ECCMID - the annual European Congress of Clinical Microbiology and Infectious Diseases (hosted by ESCMID) is currently taking place in Berlin. This year, I'm not attending, but I've been scanning the abstract book for 'Blastocystis', and it appears that an oral presentation was scheduled for yesterday in the "Emerging Infectious Diseases" section:
First of all: it's great to see fellow researchers screening larger (i.e. hundreds) of faecal DNAs by PCR for Blastocystis. I wish more people would do that to produce reliable data on prevalence and subtypes.
Now, as I've already mentioned, there are currently mainly two methods in use for subtyping, barcoding and STS PCR, and recently I evaluated these. To cut a long story short, barcoding is recommended for subtyping, since the STS method, which was used in the study by Tarasova et al. (abstract), appears to miss the majority of ST4 strains (the major genotype), and moreover, no STS primers exist for ST8 and ST9 (or any of the other 8 subtypes identified to date, but which have only been found in animals). So, the subtype data found in this study should be interpreted with this in mind.
Importantly however, I'm not sure whether the authors used the original Yoshikawa STS terminology or the terminology acknowledged in our 2007 consensus.
First, let us assume that consensus terminology is used. Then it's surprising to find ST5 in human samples in the first place, and finding a ST5 prevalence of 45% in a cohort of humans included in a larger study like this is very unlikely based on current evidence of more than 3,000 observations from all over the world, where the overall prevalence of ST5 in humans is <1%. Also, finding so much ST6 is also really striking. Also, if the consensus terminology is used, then I'm a bit puzzled why the authors put emphasis on ST7 not being found, since ST7 is relatively rare in humans.
And so let us assume that consensus terminology was not used, and the original Yoshikawa terminology was used instead. This would translate into STs 4, 6, and 7 not being detected in the CVH group. Which makes sense, since ST6 is extremely rare (at least in Europe), ST7 is only seen on occasion, and, as I said, the majority of ST4 infections are likely to go undetected by the STS method. However, ST4 appears quite common in Europe, and I suspect that it should be quite common in St Petersburg as well. But then there is one thing that comes to my mind: If ST4 infections are common, then there should be a relatively large number of samples detected by PCR which were untypable by PCR...and there is no information on untypable positive samples in the abstract...
But what is more: STS subtype 5 translates into ST2 in consensus terminology, and similarly STS subtype 6 equals ST5 (yes, it may seem confusing, but we have provided a table in the 2007 consensus paper to make this easy). This means that no matter which of the two terminologies were used, ST5 is seen in abundance in patients with CVH in St Petersburg! Which is a very remarkable observation, and maybe more interesting than the rest of the data, which I, by the way, find a bit difficult to follow (I expected to learn something about Blastocystis invasion, when I read the title of the abstract, but there is no data or information on invasiveness... and I'm very curious as to how the authors managed to obtain such a high number of samples from 'healthy people'! To evaluate the prevalence of Blastocystis in the control group, demographic data are needed, and a prevalence as low as 5.3% among healthy individuals makes me suspect that this control group consisted of newborns/toddlers who generally have a low prevalence of Blastocystis). Also, since when was ST1 'zoonotic'?
Anyway, often conference abstract are previews of upcoming articles, and so I expect that there will be a paper out soon from this group, and hopefully these issues will be clarified. The occasional confusion in Blastocystis epidemiology could be reduced to a minimum if everyone got into using barcoding and the Blastocystis 18S subtyping site (and go here for a video introduction to Blastocystis subtyping).
Now, as I've already mentioned, there are currently mainly two methods in use for subtyping, barcoding and STS PCR, and recently I evaluated these. To cut a long story short, barcoding is recommended for subtyping, since the STS method, which was used in the study by Tarasova et al. (abstract), appears to miss the majority of ST4 strains (the major genotype), and moreover, no STS primers exist for ST8 and ST9 (or any of the other 8 subtypes identified to date, but which have only been found in animals). So, the subtype data found in this study should be interpreted with this in mind.
