Showing posts with label susceptibility. Show all posts
Showing posts with label susceptibility. Show all posts

Friday, September 2, 2016

This Month in Blastocystis Research - AUG 2016

Last month, I asked about your thoughts on Blastocystis and age... is age a limiting factor with regard to the susceptibility to infection/colonisation? One of the reasons why I asked is that we and others have noted that Blastocystis becomes more and more common with age... at least until a certain stage in life (adulthood) and at least in certain countries.

Among the answers I got, I'd like to highlight one from Dr Graham Clark, London School of Hygiene and Tropical Medicine, with whom I've had the tremendous privilege of working for more than 10 years. Dr Clark writes:

In my opinion, prevalence is a mixture of three things: exposure, immunity and ‘loss’.

To illustrate the latter, infection with Entamoeba histolytica (and probably other Entamoeba species) can be lost spontaneously with a half-life of 13 months (in one study at least, Blessmann et al. 2003 J Clin Micro). There is no evidence that immunity is involved, just chance. Extrapolating to Blastocystis, I suspect the same is true – loss can occur spontaneously but what the half-life of colonisation is we have no idea and it would be difficult to evaluate without high resolution genotyping (which is what we used in E. histolytica). If you had a population of infected individuals that were followed over time and a high res[olution] typing method you could calculate rates of loss and of new infections, as we did for E. histolytica.

I do not think there is any evidence for immunity to Blastocystis infection, which leaves exposure. If the rate of exposure is greater than the rate of spontaneous loss then you would get an increase in prevalence with age. I suspect (hope!) exposure is higher in children for behavioural reasons than in adults so you will get a levelling out of prevalence with age to a point where the rate of new infection balances the rate of spontaneous loss. If exposure stays high then this plateau may not be reached.

So why are other parasites different? Probably there is immunity leading to clearance of the infection in children.
Certainly, we have quite good data from Ireland indicating that once established, Blastocystis will keep colonising the GI tract for ages (i.e., many years). Regarding loss, I hope that I'll soon be able to refer to some data that we have obtained in a study on experimental animals.

Exposure is obvisouly something to think about - Blastocystis is transmitted by ingesting food/drink contaminated with Blastocystis and/or improper hygiene. The fact remains that studies using molecular methods for detection have identified most children in Sub-Saharan African study populations as being colonised, while in more developed countries, parasites such as Dientamoeba fragilis are more common in toddlers and smaller children; Blastocystis emerges and gains in prevalence only in teenagers and older individuals... roughly speaking.

One aspect of colonisation that I've been very interested in myself, is whether it's each individual's gut microbiota that "decides" whether Blastocystis colonisation/infection establishes upon exposure. We are seeing some data emerging from a couple of labs on this, and maybe next month I'll be able to bring you up to speed in this matter. To my knowledge, there is not a lot of precedence–if any–for such a situation, but the fact that we have seen quite clear differences in gut microbiota patterns between those who have Blastocystis and those who have not, has left me thinking...

If you want to contribute to the discussion, please drop a line. 


Blessmann, J., Ali, I., Ton Nu, P., Dinh, B., Ngo Viet, T., Le Van, A., Clark, C., & Tannich, E. (2003). Longitudinal Study of Intestinal Entamoeba histolytica Infections in Asymptomatic Adult Carriers Journal of Clinical Microbiology, 41 (10), 4745-4750 DOI: 10.1128/JCM.41.10.4745-4750.2003

Scanlan PD, Stensvold CR, Rajilić-Stojanović M, Heilig HG, De Vos WM, O'Toole PW, & Cotter PD (2014). The microbial eukaryote Blastocystis is a prevalent and diverse member of the healthy human gut microbiota. FEMS microbiology ecology, 90 (1), 326-30 PMID: 25077936