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| image = Four_common_forms_of_Blastocystis_hominis_Valzn.jpg
| image_upright = 1.2
| image_caption = ''Blastocystis'' sp.
| taxon = Blastocystis
| authority = (Alexieff 1911) Brumpt 1912<ref>{{cite journal | vauthors = Alexeieff A | year = 1911 | title = Sur la nature des formations dites "kystes de ''Trichomonas intestinalis''" | journal = CR Soc Biol | volume = 71 | pages = 296–298 }}</ref>
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'''''Blastocystis''''' is a genus of single-celled [[parasite]]s belonging to the [[Stramenopiles]] that includes [[algae]], [[diatom]]s, and [[water mold]]s. There are several species, living in the [[gastrointestinal tract]]s of species as diverse as humans, farm animals, birds, rodents, reptiles, amphibians, fish, and cockroaches.<ref name="YOSHIKAWA_2007">{{cite journal |vauthors=Yoshikawa H, Wu Z, Howe J, Hashimoto T, Geok-Choo N, Tan KS |title=Ultrastructural and phylogenetic studies on ''Blastocystis'' isolates from cockroaches |journal=The Journal of Eukaryotic Microbiology |volume=54 |issue=1 |pages=33–37 |year=2007 |pmid=17300516 |doi=10.1111/j.1550-7408.2006.00141.x|s2cid=45995362 }}</ref> ''Blastocystis'' has low host specificity, and many different species of ''Blastocystis'' can infect humans,<ref name="NOEL_2005">{{cite journal |vauthors=Noël C, Dufernez F, Gerbod D |title=Molecular Phylogenies of Blastocystis Isolates from Different Hosts: Implications for Genetic Diversity, Identification of Species, and Zoonosis |journal=Journal of Clinical Microbiology |volume=43 |issue=1 |pages=348–55 |date=January 2005 |pmid=15634993 |pmc=540115 |doi=10.1128/JCM.43.1.348-355.2005|language=zh|display-authors=etal}}</ref> and by current convention, any of these species would be identified as '''''Blastocystis hominis'''''.
''Blastocystis'' is one of the most common human parasites in the world and has a global distribution.<ref name="Primary: Blastocystosis 100% prevalence">{{cite journal | vauthors = El Safadi D, Gaayeb L, Meloni D, Cian A, Poirier P, Wawrzyniak I, Delbac F, Dabboussi F, Delhaes L, Seck M, Hamze M, Riveau G, Viscogliosi E | title = Children of Senegal River Basin show the highest prevalence of Blastocystis sp. ever observed worldwide | journal = BMC Infect. Dis. | volume = 14 | pages = 164 | date = March 2014 | pmid = 24666632 | pmc = 3987649 | doi = 10.1186/1471-2334-14-164 | doi-access = free }}</ref><ref name="Efficacy against Blastocystis">{{cite journal | vauthors = Roberts T, Stark D, Harkness J, Ellis J | title = Update on the pathogenic potential and treatment options for Blastocystis sp | journal = Gut Pathog | volume = 6 | pages = 17 | date = May 2014 | pmid = 24883113 | pmc = 4039988 | doi = 10.1186/1757-4749-6-17 | quote = Blastocystis is one of the most common intestinal protists of humans. ... A recent study showed that 100% of people from low socio-economic villages in Senegal were infected with Blastocystis sp. suggesting that transmission was increased due to poor hygiene sanitation, close contact with domestic animals and livestock, and water supply directly from well and river [10]. ... | doi-access = free }}</ref><ref name="Parasites in representative sample of US population">{{cite journal | vauthors = Amin OM | title = Seasonal prevalence of intestinal parasites in the United States during 2000 | journal = Am. J. Trop. Med. Hyg. | volume = 66 | issue = 6 | pages = 799–803 | year = 2002 | pmid = 12224595 | doi = 10.4269/ajtmh.2002.66.799 | s2cid = 3767711 | url = http://www.ajtmh.org/content/66/6/799.full.pdf | access-date = 3 January 2016 | quote = Parasitologic investigations of large patient populations are rarely conducted in the United States, where the illusion of freedom from parasitic infections still predominates. Such investigations are considerably more common in third-world countries where endemic parasitoses are more readily documented.<sup>1</sup> In an attempt to address this problem we reported the results of routine examination of fecal specimens for parasites from 644 patients in the United States during the summer of 1996. ...