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Clin Gastroenterol Hepatol. 2015 Mar;13(3):507-513.e2. doi: 10.1016/j.cgh.2014.07.065. Epub 2014 Sep 16.

The prevalence of intestinal parasites is not greater among individuals with irritable bowel syndrome: a population-based case-control study.

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  • 1Department of Medical Gastroenterology, Køge University Hospital, Køge, Denmark. Electronic address:
  • 2Department of Medical Gastroenterology, Køge University Hospital, Køge, Denmark; Department of Clinical Microbiology, Hvidovre University Hospital, Hvidovre, Denmark.
  • 3Department of Microbiology and Infection Control, Statens Serum Institut, Copenhagen S, Denmark.
  • 4Department of Medical Gastroenterology, Køge University Hospital, Køge, Denmark.



The parasites Dientamoeba fragilis and Blastocystis have been detected in feces from patients with irritable bowel syndrome (IBS), therefore these parasites may be involved in IBS pathogenesis. We proposed that a higher prevalence of the parasites in IBS subjects compared with asymptomatic controls would support such a mechanism. We aimed to determine the prevalence of these parasites in IBS subjects (cases) and controls and to identify risk factors associated with parasite carriage.


We performed a population-based, case-control study of an adult population from an internet-based research institute in Denmark. In January 2010, subjects completed a questionnaire based on the Rome III criteria for IBS and answered questions on factors associated with parasite carriage. Respondents (n = 483) were asked to submit fecal samples for parasite testing; samples were analyzed from 124 cases and 204 controls.


A greater proportion of controls than cases carried the parasites (50% vs 36%; P = .01). D fragilis was detected in a greater proportion of fecal samples from controls than cases (35% vs 23%; P = .03), as was Blastocystis (22% of controls vs 15% of cases; P = .09), and a greater percentage of controls carried more than 1 species of parasite (16% of controls vs 8% of cases; P = .05). D fragilis infection was associated with having children 5 to 18 years old in the household and Blastocystis infection was associated with high income (≥600,000 Danish Kroner/y, approximately $100,000 US dollars/y), no animals in the household, and drinking bottled water.


D fragilis and Blastocystis were detected in a greater proportion of fecal samples from the asymptomatic background population in Denmark than from subjects with IBS symptoms. These findings indicate that these parasites are not likely to have a direct role in the pathogenesis of IBS. Longitudinal studies are required to understand their role in gastrointestinal health.


Blastocystis; Dientamoeba fragilis; Microbiota; Risk Factors

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