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Dear Sir:
We read with interest the recent report describing a possible role for Blastocystis hominis in the etiology of irritable bowel syndrome (IBS).1 Although the study population was small, B. hominis was frequently detected in the stools of patients with IBS. Blastocystis hominis has a somewhat controversial history with regard to its pathogenicity. Despite being linked with symptoms in many patients, it has also been frequently found in asymptomatic individuals.2 Conflicting reports abound, and indeed many of the case reports of infection with B. hominis are anecdotal and uncontrolled.3 More importantly, some studies have failed to eliminate all other causes and infective agents that may produce intestinal symptoms. We believe that the diagnostic methods used by Yakoob and others1 were not optimal for the detection of all intestinal protozoa. Although these techniques are more than adequate for detection of B. hominis, they are not suitable for detection of Dientamoeba fragilis, another human parasite that has also been linked with IBS.
Dientamoeba fragilis has puzzled microbiologists since it was described by Jepps and Dobell in 1918.4 Originally thought to be an ameba, it is now classified as a flagellate, albeit one without a flagellum, and has been associated with a variety of intestinal symptoms.5 It has also been found in asymptomatic individuals, and although D. fragilis is accepted as a pathogen in some countries, it is often overlooked or disregarded in others.6 In 2002, investigators in Australia reported the presence of D. fragilis in patients with IBS-like symptoms.7 Symptoms included abdominal cramping, bloating, constipation, diarrhea (2–15 bowel movements motions per day), flatulence, nausea, fatigue, and anorexia. In 14 (67%) of 21 patients, successful eradication of D. fragilis with iodoquinol and doxycycline resulted in resolution of symptoms.
Dientamoeba fragilis does not have a resistant cyst stage and is difficult to detect unless suitable staining or culture methods are used.6 The trophozoites degenerate rapidly after leaving the intestine and are very easy to overlook in direct saline or iodine preparations.5 The most sensitive detection method is parasite culture, and the culture media require the addition of rice starch.8 To the best of our knowledge, culture media without rice starch have not been used successfully for the detection of D. fragilis. It is therefore unlikely that Jones medium without rice starch, as used by Yakoob and others,1 can support the growth of D. fragilis. Consequently, D. fragilis cannot be confidently excluded from either the IBS or control groups in the study of Yakoob and others.1 Since D. fragilis has been associated with IBS-like symptoms, future studies into the etiology of IBS should use sensitive methods for the detection of not only B. hominis, but also D. fragilis. However, it remains to be seen if these parasites actually contribute to the pathogenesis of IBS, or merely take advantage of the disruption in the microbial flora.
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