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.
