Chapter 16 - Parasitic Diarrhea

Abstract

Parasitic infections causing diarrhea are particularly important in young children, people that are immunocompromised, and those living in conditions of poor sanitation and hygiene, with unsafe food and water supplies. Travelers from developed countries are another risk group who are particularly susceptible to parasitic pathogens because they generally lack protective immunity acquired from previous exposure. Several of these organisms are of zoonotic origin, and there may be occupational or recreational exposure risks. Parasitic pathogens causing diarrhea include protozoa belonging to apicomplexan genera (Cryptosporidium, Cyclospora, Cytoisospora, Sarcocystis); flagellates (Giardia intestinalis); amoebae (Entamoeba histolytica); a ciliate (Balantidium coli); microsporidia (now known to be atypical fungi, but included here for convenience); and some protists of unusual origin and/or controversial pathogenicity (Dientamoeba fragilis and Blastocystis hominis). In general, helminths are less important as agents of diarrhea. One important exception is the nematode Strongyloides stercoralis, which has the unusual capacity to maintain subclinical infections for years or decades, only to be unmasked when the patient is immunosuppressed for various reasons, when severe or potentially fatal disseminated infection may occur. Some helminths may not routinely be associated with diarrhea, but may produce loose or even dysenteric stools in acute or heavy infections (for example, Trichuris (whipworm) and intestinal schistosomiasis, and some cestodes). While routine laboratory diagnosis of parasitic diarrhea still depends largely on microscopic examination of fecal samples, immunological and molecular methods are becoming increasingly commercially available, and in well-resourced settings will ultimately displace traditional methods.