonmotile cyst, which is transmissible and environmentally stable due to its thick cell wall. 

Protozoa that do not form cysts are transmitted person-to-person (e.g., Trichomonas vaginalis) or use an arthropod vector for transmission (e.g., Plasmodium species). 

Diagnosis of protozoan infections is often based on the morphology of the trophozoites or cysts in the clinical samples.

3. Physiology. The nutritional requirements of the parasitic protozoa are generally acquired by pinocytosis/phagocytosis or via simple diffusion. With respect to respiration, most are facultative anaerobes.

4. Pathogenesis of selected protozoan infections
a. Intestinal and urogenital protozoa
(1) Entamoeba histolytica causes amebic dysentery and liver abscess. It is acquired by the fecal–oral route in contaminated food or water. Other amoebae that infect humans tend to cause a mild, transient diarrhea.

(2) Giardia lamblia, a flagellate, is responsible for giardiasis, which is a watery diarrhea that can persist for weeks or months and can be associated with malabsorption.

(3) Cryptosporidium species are acquired via contaminated water, although person-to-person spread can occur. Infections of healthy people usually results in a mild, self-limiting diarrhea, although infections can last for months in immunocompromised individuals, resulting in major fluid loss or disseminated infections.

(4) T. vaginalis, transmitted by sexual contact, causes vaginitis or urethritis, although many infections are asymptomatic.

b. Blood and tissue protozoans
(1) Malaria is caused by five plasmodia species (Plasmodium vivax, P. ovale, P. malariae, P. falciparum, and the newly described P. knowlesi), which are transmitted through the bite of an infected mosquito. Much of the pathology of malaria is due to the destruction of the red blood cells during the infection.

(2) Toxoplasma gondii is usually acquired by ingestion of cysts in undercooked meat or ingestion of cat fecal contamination. Infections are usually mild, although, when acquired during pregnancy, can be transmitted to the fetus resulting in abortion, stillbirth, or neonatal disease (chorioretinitis, blindness, encephalitis, and hepatosplenomegaly).

5. Therapy. (will be discussed in more detail later) 

Drugs from the nitroimidazoles class, such as metronidazole (Flagyl) and tinidazole (Tindamax), tend to be the preferred treatment for most protozoan infections (Giardia, Trichomonas, and Entamoeba), 

Although pyrimethamine and sulfonamides are often alternative therapies. 

Chloroquine has been the treatment of choice for malaria (Plasmodium species), although chloroquine resistance is common in some parts of the world. 

Recommendations for chemoprophylaxis and treatment of malaria vary depending on the profile of drug resistances in the area of travel, the species of Plasmodium, and the health status of the patient.

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Ref

http://en.wikipedia.org/wiki/Protozoa

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