Parasites occur in two distinct forms: single-celled protozoa and multicellular metazoa called helminths or worms. For medical purposes, protozoa can be subdivided into four groups: Sarcodina (amebas), Sporozoa (sporozoans), Mastigophora (flagellates), and Ciliata (ciliates). Metazoa are subdivided into two phyla: the Platyhelminthes (flatworms) and the Nemathelminthes (roundworms, nematodes). The phylum Platyhelminthes contains two medically important classes: Cestoda (tapeworms) and Trematoda (flukes). This classification is shown in Figure VI–1.
FIGURE VI–1Relationships of the medically important parasites.
Understanding the life cycle and pathogenesis of protozoa and helminths requires an explanation of certain terms. Many protozoa have a life cycle consisting of a trophozoite, which is the motile, feeding, reproducing form surrounded by a flexible cell membrane, and a cyst, which is the nonmotile, nonmetabolizing, nonreproducing form surrounded by a thick wall. The cyst form survives well in the environment and so is often involved in transmission. Certain protozoa, such as Leishmania and Trypanosoma, have flagellated forms called promastigotes or trypomastigotes and nonflagellated forms called amastigotes.
Many helminths have a life cycle that progresses from egg to larva to adult. The egg contains an embryo that, upon hatching, differentiates into a larval form, which then matures into the adult form that produces the eggs.
There are special terms applied to the host of certain parasites as they proceed through their life cycle. A definitive host is one in which the sexual cycle occurs or the adult is present, and the intermediate host is one in which the asexual cycle occurs or the larva is present.
Regarding the laboratory diagnosis of these infections, examination of the stool for ova and parasites (O&P) is often done. Ova refers to the eggs, and parasites refers to the larval or adult forms.
Eosinophilia is associated with several helminth infections, especially when roundworm larvae migrate through tissue. High eosinophil counts are seen in infections caused by the following roundworms: Ascaris, Strongyloides, Trichina, Toxocara, and the hookworms, Necator and Ancylostoma. Infections with the flatworm (fluke) Schistosoma (a trematode) also elicit eosinophilia. Eosinophils are an important component of the host defense against these parasites.