Morphology of the cyst of Blastocystis hominis.

Blastocystis horninis is a common human parasite which is probably transmitted through a cystic stagevia the oral-fecal route (Boreham and Stenzel, 1993). A number of publications have appeared on the morphology of the cyst at the ultrastructure level (Stenzel and Boreham, 199 1 ; Zaman et 01, 1993; Stenzel et 01, 1993) but so far its light microscopic appearance has not been adequately described. This is clearly needed, as laboratory diagnosis is based on light microscopy. There are two possible reasons for the lack of this information; the first is the relatively small size of the cyst (3-5 pm in the electron microscope) and the difficulty in separating it from the non-cystic stages of the parasite. A concentration technique has recently been described which obviates this problem by removing all the non-cystic stages and selectively concentrating the cysts (Zaman and Khan, 1994). Briefly the technique is as follows: