Parasites occur in two distinct forms: single-celled protozoa and multicellular metazoa called helminths or worms. For medical purposes, protozoa are classified according to their most important site of infection, namely, the intestinal protozoa such as Giardia, the urogenital protozoa such as Trichomonas, the blood protozoa such as Plasmodium (the cause of malaria), and tissue protozoa such as Toxoplasma. This book discusses the protozoa according to these categories. In some contexts, the protozoa are classified 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. Examples of medically important flatworms include Taenia solium, the tapeworm that causes cysticercosis, and Schistosoma mansoni, the fluke that causes schistosomiasis. Medically important roundworms (nematodes) include the pinworm (Enterobius), the hookworms (Ancylostoma and Necator), the threadworm (Strongyloides; the cause of strongyloidiasis), and Trichinella (the cause of trichinosis).
Relationships 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.
Transmission of the intestinal protozoa typically occurs by ingestion of cysts, whereas transmission of the blood protozoa occurs via insect vectors such as the mosquito in the case of Plasmodium (malaria), the reduvid bug in the case of Trypanosoma cruzi (Chagas’ disease), the tsetse fly in the case of Trypanosoma brucei (sleeping sickness), and the sandfly in the case of Leishmania donovani (visceral leishmaniasis or kala-azar).
Prevention of these diseases involves interrupting the chain of transmission, in particular via proper sewage disposal and water purification in the case of the intestinal protozoa and insect control in the case of the blood protozoa.
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. In some helminthic infections, humans are dead-end hosts—that is, the larval form in the human is not transmitted to other humans or animals. Humans are dead-end hosts for Taenia solium (cysticercosis), Echinococcus (hydatid cyst disease), and Trichinella (trichinosis).
Transmission occurs by three main modes: ingestion of eggs (T. solium, Enterobius, Ascaris), penetration of the skin by larvae (Schistosoma, hookworms, Strongyloides), or insect bite (Wuchereria, Onchocerca, Dracunculus). As with the protozoa, prevention of these diseases involves interrupting the chain of transmission, in particular via proper sewage disposal, water purification, insect control, and personal protection such as insect repellent and bed nets. In addition, avoiding bathing or swimming in certain fresh water sources (Schistosoma) and wearing shoes (hookworms and Strongyloides) prevent infection by these organisms.
Regarding the laboratory diagnosis of helminthic infections, examination of the stool for ova and parasites (O&P) is often done. The term ova refers to the eggs, and the term parasites refer 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, Trichinella, Toxocara, and the hookworms, Necator and Ancylostoma. Infections with the flatworm (fluke) Schistosoma also elicit eosinophilia. Eosinophils are an important component of the host defense against these parasites. Immunoglobulin (Ig) E is also elevated in these infections. In addition, cell-mediated immunity (CMI) is important in some helminth infections. For example, in Strongyloides infection, reduced CMI (as a result of, for example, acquired immunodeficiency syndrome [AIDS] high-dose corticosteroids) may result in disseminated strongyloidiasis, a life-threatening complication.