**2. Recognition** *D. fragilis* **as a pathogen**

*D. fragilis* is a ubiquitous protozoan parasite found in the gastrointestinal tract of humans. Electron microscopy [1] and molecular phylogenetic studies of the SSU rRNA gene [2,3] have recently confirmed the relationship of this parasite to trichomonads (lacking flagella). Al‐ though its pathogenic potential is still controversial, Jepps and Dobell in1918 were the first to report its pathogenicity when it was found to be the only agent detected in three patients with gastrointestinal clinical symptoms [5].

**Figure 1.** Trophozoite of *D. fragilis* stained by iron hematoxylin stain (Photo by Adnan Al-Hindi, 2005). Photo extracted from Al-Hindi and Abu Shammala, 2013 [4].

Since then, many investigators have shown that patients infected with *D. fragilis* generally presented with bowel disorders with symptoms such as diarrhea, loose stools, and epigastric abdominal pains [6–12]. Furthermore, mounting evidence is accumulating reinforcing the pathogenicpotentialof*D.fragilis*[13–20].Lately,irritablebowel syndrome (IBS)hasbeenlinked to *D. fragilis* infections as a possible cause [21, 22], further underscoring its role in the causa‐ tion of disease. A great deal of controversy exists on the mode of transmission of *D. fragilis*, and while*Enterobius vermicularis*nematodehasbeenacceptedtoplaya role inits transmission,more recently a report described the discovery of a new cyst stage in its life cycle [23].

Globally, the prevalence rates of *D. fragilis* infections vary depending on the identification tool used [6, 24, 25, 26]. Using the traditional light microscope, the rates of infections oscillate between 0.4% and 52% [26]. Nevertheless, using indirect immunoflourescent assay, Chan et al. (1996) reported a prevalence rate of 91% [27]. The application of more sensitive identification tools such as PCR and culture has the extra advantage of providing accurate prevalence data [28]. Considered as a pathogen by several researchers, numerous reports have revealed that *D. fragilis* elimination with parasitic drugs normally relieves the clinical symptoms in the absence of other pathogens. However, there is currently no consensus as to the ideal treatment regimen [20, 29, 30]. The aim of this chapter is to review the recent developments and advances made on this frequently overlooked parasite and the disease dientamoebiasis.
