Diversity and frequency of Blastocystis subtypes in patients with gastrointestinal symptoms: a multicentre clinical study, Spain 2016–2019

Blastocystis is a highly prevalent enteric protist whose clinical significance remains controversial. The organism exhibits substantial genetic diversity, yet molecular epidemiological data from clinical settings in Spain are still scarce. To determine the frequency and subtype diversity of Blastocystis in stool samples from patients with gastrointestinal symptoms who tested positive for the protist by routine diagnostic methods. This descriptive, prospective multicentre study was conducted between February 2016 and August 2019 across four tertiary hospitals located in three Spanish regions. Among 281 positive samples, 249 (88.6%) were successfully subtyped, revealing seven subtypes: ST1 (27.3%, 68/249), ST3 (22.9%, 57/249), ST2 (22.5%, 56/249), ST4 (19.3%, 48/249), ST6 (6.4%, 16/249), ST7 (1.2%, 3/249), and ST8 (0.4%, 1/249). Human-associated subtypes (ST1–ST4) accounted for 92.0% (229/249) of isolates. Geographic variability in subtype distribution was observed, with less frequent subtypes (ST6-ST8) mainly detected at a single centre. Available clinical data revealed that diarrhoea (47.6%, 128/269), abdominal pain (41.6%, 112/269), and anal pruritus (11.9%, 32/269) were the most common symptoms reported. Overall, 16.0% (43/269) of Blastocystis -positive patients had eosinophilia and were co-infected with other gastrointestinal parasites (28.1%, 79/281). These findings indicate substantial Blastocystis subtype diversity and variation among participating centres. The detection of ST6 and ST7, commonly reported in birds, and ST8, frequently reported in non-human primates, raises the possibility of zoonotic transmission but does not establish it.