The protozoa

5 The protozoa

Protozoa are single-celled animals, ranging in size from 2 to 100 nm. Many species are free-living, but others are important parasites of humans. Some free-living species can infect humans opportunistically. Protozoa continue to multiply in their host until controlled by its immune response or by treatment and thus may cause particularly severe disease in immunocompromised individuals. Protozoal infections are most prevalent in tropical and subtropical regions, but also occur in temperate regions. Protozoa may cause disease directly (e.g. the rupture of red cells in malaria), but more often the pathology is caused by the host’s response. Of all parasites, malaria presents the biggest and most severe global problem and kills > 1.5 million people each year, mostly young children.

Protozoa can infect all the major tissues and organs of the body

Protozoa infect body tissues and organs as:

• intracellular parasites in a wide variety of cells (red cells, macrophages, epithelial cells, brain, muscle)

• extracellular parasites in the blood, intestine or genitourinary system.

The locations of the species of greatest importance are shown in Figure 5.1. Intracellular species obtain nutrients from the host cell by direct uptake or by ingestion of cytoplasm. Extracellular species feed by direct nutrient uptake or by ingestion of host cells. Reproduction of protozoa in humans is usually asexual, by binary or multiple division of growing stages (trophozoites). Sexual reproduction is normally absent or occurs in the insect vector phase of the life cycle, where present. Cryptosporidium is exceptional in undergoing both asexual and sexual reproduction in humans. Asexual reproduction gives the potential for a rapid increase in number, particularly where host defence mechanisms are impaired. For this reason some protozoa are most pathogenic in the very young (e.g. Toxoplasma in the fetus and in neonates). The AIDS epidemic has focused attention on a number of protozoa which give rise to opportunistic infections in immunocompromised individuals. These include Cryptosporidium, Isospora and members of the Microsporidia. New parasites continue to emerge, e.g. Cyclospora cayetanensis, a food-borne and water-borne cause of diarrhea, which became recognized in the early 1990s.