Blastocystis Facts Sheet

I've tried to summarise a few facts here:

• Blastocystis is a single-celled, microbial parasitic protist colonising mainly the large intestine of man and other mammals, birds, reptiles, and other animals, even insects.
• The parasite is extremely common in humans, and possibly the most common microbial non-fungal eukaryote in the human intestine. More than one billion people may be colonised.
• Blastocystis comprises many ribosomal lineages, most or all of which are comparable to separate species; they are currently known as subtypes (ST).
• Humans are common hosts of ST1, ST2, ST3 and ST4, whereas other subtypes such as ST6, ST7 and ST8 are seen occasionally. ST5 and ST9 are very rare in humans. 
• Almost all subtypes found in humans are also found in animals; however, zoonotic transmission is probably uncommon, at least for the most common subtypes (ST1—ST4).
• Most carriers do probably not experience more intestinal symptoms than the average individual.
• We do not know when to seek to eradicate Blastocystis and there are no valid treatment guidelines. The effect of metronidazole may be very limited.
•  ST3 is probably the most common subtype in humans.
• ST4 may be more much more common in Europe than outside Europe. 
• ST4 has been seen frequently in patients with different types of diarrhoea or other intestinal problems, but appears uncommon in healthy individuals.
• Blastocystis is best detected by (real-time) PCR and culture; conventional parasitological techniques have generally poor sensitivity.

·         Ongoing epidemiological studies seek to analyse subtype distributions in various cohorts, e.g. IBS patients and the background population. We also continuously explore the genetic variation and host specificity of Blastocystis. Genome studies seek to unravel virulence genes that may be involved in pathogenesis, but only the genome for ST7 has been sequenced so far.

Blastocystis Treatment

In my opinion, in many cases we should "leave Blastocystis alone". In some cases, however, treatment may be warranted. However, currently there are no convincing drug regimens. RCTs needed.
For more information, please consult this review. 

Some updates on Blastocystis

Blastocystis is a micro-eukaryote, a so-called protist, parasitising the intestine of humans and a variety of animals.

We estimate that at least 1 billion people worldwide are colonised by this parasite, and we believe that the majority experience no more episodes of intestinal upset, e.g. diarrhoea, than the average individual.

Blastocystis colonises the intestine for a long time (probably months or years).

Many species of Blastocystis are known, of which at least 9 have been found in humans. Such species are currently termed "subtypes" (STs). ST1, ST2, ST3 and ST4 are common in Europe. While ST1, ST2, and ST3 appear to have equal prevalences in patients with diarrhoea and healthy individuals, ST4 appears to be linked to diarrhoea and/or chronic conditions such as irritable bowel syndrome (IBS).

There is no known efficient treatment of Blastocystis. Although metronidazole is often prescribed for Blastocystis infections, there is conflicting reports on its efficacy. Even in combination with a luminal agent, such as paromomycin, Blastocystis eradication cannot be guaranteed.

Whether Blastocystis causes symptoms in humans may depend on factors such as co-evolution. ST3 is the most common subtype in humans and ST3 may account for 30-50% of Blastocystis in humans. ST3 shows substantial intra-subtype genetic variation, and we believe that Blastocystis ST3 has co-evolved with humans, and therefore we may have adapted to ST3 colonisation. ST4 on the other hand is almost clonal and may have entered the human population relatively recently. This could partly explain why ST4 colonisation has been linked to intestinal symptoms.

Further reading:
1. Stensvold CR, Alfellani M, Clark CG. Levels of genetic diversity vary dramatically between Blastocystis subtypes.
2. Stensvold CR, Christiansen DB, Olsen KE, Nielsen HV. Blastocystis sp. ST4 is common in Danish Blastocystis-positive patients presenting with acute diarrhea.