Diseases and conditions
Medical Protozology: Morphology and classification

Medical Protozology: Morphology and classification

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Medical Protozology: Morphology and classification

Protozoa (singular, protozoan), from the Greek ‘protos’ and ‘zoon’ meaning “first animal”, are members of eukaryotic protists. They may be distinguished from other eukaryotic protists by their ability to move at some stage of their life cycle and by their lack of cell wall.

Protozoa are found in all moist habitats. They are common in sea, in soil and in fresh water. These organisms occur generally as a single cell. Colonies of protozoa might also occur in which individual cells are joined by cytoplasmic threads and form aggregates of independent cells.

However, distinct types of protozoa, include a resistant cyst (non-motile) stage to survive adverse environmental conditions, such as desiccation, low nutrient supply, and even anaerobiosis. For example, the soil amoeba, Naegleria is a resistant cyst in dryweather, a naked amoeba in moist soil, and becomes flagellated when flooded with water.

Morphology of protozoa

Protozoa are predominantly microscopic, ranging in size from 2 to more than 100μm. Morphologically, they are within a mass of protoplasm, consisting of a true membrane – bound nucleus and cytoplasm. The nucleus contains clumped or dispersed chromatin and central nucleolus or karyosome, which are useful structures to distinguish protozoan species from one another based on the shape, size and distribution of these structures.

Importance of protozoa

Protozoa serve as an important link in the food chain and ecological balance of many communities in wetland & aquatic environments. They are also important in biological sewage treatment, which involves both anaerobic digestion and/or aeration. In addition, protozoa are important laboratory organisms in research areas, by which their asexual reproduction enables clones to be established with the same genetic make-up. These are useful in the study of cell cycles and nucleic acid biosynthesis during cell division.

Medical concern of protozoa

Protozoa are ubiquitous in moist areas, including the human alimentary canal. From an ecological standpoint, protozoa may be divided into free-living forms and symbiotic forms. Some of the symbiotic ones are parasitic and may cause disease.

Although most amoebas are free-living, several are found as commensal inhabitants of the intestinal tract in humans. One of these organisms Entamoeba histolytica may invade tissue and produce disease. The majority of ciliates are free living and seldom parasitize humans. 

Flagellates of the genus Trypanosomes and Leishmania are capable of invading the blood & tissue of humans, where they produce severe chronic illness. Others such as Trichomonas vaginalis

and Giardia lamblia, inhabit the urogenital and gastrointestinal tracts and initiate disease characterized by mild to moderate morbidity but no mortality.

Sporozoan organisms, in contrast, produce two of the most potentially lethal diseases of humankind: malaria and toxoplasmosis. With the advent of HIV a new and important chapter has been opened; i.e. ‘opportunistic’ parasitosis. Most of the parasitic incidents belong to endocellular protozoa of different genera or species.

Reproduction and regeneration of protozoa

As a general rule, protozoa multiply by asexual reproduction. This is not to say that sexual processes are absent in the protozoa. Some parasitic forms may have an asexual phase in one host and a sexual phase in another host


In most parasitic protozoa, the developmental stages are often transmitted from one host to another within a cyst. The reproduction process is also related to the formation of the cyst. Asexual reproduction of some ciliates and flagellates is associated with cyst formation, and sexual reproduction of Sporozoa invariably results in a cyst. Pathogenic protozoa can spread from one infected person to another by:

Fecal – oral transmission of contaminated foods and water. ·      

Insect bit inoculums or rubbing infected insect feces on the site of bite. ·     

Sexual intercourse


Protozoan organisms are virtually always acquired from an exogenous source, and as such, they have evolved numerous ways to enter the body of the human host. Factors that are important for pathogenicity include: ·        Attachment to the host tissue followed by replication to establish colonization.

Toxic products released by parasitic protozoa.

Shifting of antigenic expression to evade the immune response and inactivate host defenses.

Antiprotozoal agents

Generally the antiprotozoal agents target relatively rapidly proliferating, young, growing cells of the parasite. Most commonly, these agents target nucleic acid synthesis, protein synthesis, or specific metabolic pathways (e.g. folate metabolism) unique to the protozoan parasites.

Classification of Protozoa

Protozoa of medical importance are classified based on their morphology and locomotive system as described below:

Amoebas – Entamoeba histolytica

FlagellatesGiarda lamblia, Trichomonas vaginalis, Trypanosoma spp, Leishmania spp

Cliliophora – Balantidium coli

Coccidian Isospora belli, Cryptosporidium parvum, Toxoplasma gondii, Plasmodium species Protozoan pathogens can also be grouped according to the location in the body where they most frequently cause disease.

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