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Infections caused by intestinal parasites represent major causes of global morbidity, including malnutrition, diarrhea with dehydration, and anemia. For example, nearly 2 billion people, mostly in resource-poor countries, are infected with one or more of the soil-transmitted nematodes, while cestodes and trematodes are common food-borne infections worldwide. Intestinal protozoa are important causes of diarrhea in travelers, as well as immigrants, refugees, and international adoptees. With the increased global mobility of persons and populations, physicians in the United States are encountering parasitic diseases with increased frequency, requiring familiarity with their clinical features and management. This chapter will focus on the four major classes of intestinal parasite: nematodes, trematodes, cestodes, and protozoa.
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Nematodes are unsegemented roundworms belonging to the phylum Nematoda. Many species are found worldwide, but most favor tropical climates. In general, nematodes are cylindrical in shape with tapered ends. They vary greatly in size from up to 40 cm in length (Ascaris lumbricoides) to less than 1 cm (Strongyloides stercoralis, Enterobius vermicularis, and hookworm species). The common finding in this group of worms is the antigenically inert outer layer called the cuticle, which serves as a barrier of protection from host antibodies and digestive enzymes.1,2 Nematodes have separate adult male and female genders, with females typically larger in size.
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