Blastocystis is one of the most widespread intestinal microbial eukaryotes in humans and animals with extraordinary genetic diversity. Based on the small ribosomal subunit (SSU rRNA) gene, it is divided into at least 30 subtypes, with ST1-ST4 being the most common in humans. It is unclear whether this eukaryote is a pathogen, as it occurs in patients with gastroenteritis and the healthy population. To determine the occurrence of Blastocystis spp. in patients with diarrhoea, we analysed faecal samples from 249 child and adult patients who had been tested for gastrointestinal pathogens in January and February 2023. To determine possible differences in the occurrence of Blastocystis spp. in sick and healthy populations of children, 379 DNA extracts from faecal specimens of the same number of children aged 0 to 6 years collected between October 2011 and October 2012 were analysed. Of the 379 children, 292 had diarrhoea and 87 represented a healthy control group without diarrhoea. The occurrence of Blastocystis spp. in dogs was determined by analysing 79 faecal samples from dogs with diarrhoea. The faecal samples were tested for the presence of Blastocystis spp. using Taq-man-based real-time PCR targeting the SSU rRNA gene, and the positive samples were sequenced using barcoding primers to determine the subtypes. Of the 249 child and adult patients with diarrhoea, 20 (8.0 %) tested positive. Subtypes were successfully determined in 11 of them (ST2-ST4). Of 379 children, nine (3.1 %) children with diarrhoea and six (6.9 %) children in the control group were positive; the difference is not statistically significant (p=0.11). We identified ST2-ST4 in three children with diarrhoea and ST2, ST3 and ST7 in five healthy children. Blastocystis spp. was detected in seven (8.9 %) samples from dogs, but the subtypes could not be determined, suggesting that dogs are only incidental hosts of the protist.
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