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Blastocystis is a polymorphic intestinal parasite that is common in humans. A total of 51 asymptomatic and symptomatic patients positive for Blastocystis only were included in the study. Symptoms were mainly nonspecific gastrointestinal symptoms. Blastocystis isolates were xenically cultured and subtyped. Blastocystis species subtype 3 was the predominant subtype. Intrasubtype differences (vacuolar/amoeboid presence) in subtype 3 morphotypes were observed in 32 asymptomatic and symptomatic subtype 3 cases and could possibly be related to Blastocystis pathogenic potential. Diverse morphologic features (vacuolar transiting to amoeboid), probably reflecting the progression from an asymptomatic to a symptomatic state, were observed in an asymptomatic subtype 3 carrier who later had symptoms. Searching for amoeboid forms might be helpful to presumptively screen symptomatic patients with subtype 3 or to follow up an asymptomatic subtype 3 carrier in case symptoms become evident before antiprotozoal treatment was attempted. Further studies on the roles of morphologic features and variation within Blastocystis species subtypes as predictors of symptoms are encouraged.

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