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Parasites—Basic Concepts

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INTRODUCTION

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The discipline of Medical Parasitology encompasses a broad spectrum of organisms belonging to the kingdoms Protista and Animalia that include diverse phylogenetic groupings such as the subkingdoms Protozoa and Metazoa, respectively. The latter include the trematodes, cestodes (Platyhelminthes), and nematodes (Nemathelminthes). Although often quite dissimilar, many parasites do share some important traits. They are opportunists by nature and exploit environmental niches and lifestyles within their hosts that suit their individual needs. Many have high prevalence rates, given the right set of circumstances, and may cause significant morbidity and mortality. All have exceedingly complex life cycles. The purpose of this chapter, and Chapters 49 and 50 that follow, is to lay a foundation of basic definitions and concepts that hopefully will aid the student in better understanding the specific diseases that will be described in succeeding chapters.

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DEFINITION

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Within the context of this section of the book, the term parasite refers to organisms that are physiologically dependent upon their host for survival and belong to the major taxonomic groupings mentioned above: Protozoa, Platyhelminthes and Nemathelminthes. Parasitism, however, denotes a relationship in which one organism, the parasite, usually benefits at the expense of the other, the host. Protozoa are microscopic, single-celled eukaryotes with a membrane-bound nucleus and organelles. Helminths, comprising both Platyhelminthes and Nemathelminthes, in contrast, are macroscopic, multicellular worms possessing differentiated tissues and complex organ systems; they vary in length from more than 1 m to less than 1 mm. The majority of both Protozoa and helminths are free living, play a significant role in the ecology of the planet, and seldom inconvenience the human race. The less common disease-producing species are typically obligate parasites, dependent on vertebrate hosts, arthropod hosts, or both for their survival. The majority of parasites are perfectly happy living in a commensalistic relationship with their host, producing little or no injury. Of importance to us are those that disturb this relationship, leading to pathogenesis and, occasionally, to death of both the host and parasite.

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Eukaryotic, single-celled Protozoa and multicellular, macroscopic helminths

Most are free living

Disease-producing species usually obligate parasites

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SIGNIFICANCE OF HUMAN PARASITIC INFECTIONS

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The relative infrequency of parasitic infections in the temperate societies of the industrialized world with strict sanitation has sometimes led to the parochial view that knowledge of parasitology has little relevance for physicians practicing in these areas. In spite of this view, parasites such as Toxoplasma gondii, Cryptosporidium spp., Giardia, and Enterobius vermicularis thrive in our midst. Many others pose risks as imported agents and our medical communities are continuously challenged to both identify and treat them. In addition, the continuing presence of parasitic disease among the impoverished, immunocompromised, sexually active, and peripatetic segments of industrialized populations means that most physicians throughout the world regularly encounter those pathogens. Parasitic diseases remain among the major causes of human ...

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