Blastocystis- commensal or culprit? Do I really care?

Guest posting from Dr Hema Varadhan, Clinical Microbiologist, Pathology North.

This parasite intrigues me every time I validate a faecal PCR result.   Why do we see these bugs more often than the others? Do we care? Do we need to treat?  The RCPA recently provided relevant guidance concerning Blastocystis and Dientamoeba which is also useful to consider.

Some background….

Blastocystis is a frequently encountered protozoan in faeces. Its pathogenicity remains unclear as there are no good animal models studied thus far. The recent (pseudo)epidemic of this parasite relates to the advent of tests with improved sensitivity such as faecal PCR.

There have been very few longitudinal studies describing the natural history of colonisation/infection with this parasite. It appears that the carriage in formed stools is as common as in liquid stools and in addition, the duration of carriage is long- for many months1,2.

So, should we really be worried? Well, some people believe that this parasite could be associated with irritable bowel syndrome. However, there is no conclusive evidence available in the current literature supporting this hypothesis 3. Nevertheless, it is believed to play a role in causing non-specific abdominal symptoms such as pain and bloating.

On a positive note, a recent study on Blastocystis interaction with the intestinal microbiome indicated that its colonisation is associated with increased bio-diversity, thereby offering a protective effect4.

That brings us to the question regarding management- should we treat or not treat? Well, given the lack of compelling evidence for pathogenicity, I would rather consider this organism as being a ‘commensal’ than a ‘culprit’ in a majority of cases. It is however unclear whether we would have to follow the same approach with immunosuppressed patients.   Perhaps, treatment may be considered after excluding other causes-infectious and non-infectious.  Treatment is not so straightforward- metronidazole should be given in optimal doses to not only achieve good concentration but also to have sustained inhibitory effect on this parasite. Cotrimoxazole is a suitable alternative5.

As always, please call one of us (microbiologists) on (02)49214000 if you wish to discuss further.


  1. Chunge, R. N et al. 1991. Longitudinal study of young children in Kenya: intestinal parasitic infection with special reference to Giardia lamblia, its prevalence, incidence and duration, and its association with diarrhoea and with other parasites. Acta Trop. 50:39-49.
  2. Herwaldt, B. L et al. 2001. Multiyear prospective study of intestinal parasitism in a cohort of Peace Corps volunteers in Guatemala.  Clin. Microbiol. 39:34-42
  3. Nourrisson C et al. 2014. Blastocystis Is Associated with Decrease of Fecal Microbiota Protective Bacteria: Comparative Analysis between Patients with Irritable Bowel Syndrome and Control Subjects. PLoS ONE 9(11): e111868. doi:10.1371/journal.pone.0111868
  4. Christophe Audebert et al. 2016.  Colonization with the enteric protozoa Blastocystis is associated with increased diversity of human gut bacterial microbiota. Nature Scientific Reports 6, Article number :25255
  5. Therapeutic Guidelines, Antibiotic version 15, 2014.


Image: The different morphotypes of Blastocystis  – Wikipedia 

One comment

  1. […] below) on a variety of areas of AMS and ID, including AIMED’s first ever quiz challenge and a guest posting from Dr Hema Varadhan, Clinical Microbiologist at Pathology North […]


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