5. Protozoa

the basic facts

Questions

How do protozoa move?

Which protozoa are transmitted by the faecal–oral route?

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, binary fission, in which two daughter cells are formed or multiple fission, in which many daughter cells are formed, 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. Only a small number of protozoa cause human disease but those that do affect millions of people worldwide, causing considerable suffering, mortality and economic hardship (Table 3.5.1). Protozoal diseases are largely confined to countries with poor economic and social structure. However, trichomoniasis, crytosporidiosis and toxoplasmosis are common in developed countries. Some protozoa of medical importance are described in Chapters 32 and 33.

Table 3.5.1 Protozoal infections

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). Examples of common pathogenic protozoa are shown in Fig. 3.5.1.

Fig. 3.5.1 Examples of human pathogenic protozoa. A. Amoebae (e.g. Entamoeba histolytica); B. flagellates (e.g. Giardia lamblia); C. ciliates (e.g. Balantidium coli); D. apicomplexa (e.g. Plasmodium malariae).