Seven Blastocystis Subtypes Found in Symptomatic Patients Across Spanish Hospitals

Blastocystis, one of the most common enteric protists in humans, has long divided opinion among clinicians and researchers: it is frequently detected in stool samples, yet its role in causing disease remains disputed. A new multicentre study from Spain, published in the journal Gut Pathogens, has now provided one of the most detailed molecular portraits of the organism in symptomatic patients from that country, documenting seven distinct genetic subtypes and revealing striking differences in their distribution across hospitals and regions.

The study, conducted between February 2016 and August 2019, was a descriptive, prospective investigation carried out across four tertiary hospitals located in three Spanish regions. The researchers enrolled patients with gastrointestinal symptoms who had tested positive for Blastocystis by routine diagnostic methods. In total, 281 positive samples were collected, and molecular subtyping succeeded for 249 of them, a success rate of 88.6 percent. That figure alone underscores a technical point: routine microscopy or culture can flag the organism, but only sequencing-based subtyping can reveal which genetic variant is actually present.

The subtype results were dominated by variants commonly associated with human carriage. ST1 was the most frequent, found in 27.3 percent of successfully subtyped samples (68 of 249), followed closely by ST3 at 22.9 percent (57 of 249) and ST2 at 22.5 percent (56 of 249). ST4, a subtype with a more complicated reputation in the literature, accounted for 19.3 percent (48 of 249). Together, the human-associated subtypes ST1 through ST4 made up 92.0 percent of all isolates, or 229 of 249 samples. The remaining isolates belonged to rarer subtypes: ST6 was detected in 6.4 percent (16 of 249), ST7 in 1.2 percent (3 of 249), and ST8 in a single sample, 0.4 percent of the total.

Geography mattered. The authors report clear variability in subtype distribution among the participating centres, with the less frequent subtypes, ST6 through ST8, detected mainly at a single hospital. This concentration pattern is consistent with local transmission dynamics or exposure differences rather than a uniform national picture, and it illustrates why multicentre designs are valuable for organisms whose prevalence and genetic makeup can vary sharply from one setting to another.

The clinical side of the study adds context to the long-running debate over whether Blastocystis causes symptoms. Among the 269 patients for whom clinical data were available, diarrhoea was the most commonly reported symptom, affecting 47.6 percent (128 of 269). Abdominal pain followed at 41.6 percent (112 of 269), and anal pruritus was reported by 11.9 percent (32 of 269). These figures describe the symptom profile of a Blastocystis-positive, symptomatic population; they do not, by themselves, establish that the protist caused the complaints, a distinction the authors of such observational studies typically emphasise.

Two additional findings complicate the clinical interpretation. Eosinophilia, an elevated count of a white blood cell type often associated with parasitic infections, was present in 16.0 percent of Blastocystis-positive patients with available data (43 of 269). Meanwhile, co-infection with other gastrointestinal parasites was documented in 28.1 percent of the positive samples (79 of 281). Both figures suggest that many patients in this cohort carried more than one parasite, meaning that symptoms attributed to Blastocystis could in some cases reflect a mixed parasitic burden.

The detection of the rarer subtypes carries its own evolutionary and epidemiological interest. ST6 and ST7 are subtypes commonly reported in birds, while ST8 is frequently reported in non-human primates. Their presence in human patients raises the possibility of zoonotic transmission, the authors note, but they are careful to state that this possibility is not established by the data. Confirming a zoonotic route would require additional evidence, such as matching subtypes in animals and humans within the same transmission setting.

From a technical standpoint, the study’s strength lies in its scale and molecular resolution. Nearly 90 percent of positive samples were successfully subtyped, allowing frequency estimates with explicit numerators and denominators rather than vague prevalence claims. The seven-subtype inventory, spanning the dominant human-associated variants and three rarer animal-associated ones, gives Spanish clinical microbiology laboratories a reference point for interpreting their own subtyping results. The authors conclude that the findings indicate substantial subtype diversity and variation among the participating centres, a conclusion that aligns with the growing international recognition of Blastocystis as a genetically heterogeneous organism whose clinical significance may depend on the subtype involved.

For now, the study stops short of settling the virulence question. What it does deliver is a rigorous frequency map: seven subtypes, a clear dominance of ST1, ST2 and ST3, a notable share of ST4, and a geographically clustered tail of rare variants. As molecular surveillance of enteric protists expands, datasets of this kind will be essential for determining whether particular subtypes behave differently in symptomatic patients, and whether the animal-associated variants signal occasional spillover from reservoirs or simply reflect underappreciated human diversity.

Subject of Research: Subtype diversity and frequency of Blastocystis in patients with gastrointestinal symptoms at four Spanish hospitals

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

Article References: Köster, P. C., García-Hita, M., Cifre, S., Trelis, M., Azcona-Gutiérrez, J. M., Pérez-Ayala, A., Domínguez-Márquez, V., de Lucio, A., Bailo, B., Castro-Companioni, L., Molina, I., Muñoz-de-Mier, G. J., Sánchez, S., & Carmena, D. (2026). Diversity and frequency of Blastocystis subtypes in patients with gastrointestinal symptoms: a multicentre clinical study, Spain 2016–2019. Gut Pathogens. https://doi.org/10.1186/s13099-026-00895-5

Image Credits: AI Generated

DOI: 10.1186/s13099-026-00895-5

Keywords: Blastocystis, subtypes, gastrointestinal symptoms, Spain, multicentre study, zoonotic transmission, eosinophilia, co-infections, ST1, ST4, parasitology, Diversity