Protozoa, from the Greek meaning ‘first animal’, refers to simple, eukaryotic organisms composed of a microscopic single cell. Reproduction is through simple asexual cell division, or binary fission, in which two daughter cells are formed or, if many daughter cells are formed, multiple fission. Certain protozoa have complex life cycles involving both asexual (schizogony) and sexual reproduction. Some protozoa form resistant cysts that can survive in the environment.
There are over 65 000 known species of protozoa, of which approximately 10 000 are parasites, deriving nourishment and environmental protection from inhabiting a living animal host. However, the majority of parasites are non-pathogenic, living as harmless commensals within the host. Some animals can harbour parasites and serve as reservoirs for human disease. Infections that are naturally transmitted between animals and humans are termed zoonoses. These may be acquired either by direct contact with an animals or indirectly through the ingestion of contaminated water and food. Some zoonoses are spread by the bite of an insect, termed a vector in which part of the organism’s life cycle is completed.
Only a small number of protozoa cause human disease but those that do affect millions of people world-wide, causing considerable suffering, mortality and economic hardship. Protozoal diseases are largely confined to countries with poor economic and social structure. However, trichomoniasis, crytosporidiosis, and toxoplasmosis are common in developed countries.
The pathogenic protozoa are part of the subkingdom Protozoa. Those of medical importance are placed in the phyla Sarcomastigophora, Apicomplexa and Ciliophora. Within these phyla the protozoa are divided into four major classes based on their locomotive form: the amoebae (Sarcodina), the flagellates (Mastigophora), the sporozoa, and the ciliates (Kinetofragminophorea).
Amoebae: These are the simplest of the protozoa and are characterised by a feeding and dividing trophozoite stage that moves by temporary extensions of the cell called pseudopodia (‘false feet’). In some species the trophozoite can form a resistant cyst stage able to survive in the environment. Those that infect the gut are true parasites being unable to reproduce except in a living host. Others occur naturally in soil and water and are not true parasites. They are termed ‘free-living’ and infect humans as opportunistic pathogens.
Flagellates: These organisms have a trophozoite form but also possess flagella for locomotion and food gathering. All pathogenic species are true parasites, being unable to reproduce outside the host.
Ciliates: These possess rows of hair-like cilia around the outside of the body for motility and also to direct food into a primitive mouth termed a cytostome. All ciliates possess two nuclei: a large polyploid micronucleus and a small micronucleus active only during sexual reproduction. Some species form cysts.
Apicomplexa: This is a unique group lacking any visible means of locomotion. They are all parasitic and most are intracellular, having a life cycle involving both sexual and asexual
reproduction. The common feature of all members is the presence of an apical complex (visible only by electron microscopy) at the anterior pole in one or more stages of the life
cycle. The exact components of the apical complex vary among members. Its function is thought to enable cell penetration.
Diagnosis and treatment The relatively large size of the protozoa enables most human pathogens to be easily identified by microscopic examination of clinical material. Those of the gut are observed in freshly taken faecal samples. Blood and tissue protozoa are visualised after staining. Detection of elevated antibodies to the infecting organism may be diagnostic in some instances (e.g. toxoplasmosis). Culture methods are not routinely used, as they are technically demanding and time consuming: an exception is the diagnosis of trichomoniasis.
Antiprotozoal therapy is generally unsatisfactory. Treatment is hampered by lack of effective agents for many diseases, their potential toxicity to humans and inability to destroy all forms of the organism. The emergence of drug resistance has also limited the therapeutic potential of many agents.