Blastocystis is a single-celled, alga-like intestinal parasite. Apart from yeasts, Blastocystis is the most common eukaryotic (i.e. non-bacterial) organism found in our intestine, and more than 1 billion people may be colonised.
The public health significance of Blastocystis colonisation, however, is incompletely known. Irritable bowel syndrome (IBS) has been linked to Blastocystis colonisation. This may be due to fact that the symptoms that may arise during colonisation are quite reminiscent of IBS symptoms and both conditions are common. While some studies have found association between Blastocystis and IBS, quite a few have not.
Once established, this parasite can reside in the gut for months-years. Although metronidazole is often prescribed for symptomatic infection (and where other causes of symptoms have been ruled out), the use of sensitive diagnostic methods such as PCR has shown us, that Blastocystis is most often not eradicated by this drug even after 10 days of max dosage, and currently, there is no convincing drug regimen.
Blastocystis comprises many different species (subtypes (ST)), some of which are common in humans. While subtype 1, 2 and 3 are common in all parts of world and appear to be equally prevalent in patients with diarrhoea and the background population (i.e. individuals with no intestinal complaints), ST4 seems to appear mainly in patients with diarrhoea and/or IBS, and ST4 is therefore a subtype currently under intense scrutiny. Meanwhile, I believe that most infestations with ST3 are harmless. This is supported by some of our recent data showing that the genetic diversity of ST3 is extensive, suggesting co-evolution with humans over a long period. Contrary to this stands ST4, which has an almost clonal population structure, suggesting recent entry into the human population. Furthermore, ST4 appears to have a restricted geographical distribution, being relatively rare outside Europe. However, we are still in lack of data, and strict inferences on ST distribution and role in disease are still premature.
If ST4 is pathogenic, while other common subtypes are harmless commensals, this is not the first time parasites that cannot by distinguished by morphology differ in terms of the ability to cause disease. A similar situation is seen in those species of amoebae called Entamoeba histolytica and Entamoeba dispar. While E. dispar by most experts is considered a commensal mostly indicating relatively recent exposure to faecal-oral contamination, E. histolytica can lead to potentially fatal invasive disease, including abscess formation mainly in the liver.
Many of us harbour Blastocystis, and by far most of us without knowing it. One of the interesting things about Blastocystis is why so many people are hosting the parasite, while others do not. Very little is known about Blastocystis in the environment, and whether we are exposed to Blastocystis in food items, such as vegetables, or drinking water. The prevalence of Blastocystis appears to be higher among adults and the elderly.
Until recently, Blastocystis was quite difficult to detect. Still today, inappropriate methods are being used for detection, while sensitive tools such as culture and PCR are being increasingly employed in modern clinical microbiology labs to distinguish between carriers and non-carriers and to evaluate patients after treatment. There is no doubt that diagnostic challenges and failure to acknowledge Blastocystis‘ extensive genetic diversity have hampered attempts to get to grips with the clinical significance of Blastocystis.
Impartial information on Blastocystis for laymen is quite difficult to obtain and there are plenty of sites on the internet trying to make a commercial success of Blastocystis, perpetuating anecdotal data and information on the parasite for which there is currently no epidemiological, genetic or biochemical support.
For more information and updates on Blastocystis, please visit http://www.blastocystisblog.blogspot.com