Many different types of Blastocystis exist, and they can infect humans, farm animals, birds, rodents, amphibians, reptiles, fish, and even cockroaches.

[5] A lively debate ensued in the early 1990s, with some physicians objecting to publication of reports that Blastocystis caused disease.

An Italian researcher reported differences in the protein profiles of isolates associated with chronic and acute infection.

[citation needed] The studies that followed generally found that no specific "pathogenic" or nonpathogenic species of Blastocystis exists.

The researchers concluded that host factors, such as age and genetics, may play the dominant role in determining the symptoms seen in the disease.

[30][59][60][61][62] For identification of infection, the only method clinically available in most areas is the ova and parasite (O&P) exam, which identifies the presence of the organism by microscopic examination of a chemically preserved stool specimen.

In the United States, pathologists are required to report the presence of Blastocystis when found during an O&P exam, so a special test does not have to be ordered.

Direct microscopy is inexpensive, as the same test can identify a variety of gastrointestinal infections, such as Giardia, Entamoeba histolytica, and Cryptosporidium.

[67] The study used blood samples to measure the patient's immune reaction to chemicals present on the surface of the Blastocystis cell.

[72] Historical reports, such as one from 1916, note difficulty associated with eradication of Blastocystis from patients, describing it as "an infection that is hard to get rid of.

[76][83] Rifaximin,[84] and albendazole have shown promise as has ivermectin which demonstrated high effectiveness against blastocystis hominis isolates in an in vitro study.

[85] There is also evidence that the probiotic yeast Saccharomyces boulardii,[79] and the plant Mallotus oppositifolius[86] may be effective against Blastocystis infections.

Emetine was reported as successful in cases in early 20th century with British soldiers who contracted Blastocystis infection while serving in Egypt.

Clioquinol (Entero-vioform) was noted as successful in treatment of Blastocystis infection but removed from the market following an adverse event in Japan.

A number of different species groups of Blastocystis infect humans,[69] with some being reported to cause disease while others do not.

[68][20] To date, surveys have not distinguished between different types of Blastocystis in humans, so the significance of findings may be difficult to evaluate.

[citation needed] Experimental infection in immunocompetent and immunocompromised mice has produced intestinal inflammation, altered bowel habits, lethargy, and death.

[96] While many enteric protists are the subject of research, Blastocystis is unusual in that basic questions concerning how it should be diagnosed and treated and how it causes disease remain unsettled.