Dientamoeba fragilis: a story of contradictions

Dientamoeba fragilis is a gastrointestinal parasite of controversial clinical significance. From its discovery until today, contradictory articles have been published on whether infection is correlated with symptoms, treatment is associated with recovery and whether infection is associated with elevated intestinal inflammatory markers (faecal calprotectin). Additionally, there is no consensus on the infective stage of the lifecycle. Competing theories propose that either Enterobius vermicularis ova act as a vector for the transmission of trophozoites or that the cyst stage, which is rarely found, is responsible for infection. In this review, we aim to critique these contradictions to determine if D. fragilis should be considered a pathogen in clinical practice. The frequent limitation of studies is challenges in setting up a reliable, healthy control group and the reliability of diagnostic methods. Many studies are opportunistic in design, using samples that have been submitted for routine pathology testing. Even if all pathology tests are negative for infectious agents, the current health status of people who are submitting samples for pathology testing is unlikely to be the best option, just the most available one. Of greater concern is the reliability of some diagnostic methods. Some studies have suggested that at least one of the lab-based real-time PCR assays used for the diagnosis of D. fragilis has issues with false positives in human samples. This calls into question much of the evidence that has been published on D. fragilis being a commensal instead of a pathogen. As such, D. fragilis should be considered a potential pathogen when investigating gastrointestinal illness. Developing better guidelines on determining when D. fragilis is the causative agent of symptoms and when to treat are important topics for future research.