Thomas N. Oeltmann

Blood and Tissue Protozoa II: Human Malaria

Chapter seven deals with the second phase of “Blood and Tissue Protozoans, Human Malaria.” This section opens with a overview of the group of organisms that possess structures known collectively as the apical complex. Within this group are the protozoans that belong to the genus Plasmodium , the causative agents of the disease termed malaria. Four species of Plasmodium are the primary causative agents for human malaria, P. vivax, P. malariae, P. ovale, and P. falciporum. A fifth species, P. knowlesi , hitherto considered a parasite limited to monkeys, has been implicated with the human form of the disease. Following a brief study of the effect that human malaria has had in human history, a generalized life cycle of the organism is presented. Ample electron micrographs illustrate the various stages of the protozoan as it passes through the mosquito and human hosts. This generalized depiction of the life cycle is followed by a discussion of the life cycle variations that exist among the four species. Following a review of the epidemiology of the disease, the concepts of relapse and recrudescence are presented. The chapter concludes with a detailed discussion of the history of the chemotherapy that has been used through the ages and the latest regimens in use. Ample maps displaying world incidence of the four species is presented.

Visceral Protozoa II: Flagellates

Chapter five introduces eight species of flagellated protozoans that infect the human digestive and reproductive systems. In the ensuing discussion, this group of organisms is divided into two groups, the nontrichomonad flagellates and the trichomonad flagellates. Within these two groups, eight species of flagellates inhabit the aforementioned sites, but only two organisms are usually considered pathogenic, Giardia lamblia and Trichomonas vaginalis . The former infects the small intestine of humans while the latter infects the reproductive system. The epidemiology of G . lamblia is discussed emphasizing the wide variety of wild animals that can serve as reservoir hosts for the organism. Backpacker’s disease is presented as an example of the role that wild animals can play in transmitting the organism to humans. The host’s immune response is considered as is the current chemotherapeutic regimen. Of the trichomonads, only T. vaginalis is considered pathogenic to humans. Following a brief overview of the trichomonad morphology, the physiology and epidemiology of T. vaginalis is discussed in detail. In considering the latter, the roles of males and females in the spread of the infection is considered. A brief discussion of the host’s immune response is followed by a paragraph outlining the current chemotherapeutic regimen for T. vaginalis.

Visceral Protozoa II

Chapter four focuses mostly on Entamoeba histolytica , an anaerobic parasitic protozoan, part of the genus Entamoeba . Predominantly infecting humans and other primates, E. histolytica is estimated to infect about 50 million people worldwide. Previously, it was thought that 10% of the world population was infected, but these figures predate the recognition that at least 90% of these infections were due to a second species, the nonpathogenic E. dispar . Mammals such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission. Only one ciliophoran infects humans, Balantidium coli. This parasitic species of ciliate protozoan causes the disease balantidiasis. Balantidiasis is a zoonotic disease and is acquired by humans via the fecal-oral route from the normal host, the pig, where it is asymptomatic. Contaminated water is the most common mechanism of transmission. Several pathogenic free-living amoebae are also discussed including Acanthamoeba and Balamuthia.

Blood and Tissue Protozoa III

Chapter 6 discusses the family of blood feeding hemoflagellates that includes two genera, Leishmania and Trypanosoma, both of which require a blood feeding insect vector and infect humans. The chapter discusses the defining characteristics of the hemoflagellates including the subpellicular microtubular network, the kinetoplast, and the metamorphic transitions of the four morphologic forms observed in this family that include the amastigote, the promastigote, the epimastigote, and the trypomastigote. The chapter provides electron micrographic images and illustrative drawings of the various stages of development and key organelles like the flagellum and the mitochondrion with the associated kinetoplast DNA. The morphologic changes during the life cycle are well illustrated. This chapter further provides a more lengthy discussion of the physiology, pathology, epidemiology, symptomatology, and the host immune response of both genera of hemoflagellates.

Visceral Protozoa I: Rhizopods (Amoebae) and Ciliophorans

Chapter four focuses mostly on Entamoeba histolytica , an anaerobic parasitic protozoan, part of the genus Entamoeba . Predominantly infecting humans and other primates, E. histolytica is estimated to infect about 50 million people worldwide. Previously, it was thought that 10% of the world population was infected, but these figures predate the recognition that at least 90% of these infections were due to a second species, the nonpathogenic E. dispar . Mammals such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission. Only one ciliophoran infects humans, Balantidium coli. This parasitic species of ciliate protozoan causes the disease balantidiasis. Balantidiasis is a zoonotic disease and is acquired by humans via the fecal-oral route from the normal host, the pig, where it is asymptomatic. Contaminated water is the most common mechanism of transmission. Several pathogenic free-living amoebae are also discussed including Acanthamoeba and Balamuthia.

