Blastocystis is a common intestinal micro-eukaryote found in both humans and non-human hosts and known to be genetically very diverse. It has been divided into numerous subtypes (STs), nine of which have been identified in humans to date. Surveys of ST prevalence have started to emerge over the past few years but to date no data are available for any African country except Egypt and Tanzania. In this study, we determined the prevalence of Blastocystis STs in populations from Libya, Liberia and Nigeria, as well as expanding the dataset available for the UK. A total of 356 Blastocystis STs were identified in this study, 271 from the UK, 38 from Libya, 25 from Liberia and 22 from Nigeria. SSU rRNA gene sequences revealed the presence of eight of the nine STs known from humans but at varying frequencies between countries. ST1 was the most common ST in Libya and Nigeria whereas ST3 showed the highest frequency in the other two countries, as indeed is the case in most populations around the world. ST4 was absent in Libya and ST2 in Nigeria, while no ST5, ST6, ST8 or ST9 infections were detected in any of the three African populations. The picture emerging from this and other surveys suggests that there is significant variation in ST prevalence between populations. Some of the possible reasons for and implications of this diversity are discussed.

23Blastocystis is a common unicellular anaerobic eukaryote that inhabits the large intestine of 24 many animals worldwide, including humans. The finding of Blastocystis in faeces in 25 mammals and birds has led to proposals of zoonotic potential and that these hosts may be the 26 source of many human infections. Blastocystis is, however, a genetically diverse complex of 27 many distinct organisms (termed subtypes; STs), and sampling to date has been limited, both 28 geographically and in the range of hosts studied. In order to expand our understanding of host 29 specificity of Blastocystis STs, 557 samples were examined from various non-primate animal 30 hosts and from a variety of different countries in Africa, Asia and Europe. STs were 31 identified using 'barcoding' of the small subunit rRNA gene using DNA extracted either 32 from culture or directly from faeces. The host and geographic range of several STs has 33 thereby been greatly expanded and the evidence suggests that livestock is not a major 34 contributor to human infection. Two new STs were detected among the barcode sequences 35 obtained; for these, and for three others where the data were incomplete, the corresponding 36 genes were fully sequenced and phylogenetic analysis was undertaken. 37 38

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Despite Blastocystis being one of the most widespread and prevalent intestinal eukaryotes, its role in health and disease remains elusive. DNA-based detection methods have led to a recognition that the organism is much more common than previously thought, at least in some geographic regions and some groups of individuals. Molecular methods have also enabled us to start categorizing the vast genetic heterogeneity that exists among Blastocystis isolates, wherein the key to potential differences in the clinical outcome of Blastocystis carriage may lie. In this review we summarize some of the recent developments and advances in Blastocystis research, including updates on diagnostic methods, molecular epidemiology, genetic diversity, host specificity, clinical significance, taxonomy, and genomics. As we are now in the microbiome era, we also review some of the steps taken towards understanding the place of Blastocystis in the intestinal microbiota.