Blastocystis hominis is a common intestinal protozoon with worldwide distribution. It does not have a dedicated host and since it is common between humans and most animals It can be transmitted from animal species to humans. It is common in seemingly healthy people with no intestinal symptoms, as well as in people with symptoms. Few research has been done on the incidence of Blastocystis in children with and without symptoms and there is few information about the relationship between genotypes and clinical signs .Therefore, this study aimed molecular diagnosis and genotype analysis of Blastocystis hominis in symptomatic children by PCR-sequencing

Stool samples of 150 children between the ages of 2-18 including 50 samples with gastrointestinal symptoms (in the case group) and 100 similar samples of children without gastrointestinal symptoms (in the control group) who were referred to the teaching hospitals of Mashhad University of Medical Sciences between October 2020 and April2021 was collected. First, stool test was performed by direct slide and formalin ethyl acetate to identify parasitic agents. Then DNA was extracted from the collected samples and then molecular tests were performed using specific primers and at the end of the positive samples were sequenced to determine the type of parasite subtypes.

Out of 150 samples, including 50 samples of children with gastrointestinal symptoms and 100 samples of children without gastrointestinal symptoms, only 6 cases in symptomatic children and 7 cases in asymptomatic children were positive for Blastocystitis hominis by molecular PCR and subtypes were obtained. ST1 was determined in the case group and ST 1-2-3-7 in the control group. The most common subtype in two groups was ST1. The most common gastrointestinal symptom in the symptomatic group was abdominal pain.

ST1 was the most common subtype in both groups of children with and without gastrointestinal symptoms. Due to the existence of different subtypes of this parasite in different regions, it seems that by using the pattern of subtypes obtained from the parasite, in future studies, the relationship between clinical symptoms and ways of transmission of this parasite can be discussed and studied