The open access journal 'Tropical Parasitology' (published by the Indian Academy of Tropical Parasitology) has included a symposium on
Blastocystis in their January-June (Vol. 3) issue (available
here). The symposium comprises three papers; one is on "taxonomy, biology and virulence", the next is on genetic diversity and molecular methods for diagnosis and epidemiology, and the last one is on treatment controversies. I believe that it may take quite a while before these papers will appear in PubMed.
The first paper written by Drs Parija and Jeremiah sums up a few of the aspects related to (especially historical) taxonomic issues and very little on the actual biology of
Blastocystis. Meanwhile, there is quite a substantial section on
Blastocystis morphology. Regarding virulence, the authors mention the possibility that differences in virulence may be due to differences in subtypes, but that subtyping alone does not predict pathogenicity which in part may be due to varying levels of intra-subtype genetic variation. The authors also briefly mention some of the morphological and phenotypical observations that have been associated with 'pathogenic
Blastocystis', such as the amoeboid stage, large cells, rough surface, slow growth rate, and increased binding to lectins. It is always interesting to speculate on such associations, but it must be kept in mind that results from in-vitro experiments may not necessarily reflect in-vivo situations.
One topic that keeps popping up in the literature - and also in two of the papers here in this symposium - is the possibility of 'amoebic forms' of
Blastocystis being associated with symptomatic infection. This hypothesis was introduced in 2006 by Tan and Suresh, I believe; Scanlan (2013) speculated that amoeboid forms might be the nutrient acquiring form potentially selecting for bacterial virulence or certain bacterial communities through grazing; please go
here for more thoughts from a previous blog post.
My own experience on
Blastocystis morphology mainly stems from looking at cultures, and since we practically only get isolates from patients with gastrointestinal disease, I don't know what
Blastocystis cultures from asymptomatic individuals look like. A dear colleague of mine - Marianne Lebbad, a brilliant Swedish parasitologist with many years in business - sent me the picture below (light microscopy of a faecal concentrate) and speculates that
Blastocystis might be able to form groups/clusters of cells, maybe even with the ability to form a mono-layer on the surface of the gut mucosa? I've never observed the cluster formation in cultures, but then again, we have no idea of whether the stages seen in in vitro cultures (microaerophilic environment) are identical to the in vivo stages (strictly anaerobic), and exactly how
Blastocystis lives and multiplies in the colon... Anyway, the idea of biofilm comes into mind. It would be nice to learn more from colleagues with a similar experience.
 |
| Light microscopy of Blastocystis apparently forming a cluster of cells; we wonder whether the cells are in fact 'glued' together and if so, how? Courtesy of Dr Marianne Lebbad. |
Moving on to the next paper, this one was written by me and deals mostly with issues and developments within the field of diagnostics, molecular characterisation, and molecular epidemiology. The target audience comprises clinical microbiologists and those involved in
Blastocystis epidemiology and genetic diversity research. Included is a table, which is basically a reproduction of the one included in the recent paper by
Alfellani et al. (2013) displaying the distribution of subtypes in humans across different geographical regions. I hope that the open access feature of this paper will prompt even more researcher into
Blastocystis epidemiology! At least it is currently listed on the site as 'popular'!
The third paper in the string is written by Drs Sekar and Shanthi. These authors put emphasis on the conspicuous lack of data on the metabolic processes of
Blastocystis, making it difficult to establish how to best approach antibiotic intervention; we must anticipate that with more genomic and transcriptomic data analyses arriving within a foreseeable future we will soon know much more about this. They also reiterate what has been put forth by many, namely that differences in eradication may boil down to differences in drug susceptibility, which again may be due to a variety of reasons, including genetic diversity, which is extreme in
Blastocystis.
According to these authors, 'therapy should be limited to patients with persistent symptoms subsequent to a complete work up for alternative etiologies'; at the present stage this appears sensible, although clinicians would probably appreciate a clearer definition of 'symptoms'!
The review goes through some of the drugs most commonly used for treating
Blastocystis, including metronidazole, paromomycin and co-trimoxazole, but also includes a few data on the use of the probiotic
Saccharomyces boulardii in attempts to eradicate
Blastocystis. There is not very much on the mechanisms of drug action, - it's more like a summary of data coming out from different studies, including the few placebo-controlled ones.
Regarding co-trimoxazole (which is also known as 'Bactrim' or 'Septra') this drug combo is often administered to HIV-patients prophylactically against
Pneumocystis. In a study of parasites in Danish HIV patients, only 6/96 patients were given co-trimoxazole (unpublished data); two of these patients had
Blastocystis. Hence, one 'alternative' way of finding out about the efficacy of co-trimoxazole on
Blatocystis is to test the stools from patients undergoing long-term
Pneumocystis prophylaxis comparing these patients to a cohort not receiving
Pneumocystis prophylaxis but otherwise similar.
I find it a bit peculiar though to go through a review on treatment data that does not at one single point mention the need for sensitive diagnostics when evaluating courses of treatment and the identification of carriers and non-carriers. Also, there are some passages which are quite difficult for me to follow, for instance p. 36, second column, bottom section.
I hope that this symposium will inspire some of our colleagues and contribute to an increased understanding of
Blastocystis.
References:
SYMPOSIUM
Parija SC & Jeremiah SS (2013). Blastocystis: Taxonomy, biology and virulence Tropical Parasitology DOI: 10.4103/2229-5070.113894
Stensvold CR (2013). Blastocystis: Genetic diversity and molecular methods for diagnosis and epidemiology Tropical Parasitology DOI: 10.4103/2229-5070.113896
Sekar U & Shanthi M (2013). Blastocystis: Consensus of treatment and controversies Tropical Parasitology DOI: 10.4103/2229-5070.113901
OTHER:
Scanlan PD (2012). Blastocystis: past pitfalls and future perspectives. Trends in parasitology, 28 (8), 327-34 PMID: 22738855
Stensvold CR, Nielsen SD, Badsberg JH, Engberg J, Friis-Møller N, Nielsen SS, Nielsen HV, & Friis-Møller A (2011). The prevalence and clinical significance of intestinal parasites in HIV-infected patients in Denmark. Scandinavian Journal of Infectious Diseases, 43 (2), 129-35 PMID: 20936912
Tan TC & Suresh KG (2006). Predominance of amoeboid forms of Blastocystis hominis in isolates from symptomatic patients. Parasitology Research, 98 (3), 189-93 PMID: 16323025
