Blastocystis homonis Blastocystis hominis appear as spherical to oval cyst-like structures They can vary widely in size (5 to 30 µm; usual range 8 to 10 µm), and typically consist of a central body, or "vacuole" surrounded by a thin rim of cytoplasm containing up to six nuclei.

Blastocystis hominis has previously been considered as yeasts, fungi, or ameboid, flagellated, or sporozoan protozoa. Recent molecular studies dealing with the sequence information on the complete SSUrRNA gene, however, have placed B. hominis with an informal group: the stramenopiles (Silberman et al. 1996). Stramenopiles include unicellular and multicellular protistes including brown algae, diatoms, chrysophytes, water molds, slime nets, etc.

Blastocystis hominis cyst-like forms stained in trichrome The sizes vary from 4 µm (A) to 10 µm (F).  The vacuoles stain variably from red to blue. 

An unstained Blastocystis hominis cyst-like form.

Life cycle

Diagnosis Permanently stained smears are preferred over wet mount preparations because fecal debris may be mistaken for the organisms in the latter. 

The treatment of blastocystis hominis Many believe that this disease is self-limiting and therefore should not be treated. However, upon diagnosis with the disease, patients are usually treated with Metronidazole, which has been effective, but studies have also suggested resistance to this drug

Epidemiology This disease is not well-tracked, but appears to occur worldwide. Originally reported as being associated with diarrhea in the tropics and subtropics, more recent reports have show that infections are common in residents of tropical, subtropical, and developing countries. Young adults appear to have the highest rate of infection.