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Chapter 67. Intestinal Parasites

Nisha Manickam; Michael Cappello

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.

(Table 67–1)

Table 67–1. Nematodes
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Table 67–1. Nematodes

Organism

Epidemiology

Clinical Caveats

Ascaris lumbricoides

  • Highest prevalence occurs in tropical and subtropical regions, especially in areas with inadequate sanitation.
  • In the United States, transmission has occurred in rural areas of the southeastern states.
  • May cause Loffler's syndrome (pulmonary infiltrates, cough, wheezing, dyspnea, and fever) with peripheral eosinophilia during the migration of larvae through lung tissue.
  • Treatment may trigger movement of the adult intestinal worms and can lead to intestinal obstruction.

Strongyloides species

  • Tropical and subtropical areas including the southeastern US.
  • More frequently found in rural areas, institutional settings, among World War II and Vietnam war veterans, and lower socioeconomic groups.
  • Hyperinfection may occur in persons undergoing chemotherapy or taking systemic corticosteroids—therefore persons from endemic areas should be screened prior to therapy.

Hookworm (Necator americanus and Ancylostoma duodenale)

  • Worldwide distribution, mostly in areas with moist, warm climate.
  • Heavily infected children may have growth stunting, cognitive delays, iron deficiency anemia, hypoalbuminemia, and eosinophilia.

Enterobius vermicularis (pinworm)

  • The most common helminth infection in the United States (an estimated 40 million persons infected).
  • Worldwide distribution, with infections more frequent in young children and among persons living in crowded conditions.
  • More common in temperate than tropical countries.
  • Common in children in daycare settings, pinworm is diagnosed best using the scotch-tape test.

Trichuris trichiura

  • Worldwide distribution, favoring tropical areas, especially those with poor sanitation practices.
  • Also found in the southern US.
  • More common among children than adults.
  • Heavy infections can mimic inflammatory bowel disease, with hemorrhagic colitis known as Trichuris dysentery.
  • Rectal prolapse can also develop.

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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