Transient colonization with Blastocystis spp. after transmission via fecal microbiota transplantation.

BACKGROUND

The pathogenicity of Blastocystis spp. is still debated. Guidelines for feces donor screening differ in their advice to screen for Blastocystis spp., but when tested, its presence is a common reason for exclusion. Blastocystis spp. are correlated to increased bacterial alpha-diversity and distinct bacterial groups and therefore its presence may indicate favorable efficacy of fecal microbiota transplantation (FMT). The latest European consensus report no longer advices rejecting feces donors testing positive for Blastocystis spp. Only one paper has been published on human transmission of Blastocystis spp. via frozen FMT.

OBJECTIVE

To investigate the transmission and long-term effects of Blastocystis-positive FMT, prepared with fresh (i.e., unfrozen) feces.

METHODS

In a trial (NCT03074227) on FMT for refractory Irritable Bowel Syndrome (IBS), adolescents (age 16-20 years) received two administrations - at baseline and after 6 weeks - of fresh allogeneic FMT from a Blastocystis-positive donor via nasoduodenal tube. The follow-up was 48 weeks. Blastocystis spp. presence, viability and subtyping were determined using microscopy, culture, PCR and sequencing.

RESULTS

Three recipients received FMT from one donor colonized with Blastocystis subtype 3 (ST3). At baseline, two recipients were negative for Blastocystis spp. and one recipient carried ST2. Culturing revealed viable Blastocystis spp. in fresh donor feces but not in frozen samples. After FMT with fresh feces, the two prior-negative recipients tested positive for the donor's ST3 at 12 weeks, but had lost this subtype by week 24 and 48. The recipient initially colonized with ST2 remained colonized with ST2 and did not acquire ST3. Transient adverse events occurred, but did not differ from patients treated with Blastocystis-negative FMT. No FMT-related serious adverse events emerged.

CONCLUSION

We present the first long-term data on viable Blastocystis spp. transmission via fresh FMT in three cases. Transient colonization with Blastocystis spp. was observed, without serious FMT-related adverse events.

© 2025. The Author(s).