Intestinal Parasites



  • Parasites are divided into two groups.
    • Intestinal protozoa: single-cell organisms; typically multiply within the host; transmission by direct fecal–oral route; do not cause eosinophilia
    • Helminths (worms): multicellular organisms; rarely multiply within the host (exceptions: Strongyloides stercoralis, Hymenolepis nana); infection may cause eosinophilia, which correlates with tissue invasion.
  • Most worms require outside incubation and need a vector for transmission. Enterobius vermicularis (pinworm) eggs are infectious shortly after being passed; autoinfection occurs readily.
  • Person-to-person transmission is uncommon (except for pinworm).
  • System(s) affected: gastrointestinal (GI)

Pediatric Considerations
Children affected most commonly (1)[A]

Pregnancy Considerations
Many treatments are contraindicated during pregnancy.



  • Predominant sex: male = female
  • Predominant age: pediatric


  • United States: 5–30% of general population has at least one fecal parasite. Random testing finds at least one GI parasite in 5–10% of all people.
  • Daycare: asymptomatic 20–30%; symptoms 50–80%
  • Intestinal protozoa account for most parasitic infections in North America. Helminths account for <10% of GI parasites.

Etiology and Pathophysiology

  • Pathophysiology is host–parasite-specific.
  • Intestinal parasitic infections are generally self-limiting. Most worms have a defined life expectancy within the host.
  • Protozoan pathogens
    • Giardia lamblia: common
    • Entamoeba histolytica, Cryptosporidium sp., Cystoisospora (Isospora) belli, Balantidium coli, Cyclospora cayetanensis, Microsporidia
  • Possible protozoan pathogens: Dientamoeba fragilis
  • Probable nonpathogenic protozoa
    • Amoebas: all other Entamoeba sp., Endolimax nana; all other intestinal flagellates
  • Helminthic pathogens
    • Nematodes (roundworms): E. vermicularis, Trichuris trichiura, hookworm (Necator americanus, Ancylostoma duodenale), S. stercoralis, Capillaria philippinensis
    • Trematodes (flukes): Fasciolopsis buski, Clonorchis sinensis, Opisthorchis viverrini, Heterophyes heterophyes, Fasciola hepatica, Paragonimus westermani, Schistosoma mansoni, Schistosoma japonicum, Schistosoma haematobium, Schistosoma mekongi
    • Cestodes (tapeworms): Taenia saginata, Taenia solium, Diphyllobothrium latum, H. nana, Hymenolepis diminuta, Dipylidium caninum

Risk Factors

  • Age (children most commonly infected)
  • Low socioeconomic status, poor sanitation; food, water security; crowding: daycare, institutional care
  • International travel or immigration
  • Pregnancy; medical comorbidities: gastric hypoacidity, immunosuppression (AIDS, hypogammaglobulinemia)

General Prevention

  • Intestinal parasites acquired via fecal–oral route through ingestion of contaminated food or water. Rarely, infected arthropod vectors are involved; person-to-person transmission generally fecal–oral
  • Safe food and water precautions. Enteric and hand hygiene is the gold standard for preventing infections. Public health infrastructure systems for safe food and water processing contribute to the low prevalence of intestinal parasites in developed countries.

Commonly Associated Conditions

  • HIV/AIDS, steroid use, immune deficiencies
  • Intestinal parasite infection may protect against allergic sensitization (2,3)[A].

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