Blood and Tissue Protozoa I: Hemoflagellates

Chapter 6 discusses the family of blood feeding hemoflagellates that includes two genera, Leishmania and Trypanosoma, both of which require a blood feeding insect vector and infect humans. The chapter discusses the defining characteristics of the hemoflagellates including the subpellicular microtubular network, the kinetoplast, and the metamorphic transitions of the four morphologic forms observed in this family that include the amastigote, the promastigote, the epimastigote, and the trypomastigote. The chapter provides electron micrographic images and illustrative drawings of the various stages of development and key organelles like the flagellum and the mitochondrion with the associated kinetoplast DNA. The morphologic changes during the life cycle are well illustrated. This chapter further provides a more lengthy discussion of the physiology, pathology, epidemiology, symptomatology, and the host immune response of both genera of hemoflagellates.

General Characteristics of the Cestoda

This chapter describes the basic structural characteristics and physiology of the cestoda. Cestoda is a subclass of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. These parasites live in the digestive tract of vertebrates as adults, and often in the bodies of various animals as juveniles. A little over one thousand species have been described and all vertebrates can be infected by at least one species of tapeworm. Humans can be infected after consuming the parasite in underprepared meat such as pork ( Taenia solium ), beef ( T. saginata ), and fish ( Diphyllobothrium ). In addition, food prepared in conditions of poor hygiene can also lead to infection. ( Hymenolepis or Echinococcus ). The general life cycles of these parasites is also described.

Blood and Tissue Protozoa III: Other Protists

This chapter considers blood and tissue protozoa not considered in the prior two chapters. Babesia is a protozoan parasite of the blood that causes a hemolytic disease known as Babesiosis. People who contract Babesiosis suffer from malaria-like symptoms and as a result, malaria is a common misdiagnosis for the disease. Approximately 100 species of Babesia have been identified, only a few have been documented as pathogenic in humans. In the United States, Babesia microti is the most common strain associated with humans. Toxoplasma gondii can be carried by many warm-blooded animals (birds or mammals, including humans) but the definitive host of T. gondii is the cat. Toxoplasmosis is usually minor and self-limiting but can have serious or, in rare cases, fatal effects on a fetus whose mother contracts the disease during pregnancy or on an immuno-compromised individual.

Parasite-Host Interactions

Chapter two introduces selected aspects of the parasite host interaction with a range of topics involving their physiology, biochemistry, and immunology. Emphasis in this chapter is given to the rapidly developing field of immunology with an expanded discussion of the hosts innate immune system interactions elicited by parasites. In view of its observed importance in parasite-host interactions, this chapter includes a basic overview of the mechanisms of lymphocyte development and differentiation in response to parasite invasion. Both cellular and humoral reactions of B and T cells in response to parasite invasion are discussed with regard to the presentation of endogenous and exogenous antigens. Basic immunoglobulin structure and function are discussed along with the role of some of the more important cytokines involved in the immune response. The impact of selected opportunistic parasitic infections on immunocompromised individuals is also discussed.

General Characteristics of the Euprotista (Protozoa)

Chapter three consists of an overview of the Protozoa. After a brief introduction to the group, the chapter moves into a discussion of the various means of locomotion displayed by these single-celled organisms. These include pseudopodia, cilia, and flagella. The information is augmented by electron micrographs and line drawings. The morphology of the locomotory organelles is supplemented with a clear, concise explanation of their functions. A similar format is followed with an explanation of those organelles involved with the physiological activities of the organisms. These include a discussion of the roles of mitochondria and Golgi complexes in energy metabolism as well as the role of lysosomes in the acquisition and utilization of food. Encystation and various types of reproduction such as the several types of asexual and sexual reproduction strategies are considered as they pertain to this diverse group of organisms. Again, the information is supplemented with ample graphic material. The chapter concludes with a presentation of a reduced classification scheme. Within this scheme, the relationship between the various protozoans that will be discussed in subsequent chapters is presented.