Causal Agents

Blastocystis is a genetically diverse unicellular parasite of unclear pathogenic potential that colonizes the intestines of humans and a wide range of non-human animals. On the basis of molecular data, the organism has been classified as a stramenopile. Organisms such as diatoms, chrysophytes, water molds, and slime nets are other examples of stramenopiles.

Blastocystis organisms isolated from humans have commonly been referred to as B. hominis. However, because of extensive genetic diversity (even among organisms isolated from humans) and low host specificity, the designation Blastocystis sp. is considered more appropriate. If genetic typing is performed, the subtype (ST) also should be noted in accordance with consensus terminology.* Among the nine STs found to date in humans, the four most prevalent STs are ST1, ST2, ST3, and ST4; other STs occur sporadically and may be related to zoonotic transmission.

* Stensvold, C.R., Suresh, G.K., Tan, K.S., Thompson, R.A., Traub, R.J., Viscogliosi, E., Yoshikawa, H. and Clark, C.G., 2007. Terminology for Blastocystis subtypes–a consensus. Trends in Parasitology, 23 (3), pp.93-96.

Life Cycle

The life cycle of Blastocystis sp. is not yet understood, including the infectious stage and whether (and which of the) various morphologic forms of this polymorphic organism that have been identified in stool or culture constitute distinct biologic stages of the parasite in the intestinal tract of hosts. The cyst form (3–5 µm) is postulated to be an infectious stage, but not confirmed. The predominant form found in human stool specimens is referred to as the vacuolar (or central body) form and is of variable size (5–40 µm, occasionally much larger). Replication appears to occur via binary fission. Other morphologic forms (e.g., ameboid and granular forms) also have been noted in stool samples and/or culture; their biological role and eventual developmental fate require further investigation.


Blastocystis has been detected in stool from humans and a wide range of non-human animals (e.g. canids, swine, primates, rodents, birds, etc.).

Geographic Distribution

Blastocystis sp. is found worldwide. Efforts to characterize the geographic and host distribution of Blastocystis subtypes are ongoing.

Clinical Presentation

Whether Blastocystis sp. (or particular subtypes thereof or particular strains of certain subtypes) can cause gastrointestinal disease (symptomatic infection) in humans continues to be debated and investigated. Blastocystis sp. has been detected in both symptomatic and asymptomatic persons.

Page last reviewed: October 22, 2019