This Month In Blastocystis Research (JUN 2013)

Another paper in the string of publications coming out from the PhD study by Dr Alfellani (London School of Hygiene and Tropical Medicine) has just appeared in PubMed.

Dr Alfellani and his colleagues have done a great job in analysing a multitude of samples from humans, non-human primates and animals; I have previously blogged about their observations from studies of human and non-human primates. Moreover, they have surveyed available data in order to better discuss their own findings, and the work has contributed significantly to what today is known about the host specificity, genetic diversity, phylogeography and general molecular epidemiology of Blastocystis.

Alfellani's most recent paper is published in the journal Protist, and it deals with the 'Genetic Diversity of Blastocystis in Livestock and Zoo Animals'.

It is quite a large paper which includes a lot of new information and a comprehensive (and hopefully exhaustive) table summarising Blastocystis subtype data in all relevant hosts (humans, non-human primates, other mammals and birds).

I will highlight a couple of things from the paper:

1. Apart from reporting on virtually complete SSU rDNA sequences from a couple of subtypes for which entire SSU rDNA sequences have yet not been available, we also report on three novel subtypes. Until recently, we only knew about 14 subtypes (ST1-ST14), of which ST1-ST9 can be found in humans. Now, three additional subtypes have been identified; ST15 in artiodactyls (camel and sheep) and non-human primates (chimpanzee and gibbon), ST16 in kangaroos, and ST17 in gundis.

The Gundi (Ctenodactylus gundi) is a rodent living mainly in the deserts of Northern Africa. (Source)

2. Novel data arising from analysis of faecal samples from humans and animals in Sebha, Libya, strongly indicate that humans and animals in this area are infected by different subtypes: Humans appear to carry ST1, ST2, and ST3, while synanthropic animals (artiodactyls in this case) mostly have ST5 and ST10 infections, suggesting that livestock is not a major contributor to human Blastocystis infection.

To this end, there is growing evidence of quite a substantial degree of host specificity of Blastocystis.  Even when subtypes overlap between humans and animals, we have accumulating evidence that the strains found in humans and animals are different. This means that the hypothesis that animals constitute an important reservoir of human Blastocystis infections currently has very limited support. It is my clear impression that when a strain of ST6 or ST8 is detected in humans, this strain has most probably been transmitted from an animal source. But we very rarely see these subtypes in humans, at least in Europeans.

It will be extremely interesting to see how the universe of Blastocystis subtypes unfolds... by genetically characterising strains in humans and non-human hosts, we are building up a clearer picture of transmission patterns and evolutionary biology, including our adaptation to Blastocystis, and the parasite's adaptation to us and other hosts.

It is noteworthy that we are starting to see different subtypes in rodents. We have previously thought that generally, rodents were infected by ST4. But now we know that many rodents are not infected, and we also know that rodents may harbour subtypes other than ST4.

So,17 subtypes of Blastocystis are now known. We have probably only seen the top of the iceberg, since many host species have not yet been sampled from, and it is likely that we will see quite a few STs being identified in the nearest future. To this end it is necessary to have a consensus regarding the identification of novel subtypes. Along with the Protist paper we have uploaded a supplementary file (Appendix A, TXT format) with aligned reference sequences that can be used for phylogenetic analysis,  hoping that it will be useful to our colleagues. In a future blog post I will try to address the issues of identifying new subtypes more specifically.

ST15 is one of the more interesting subtypes since it appears to have quite a low host specificity - infecting both non-human primates and artiodactyls. Yet, we have come across it only now. ST15 and ST17 are remarkable in the way that they appear to be closer related to herptile and arthropod lineages, respectively, than to lineages from mammals.

Please note that virtually complete sequences of ST10, ST13, ST14, ST15, and ST17 analysed in the study have been released in GenBank just now.

Further reading:

Alfellani MA, Taner-Mulla D, Jacob AS, Imeede CA, Yoshikawa H, Stensvold CR, & Clark CG (2013). Genetic Diversity of Blastocystis in Livestock and Zoo Animals. Protist, 164 (4), 497-509 PMID: 23770574

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, Jacob AS, Perea NO, Krecek RC, Taner-Mulla D, Verweij JJ, Levecke B, Tannich E, Clark CG, & Stensvold CR (2013). Diversity and distribution of Blastocystis sp. subtypes in non-human primates. Parasitology, 140 (8), 966-71 PMID: 23561720

Blastocystis in Non-Human Primates

If my recent blog post "Blastocystis aux Enfers" could be described as "Blastocystis meets Dante Alighieri", then this post might come across as "Blastocystis meets Sir David Attenborough" (with all due respect to both of these gentlemen!).

Non-human primates (NHPs) include apes (hominoids), Old World monkeys (cercopithecoids), New World Monkeys (ceboids) and prosimians such as lemurs. I have been so fortunate to be involved in a study of Blastocystis in NHPs; a study which was led by Dr Alfellani with several co-investigators, and which has just appeared online in the journal Parasitology (click here to be diverted to the the website - first view article section).

The study is the first of its kind aiming to provide a substantial insight into the host specificity of Blastocystis in NHPs and included subtype observations for 441 captive and free-living animals representing no less than 30 genera; most of the data were generated during the study, while sporadic observations from similar studies were also included.

It was a huge study with a lot of interesting information, and I will try and summarise some of the points here.

