To evaluate the prevalence of intestinal protozoan parasites Blastocystis spp. and Dientamoeba fragilis in ulcerative colitis (UC) patients and healthy controls, and to assess their association with UC disease activity. Fecal samples were collected from 103 UC patients and 95 healthy volunteers during the screening of the TURN2 trial, a randomized controlled trial on the efficacy of anoxic-prepared fecal microbiota transplantation (FMT) in active UC. The presence and load of protozoa were assessed by microscopy and PCR. Disease activity was assessed using the Simple Clinical Colitis Activity Index (SCCAI), Partial Mayo Score and fecal calprotectin (FCP). The prevalence of Blastocystis spp. was significantly higher in healthy controls (33% by microscopy and 34% by PCR, respectively) compared to UC patients (6% by both methods, p < 0.001). Similarly, D. fragilis was more common in healthy controls (34% by microscopy and 37% by PCR) than UC patients (9% and 12% respectively, both p < 0.001). Microscopic detection of D. fragilis was significantly associated with lower disease activity, as measured by the Simple Clinical Colitis Activity Index (median IQR: 4 4 vs. 6 4, p = 0.023) and the Partial Mayo Score (median IQR: 4 2 vs. 6 2, p = 0.009). Amongst the D. fragilis positive samples, higher D. fragilis loads, as assessed by either microscopy or PCR, were significantly associated with lower FCP levels (p = 0.010 and p = 0.040). PCR detection of D. fragilis showed similar but non-significant associations with disease activity scores. No associations were found between Blastocystis spp. and disease activity. Our findings show a higher prevalence of Blastocystis spp. and D. fragilis in healthy individuals compared to UC patients, challenging their pathogenicity in the context of UC and in general. The reported association between D. fragilis presence and load and lower UC disease activity warrants further investigation in larger cohorts.
Bénard et al. (2026) studied this question.