The microscopic parasite Blastocystis sp. is the most common potentially disease-causing organism found alone (without any other such organism) in patients with intestinal symptoms in the United States (Boorom et al). It was first associated with symptoms of illness in the late 1800s, yet today there is still a long-standing debate about whether or not it actually causes disease.
Blastocystosis
Infection with Blastocystis sp. is known as blastocystosis. The organism is thought to be passed from one person to another – and also to and from various animals – by the fecal-oral route (swallowing food or water contaminated with feces). Reported symptoms of blastocystosis range from mild or none, to abdominal pain, diarrhea, constipation, and even skin rashes, in some cases continuing long term.
Various scientists and medical professionals have observed that the reported symptoms of blastocystosis are very similar to those of irritable bowel syndrome (IBS) – and indeed, certain other intestinal protozoan parasites produce similar symptoms. This has lead to studies of IBS patients to investigate how many of them are infected with Blastocystis.
Irritable Bowel Syndrome and Blastocystis
A number of studies have found that a significant number of IBS sufferers are infected with Blastocystis sp. compared with healthy controls. Some studies, however, have found no difference in infection rates between IBS sufferers and healthy controls.
Important caveats to remember when Blastocystis appears to correlate with IBS include:
- Even if a lot of patients with IBS are infected with Blastocystis, scientists don’t yet know whether the parasite is causing the symptoms or whether IBS in some way makes the intestine a good place for the organism to thrive.
- Those who are infected with Blastocystis have consumed fecal contamination. There may be some other organism, undetected, that is causing symptoms (Giardia lamblia, Dientamoeba fragilis).
- Even if Blastocystis explains some cases of IBS, it is unlikely that it will explain all cases. Other pathogens may be involved.
Studies that don’t link Blastocystis to IBS may be flawed as well:
- Recent research reveals that there are at least nine subtypes of Blastocystis and it’s likely that only some of them are involved in IBS, if any. Researchers must sort out the subtypes and look for the right ones in IBS studies.
- Blastocystis is often missed by the laboratory methods used to detect it, with culture being the most sensitive method. Studies that don’t use culture likely miss a significant number of positive cases.
To date, an insufficient number of published studies exist to draw a final conclusion about the role of Blastocystis in irritable bowel syndrome. Carefully conducted studies in both animals and people may yet provide the answer.
Sources:
Boorom, Kenneth F., Huw Smith, Laila Nimri, et al. 2008. “Oh my aching gut: irritable bowel syndrome, Blastocystis, and asymptomatic infection.” Parasites & Vectors 1:40.
Giacometti, O. Cirioni, A. Fiorentini et al. 1999. “Irritable Bowel Syndrome in Patients with Blastocystis hominis Infection.” European Journal of Clinical Microbiology and Infectious Disease 18: 436-439.
Yakoob, Javed, Wasim Jafri, Nadim Jafri et al. 2004. “Irritable Bowel Syndrome: In Search of an Etiology: Role of Blastocystis hominis.” American Journal of Tropical Medicine and Hygiene 70(4): 383 -385.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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