Importantly however, I'm not sure whether the authors used the original Yoshikawa STS terminology or the terminology acknowledged in our 2007 consensus.
First, let us assume that consensus terminology is used. Then it's surprising to find ST5 in human samples in the first place, and finding a ST5 prevalence of 45% in a cohort of humans included in a larger study like this is very unlikely based on current evidence of more than 3,000 observations from all over the world, where the overall prevalence of ST5 in humans is <1%. Also, finding so much ST6 is also really striking. Also, if the consensus terminology is used, then I'm a bit puzzled why the authors put emphasis on ST7 not being found, since ST7 is relatively rare in humans.
And so let us assume that consensus terminology was not used, and the original Yoshikawa terminology was used instead. This would translate into STs 4, 6, and 7 not being detected in the CVH group. Which makes sense, since ST6 is extremely rare (at least in Europe), ST7 is only seen on occasion, and, as I said, the majority of ST4 infections are likely to go undetected by the STS method. However, ST4 appears quite common in Europe, and I suspect that it should be quite common in St Petersburg as well. But then there is one thing that comes to my mind: If ST4 infections are common, then there should be a relatively large number of samples detected by PCR which were untypable by PCR...and there is no information on untypable positive samples in the abstract...
But what is more: STS subtype 5 translates into ST2 in consensus terminology, and similarly STS subtype 6 equals ST5 (yes, it may seem confusing, but we have provided a table in the 2007 consensus paper to make this easy). This means that no matter which of the two terminologies were used, ST5 is seen in abundance in patients with CVH in St Petersburg! Which is a very remarkable observation, and maybe more interesting than the rest of the data, which I, by the way, find a bit difficult to follow (I expected to learn something about Blastocystis invasion, when I read the title of the abstract, but there is no data or information on invasiveness... and I'm very curious as to how the authors managed to obtain such a high number of samples from 'healthy people'! To evaluate the prevalence of Blastocystis in the control group, demographic data are needed, and a prevalence as low as 5.3% among healthy individuals makes me suspect that this control group consisted of newborns/toddlers who generally have a low prevalence of Blastocystis). Also, since when was ST1 'zoonotic'?
Anyway, often conference abstract are previews of upcoming articles, and so I expect that there will be a paper out soon from this group, and hopefully these issues will be clarified. The occasional confusion in Blastocystis epidemiology could be reduced to a minimum if everyone got into using barcoding and the Blastocystis 18S subtyping site (and go here for a video introduction to Blastocystis subtyping).
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| Are some citizens of St Petersburg infected by Blastocystis sp. ST5, a subtype seen primarily in livestock and African apes? Source |
References:
Tarasova E, Suvorova M, Sigidaev A, Suvorov A. Blastocystis invasion in patients with chronic viral hepatitis in Saint Petersburg. ECCMID 2013 abstract O338.
Alfellani MA, Stensvold CR, Vidal-Lapiedra A, Onuoha ES, Fagbenro-Beyioku AF, & Clark CG (2013). Variable geographic distribution of Blastocystis subtypes and its potential implications. Acta Tropica, 126 (1), 11-8 PMID: 23290980
Alfellani MA, Stensvold CR, Vidal-Lapiedra A, Onuoha ES, Fagbenro-Beyioku AF, & Clark CG (2013). Variable geographic distribution of Blastocystis subtypes and its potential implications. Acta Tropica, 126 (1), 11-8 PMID: 23290980
Stensvold CR (2013). Comparison of sequencing (barcode region) and sequence-tagged-site PCR for Blastocystis subtyping. Journal of Clinical Microbiology, 51 (1), 190-4 PMID: 23115257
Stensvold CR, Suresh GK, Tan KS, Thompson RC, Traub RJ, Viscogliosi E, Yoshikawa H, & Clark CG (2007). Terminology for Blastocystis subtypes--a consensus. Trends in Parasitology, 23 (3), 93-6 PMID: 17241816