Apes such as bonobos, chimps, gorillas and orangutans were colonised by some of the most common subtypes in humans, namely ST1, ST2, and ST3, accounting for about 77% of the cases. Contrary to humans though, ST5 also appeared rather common, accounting for about almost 14% of the cases, and some of the gibbons studied had ST8. Interestingly, a chimp and a gibbon were found to be hosts of a new subtype, ST15.

Old World monkeys were studied to an even larger extent, and again, ST1, ST3 and ST2 predominated, accounting for about 95% of all cases of single subtype infection. Here ST5 was also seen (2%) but only in langurs/lutungs and vervet monkeys. Interestingly, ST8 was seen only in 1/226 cases. ST13 was found by colleagues in Tanzanian colobus monkeys (Petrasova et al., 2011), and 8% of the 226 cases represented mixed/unknown subtype infections.

Woolly monkey (Lagothrix lagotricha) (Source)

New World monkeys included in the study were mainly represented by woolly monkeys, and these were colonised first and foremost by ST8 (49%), but ST3, ST2, ST1 were also found. So was a single case of ST4, which in general appears to be surprisingly rare among NHPs.

A few observations on lemurs were included, and such animals appear to host a vast variety of subtypes with no particular predilection, hence ST1, ST2, ST4, ST8, ST10 and ST15.

Ring-tailed lemur (Lemur catta) (Source).

The most striking differences between humans and NHPs in terms of colonisation by Blastocystis subtypes is that humans are very rarely colonised by ST5, while this subtype appears common in apes and Old World monkeys. ST8 was seen only in arboreal apes and in woolly and howler monkeys, which are also tree-dwellers, and it is tempting to think that ST8 is found mainly in tree-dwelling NHPs; to my knowledge, ST8 has not been found in non-primate hosts, except for once in a bird. Human colonisation by ST8 has been demonstrated only very rarely, for instance in a Danish woman returning from holiday in Indonesia and in animal keepers. Conversely, ST4 is seen extremely rarely in NHPs, while very common in humans in some parts of the world, apparently especially in Europe. These clear discrepancies in subtype distribution in humans and NHPs may boil down to host specificity and/or apparent geographically restricted range of some subtypes.

Another striking observation was that cryptic host specificity exists in ST1 and ST3, meaning that ST1 and ST3 strains found in NHPs overall differ genetically from strains found in humans belonging to the same subtypes, adding support to our previous findings.This suggests that humans are generally colonised by other strains than those found in NHPs. It will be interesting to see, whether other types of hosts sharing these subtypes carry distinct, host-specific strains. While MLST is probably the best way of testing for this, a lot of information can be obtained simply by barcoding. Pets, for instance, may share subtypes seen in humans, and so barcoding of "pet blasto" may be one of the very interesting pathways to knowledge.

We found no evidence of those subtypes that we have nicknamed "avian subtypes", namely ST6 and ST7. In some parts of the world, these two subtypes do not appear uncommon in humans; in Denmark and Sweden, for instance, ST7 is seen on quite a few occasions. But, interestingly, both STs are apparently absent in NHPs.

Langurs - the front cover of one of my favourite books showcasing works by the magnificent Walton Ford.

Incidentally, there is a sequence in GenBank from a gorilla (JX159284) which possibly represents a novel subtype, which is related to reptilian Blastocystis, and so it appears that the host spectrum and diversity of Blastocystis in NHPs continues to unfold.

A recent study saw that faecal microbiomes of wild non-human primates co-vary with host species, hence reflecting host phylogeny. This was evidenced by higher intra-species similarity among wild primate species, which may reflect species specificity of the microbiome in addition to dietary influences. This may in part explain the differences in Blastocystis subtypes seen in different NHP host species, but it is also possible that differences in subtypes reflect differences in habitat (and thereby possibly exposure) or geographical differences in subtype distribution. Indeed Homo sapiens is host to a variety of subtypes, and while ST4 is common in Europe, it appears virtually absent in many other parts of the world. Likewise, the differences in the prevalence of ST8 may reflect differences in geographical distribution, habitat and diet (arboreal vs. ground) as well differences in host specificity.

The overall interesting thing here is the schism of exposure vs. host specificity.


Suggested reading:

Alfellani, M., Jacob, A., Perea, N., Krecek, R., Taner-Mulla, D., Verweij, J., Levecke, B., Tannich, E., Clark, C., & Stensvold, C. (2013). Diversity and distribution of Blastocystis sp. subtypes in non-human primates Parasitology, 1-6 DOI: 10.1017/S0031182013000255

Yildirim S, Yeoman CJ, Sipos M, Torralba M, Wilson BA, Goldberg TL, Stumpf RM, Leigh SR, White BA, & Nelson KE (2010). Characterization of the fecal microbiome from non-human wild primates reveals species specific microbial communities. PloS one, 5 (11) PMID: 21103066

Stensvold CR, Arendrup MC, Nielsen HV, Bada A, & Thorsen S (2008). Symptomatic infection with Blastocystis sp. subtype 8 successfully treated with trimethoprim-sulfamethoxazole. Annals of tropical medicine and parasitology, 102 (3), 271-4 PMID: 18348782

Stensvold CR, Alfellani M, & Clark CG (2012). Levels of genetic diversity vary dramatically between Blastocystis subtypes. Infection, genetics and evolution : journal of molecular epidemiology and evolutionary genetics in infectious diseases, 12 (2), 263-73 PMID: 22116021