Blastocystis hominis

Disease / Pathology

Blastocystis hominis is a unicellular parasite that has been recently reclassified as a stremenophile via molecular sequencing. The group has extensive genetic diversity and are best described as Blastocystis spp.. Within the human host there may be a variety of subtypes present which may be why there is extensive variation in the sizes of the organisms (polymorphism) when detected. Subtypes 1-4 are most commonly detected in humans.

The organism’s role in the production of disease in humans is largely unknown. There are individuals that may carry the organism and experience little or no discomfort. Some studies may indicate that subtype 3 is more prone to producing pathogenic symptoms or other possible subtypes.

Symptoms include watery diarrhea; nausea, abdominal pain, bloating, excessive gas, loss of appetite, and weight loss . An infection with Blasotcystis hominis is called blastocystosis.

Location in the Host

Blastocystis hominis colonizes the intestines of humans and other mammals.

Geographic Distribution

Blastocystis hominis has a worldwide distribution.

Life Cycle

Ingestion of infectious cyst from contaminated food or water are the likely sources of infection. Since animals can serve as a reservoir, persons working with animals are at risk. Travel to countries that are most likely to have a high incidence may put persons at risk. Exposure to infectious human feces can also be a source of infections.

Morphology & Diagnosis

Blastocystis hominis is spherical with a vacuole that takes up a majority of the organism. There are small nuclei that are located on on the periphery of the vacuole. Generally the organism can be seen either in lugol’s iodine concentrates as well as trichrome stained preparations. The size of the organisms can vary widely within the specimen being analyzed. Generally the size range is between 5-30 microns.


Blastocystis hominis – Trichrome photo 1000x oil on the left with descriptive illustration on the left. photo and illustration by W. Vientos