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

Blastocystis SSU-rDNA sequence data from 317 captive and free-living non-human primates (NHPs) representing 30 genera of apes, Old and New World (OW and NW) monkeys and prosimians were analysed to investigate subtype (ST) and allele distribution among hosts. Excluding 20 mixed ST infections, 27% of the sequences belonged to ST1, 22% to ST2, 34% to ST3, 1% to ST4, 4% to ST5, 11% to ST8, <1% to ST13 and 1% to ST15. The study confirmed cryptic host specificity of ST1 and ST3; conversely, considerable overlap in ST2 alleles exists among humans and NHPs. Subtype distribution in humans and NHPs differs mainly in that ST4 is rarely reported in NHPs while ST5 and ST8 are both unusual in humans. This may be due to host specificity and/or the apparent geographically restricted range of some subtypes. While the distribution of ST1, ST2 and ST3 was independent of NHP group or geographical association, ST5 was seen only in apes and OW monkeys and ST8 primarily in arboreal NHPs and only in species native to Asia or South America.