Taxonomy: Class Blastocystea
Animal: Blastocystis hominis 6 14a.jpg
Sites: Gut
Blastocystis hominis cyst (below centre) and vacuolar forms (see to left of cyst). Cysts arise from the avacuolar and multivacuolar forms in the colon, is about 5 microns wide, usually one nucleus, thick cyst wall and very small vacuoles. This may be the transmissible stage but this has not been proven (Trichrome stain of faecal preparation). Vacuolar form has a central vacuole with 1 - 4 nuclei and 5 - 20 microns wide. The vacuolar form is found in culture or passed from the colon and was thought to possibly be the typical form but they are not the forms present in the colon. Unless the specimen is examined fresh or fixed with PVA or formalin immediately on passing, the in vivo forms ( avacuolar, amoeboid and multi- vacuolar forms) will have deteriorated and only the CV or granular forms will be seen in wet preparations, cysts will be missed if a stained smear is not examined due to their small size - 5 microns - light microscopy being the usual way to examine faecal samples.

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