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- Parasites are divided into 2 groups:
- Protozoa: Single-cell organisms; typically multiply within the host. Intestinal protozoa: 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 a degree of eosinophilia. Level of eosinophilia is associated with the degree of tissue invasiveness. Worms have a limited life span and, without reinfection, most eventually die on their own.
- Some are invasive, and some do not release their infective forms into the bowel. This latter group (e.g., Toxoplasma gondii, Echinococcus sp., Trichinella spiralis) is not reviewed here.
- Most worms require incubation outside the host before being infectious or need a vector for transmission. Enterobius vermicularis (pinworm) eggs are infectious shortly after being passed; autoinfection occurs readily.
- Person-to-person transmission of worms is uncommon, except for pinworm.
- System(s) affected: Gastrointestinal (GI)
Most common age group affected
Many of the treatments are contraindicated.
Acquisition involves personal, food, and/or water sanitation and migration from higher-prevalence areas.Incidence
- Predominant sex: Male = Female
- Predominant age: Pediatric
- US laboratory statistics: 5–30% of general population. Random testing finds at least 1 GI parasite in 5–10% of all people.
- From daycare surveys: Asymptomatic 20–30%; symptomatic 50–80%
- Intestinal protozoa account for most parasite findings in North America. Helminths account for <10% of GI parasites.
- Blastocystis hominis is a commensal enteric fungus of no clinical significance found in 20–30% of stools.
- Age (children)
- Low socioeconomic status and poor sanitation: Personal, food, water; crowding: Daycare centers, institutional care
- International travel or migration
- Multiple medical conditions, pregnancy, gastric hypoacidity, immunosuppression (AIDS)
- Intestinal parasites are usually acquired by direct fecal–oral contact via ingested contaminated food or water. Rarely, infected arthropod vectors are involved in transmission. Person-to-person transmission may occur through this mechanism.
- Safe food and water precautions (“Wash it, cook it, peel it, or forget it”); enteric and hand hygiene is the means of preventing infections. Infrastructure systems for safe food and water processing contribute to the low prevalence of intestinal parasites.
- The pathophysiology of GI parasitic infections is host–parasite-specific.
- Most intestinal parasitic infections are eventually self-limiting. Most worms have a defined life expectancy in the host. Autoreinfection does occur in some worm infections (e.g., strongyloidiasis, pinworm).
- Protozoan pathogens:
- Giardia lamblia: Common
- Entamoeba histolytica, Cryptosporidium sp., Isospora belli, Balantidium coli, Cyclospora cayetanensis, Microsporida
- Possible protozoan pathogens: Dientamoeba fragilis
- Probable nonpathogenic protozoa:
- Amoebas: All other Entamoeba sp., Endolimax nana
- All other intestinal flagellates
- Helminthic pathogens:
- Nematodes (roundworms): Enterobius vermicularis, Trichuris trichiura, Ascaris lumbricoides, hookworm (Necator americanus, Ancylostoma duodenale), Strongyloides stercoralis, Capillaria philippinensis, Trichostrongylus sp.
- Trematodes (flukes): Fasciolopsis buski, Clonorchis sinensis, Opisthorchis viverrini, Heterophyes, Fasciola hepatica, Paragonimus westermani, Schistosoma mansoni, S. japonicum, S. hematobium, S. mekongi
- Cestodes (tapeworms): Taenia saginata, T. solium, Diphyllobothrium latum, Hymenolepis nana, H. diminuta, Dipylidium caninum