Blastocystis: to treat or not to treat...but how?

TO THE EDITOR—In their recent paper, Coyle et al [1] recommend metronidazole as the drug of choice for Blastocystis eradication. We have come across multiple cases where high-dose metronidazole treatment does not eliminate Blastocystis from the intestine. In fact, it appears that no single drug is capable of eradicating Blastocystis [2]. Here, we present an example of an intractable Blastocystis infection despite multiple courses of antibiotic intervention. A 36-year-old female presented with a 9-month history of abdominal pain, diarrhea, and bloating. Blood tests and a sigmoidoscopy with biopsies were normal, and she was diagnosed with irritable bowel syndrome (IBS) according to Rome III criteria [3]. Fecal samples revealed Blastocystis sp. subtype 9 (ST9) and Dientamoeba fragilis. During a period of almost 3 years, she sequentially received antimicrobial treatment (Table 1). Clinical and microbiological effect was systematically evaluated 2weeks after treatment. Although the patient was cleared of D. fragilis, none of these treatments successfully eliminated Blastocystis ST9, which was repeatedly isolated from her feces, nor did they alleviate her gastrointestinal symptoms. No further treatment options are available in general in Denmark. The mechanisms leading to Blastocystis eradication are unclear. Using molecular diagnostics, we have come to realize that the parasite colonizes a substantial proportion of any given population [4]. With such a high rate of colonization, we must anticipate that we are all exposed to Blastocystis regularly, and therefore the factors influencing successful Blastocystis colonization should be explored [4]. Metagenomic studies have led to advances in the understanding of the structure and function of the human intestinal microbiome [5, 6], whereas nonprokaryotic organisms remain much less studied. If Blastocystis colonization is dependent on the composition of the bacterial flora as suggested recently [4], it is striking that the parasite could be sustained throughout the many different courses of antimicrobial treatment in this IBS patient. Eradication of Blastocystis may happen directly (protistostatic or protistocidal effect) or indirectly (due to perturbations of the intestinal flora). In this case eradication failed, and our study adds to the string of papers