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  • Three major groups of parasites cause human disease:
    • Protozoa
    • Helminths
      • –Nematodes (roundworms)
      • –Cestodes (flatworms)
      • –Trematodes (flukes)
    • Arthropods
  • Virtually all organ systems are at risk for parasitic infestation, with symptoms depending on the system(s) involved. Arthropods are predominantly surface dwellers and cause pruritus and rash. Nematodes and cestodes infest the gut, producing diarrhea, abdominal pain, and nutritional derangement, but, along with trematodes, they may migrate to the lungs and solid organs.
  • Ascaris lumbricoides is the largest and most prevalent human nematode, with an estimated one billion cases worldwide. Albendazole (400 mg orally as a single dose) or ivermectin (150–200 μg/kg orally as a single dose) is curative.
  • Enterobius vermicularis (pinworm) is present in all parts of the United States and affects individuals of all ages and socioeconomic levels, with the most common presentation being that of a toddler or small child with anal itch. Scotch tape, placed sticky side to perianal skin when the child first awakens and then viewed under low power, is usually diagnostic, but repeated examination may be necessary to find the eggs.
  • Trichuris trichiura (whipworm) lives predominantly in the cecum and can cause malabsorptive symptoms, pain, bloody diarrhea, and fever but is usually asymptomatic. A heavy worm burden may cause a colitislike picture and rectal prolapse and can be associated with anemia and developmental and cognitive deficits.
  • The hookworms, Necator americanus and Ancylostoma duodenale, are found between 36-degrees north and 30-degrees south latitude and are one of the most prevalent infectious diseases of humans, with an estimated one billion individuals affected. Although a broad spectrum of symptoms is possible, the hallmark of hookworm infestation is the microcytic, hypochromic anemia of iron deficiency.
  • Symptoms of schistosome infestation appear only in those with heavy infestation and are commonly frequency, dysuria, and hematuria with Schistosoma haematobium and colicky abdominal pain and bloody diarrhea with the other agents. None of these flukes are endemic in the continental United States.
  • The avian schistosome Trichobilharzia ocellata is spread by migratory birds to the freshwater lakes of the northern United States. The cercariae cause dermatitis, known as swimmer's itch.
  • Most patients with amoebic infestation carry amebas asymptomatically in the cecum and large intestine. Heavy infestations of Entamoeba histolytica produce a colitislike picture (“gay bowel syndrome”) with nausea, vomiting, bloating, pain, bloody diarrhea, and leukocytosis without eosinophilia.
  • Infection with chloroquine resistant malaria should be high in the differential diagnosis of febrile children returning from international travel.
  • Resistance of head lice to permethrin is now common, with reported insecticidal activity down to 28% in one study. The combination of 1% permethrin with trimethoprim/sulfamethoxazole has been reported to be 95% effective, compared with 80% for permethrin alone.


Parasitic diseases are ubiquitous. In spite of advances in sanitation throughout the world, new medications, and the heightened awareness of health care providers, between one-fourth and one-half of the world's population has a parasitic infestation at any given time. An increasingly mobile society has made disease containment nearly impossible. Travel (for business and ...

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