<br />'''Prevalence.''' Nine hundred sixteen (32%) of 2,896 tested patients were infected with 18 species of intestinal parasites in the year 2000 (Table 1) in 48 states and the District of Columbia as follows ... Blastocystis hominis was the most frequently detected parasite in single and multiple infections, with Cryptosporidium parvum and Entamoeba histolytica/E. dispar ranking second and third, respectively. | archive-date = 27 May 2020 | archive-url = https://web.archive.org/web/20200527184852/http://www.ajtmh.org/docserver/fulltext/14761645/66/6/12224595.pdf?expires=1590606232&id=id&accname=guest&checksum=B25679BF0F29B7733F838445F001B1F4 | url-status = dead }}</ref><ref name="Blastocystosis prevalence">{{cite journal | vauthors = Boorom KF, Smith H, Nimri L, Viscogliosi E, Spanakos G, Parkar U, Li LH, Zhou XN, Ok UZ, Leelayoova S, Jones MS | title = Oh my aching gut: irritable bowel syndrome, Blastocystis, and asymptomatic infection | journal = Parasit Vectors | volume = 1 | issue = 1 | pages = 40 | year = 2008 | pmid = 18937874 | pmc = 2627840 | doi = 10.1186/1756-3305-1-40 | quote = Blastocystis is now by far the most prevalent mono-infection in symptomatic patients in the United States [14] and was found 28.5 times more often than Giardia lamblia as a mono-infection in symptomatic patients in a 2000 study [14]. | doi-access = free }}</ref> It is the most common parasitic infection in the United States, where it infected approximately 23% of the total population during year 2000.<ref name="Parasites in representative sample of US population" /><ref name="Blastocystosis prevalence" /> In [[developing country|less developed]] areas, infection rates as high as 100% have been observed.<ref name="Primary: Blastocystosis 100% prevalence" /><ref name="Efficacy against Blastocystis" /> High rates of infection are found in individuals in developed countries who work with animals.<ref name="Efficacy against Blastocystis" /><ref name="PARKAR_2010">{{cite journal |vauthors=Parkar U, Traub RJ, Vitali S |title=Molecular characterization of Blastocystis isolates from zoo animals and their animal-keepers |journal=Vet. Parasitol. |volume=169 |issue=1–2 |pages=8–17 |date=April 2010 |pmid=20089360 |doi=10.1016/j.vetpar.2009.12.032 |url=http://researchrepository.murdoch.edu.au/id/eprint/3809/|display-authors=etal}}</ref> Although the role of ''Blastocystis hominis'' in human disease is often referred to as controversial, a systematic survey of research studies conducted by 11 infectious disease specialists from nine countries, found that over 95% of papers published in the 10 years prior identified it as causing illness in [[immunocompetence|immunocompetent]] individuals.<ref name="Blastocystosis prevalence" /> The paper attributed confusion over pathogenicity to the existence of [[asymptomatic carrier]]s, a phenomenon the study noted is common to all gastrointestinal protozoa.<ref name="Blastocystosis prevalence" /> However, ''Blastocystis'' has never fulfilled Koch's postulate that infection of a healthy individual with ''Blastocystis'' leads to disease. The fact that ''Blastocystis''<nowiki/>' infection route is oral-anal indicates that carriers have been in contact with faecal contaminated matter which might have included other intestinal pathogens that explain the observed symptoms. A more likely explanation is the presence of virulent and non-virulent strains since there exists an enormous genetic variation between different strains (or genotypes). See the genotype paper by Rune Stensvold<ref>{{Cite journal|title=Terminology for Blastocystis subtypes—a consensus.|journal=Trends in Parasitology|volume=23|issue=3|pages=93–6|pmid = 17241816|year = 2007|last1 = Stensvold|first1 = C. R.|last2=Suresh|first2=G. K.|last3=Tan|first3=K. S.|last4=Thompson|first4=R. C.|last5=Traub|first5=R. J.|last6=Viscogliosi|first6=E.|last7=Yoshikawa|first7=H.|last8=Clark|first8=C. G.|doi=10.1016/j.pt.2007.01.004}}</ref> and the
A 2024 study found that the presence of ''Blastocystis'' was found with individuals with higher plant consumption and associated with lower obesity, better cardiovascular health, and less metabolic abnormalities. Diet modification in participants that led to a reduction in obesity also led to greater amounts of ''Blastocystis.''<ref>Piperni, Elisa; Nguyen, Long H; Manghi, Paolo; [[Tim Spector|Spector, Tim D.]]; Segata, Nicola; ''et al.'' (2024) Intestinal ''Blastocystis'' is linked to healthier diets and more favorable cardiometabolic outcomes in 56,989 individuals from 32 countries. ''Cell, 187'' (17), 4,554–4,570.</ref>
==Classification==
The current [[protozoa]]n classification of ''Blastocystis'' was only resolved in 2007. The original description of ''Blastocystis'' was as a [[yeast]] due to its yeast-like glistening appearance in fresh [[Microscope slide|wet mounts]] and the absence of [[pseudopod]]ia and [[Animal locomotion|locomotion]].<ref name="BRUMPT_1912">{{cite journal |author=Brumpt E |title=''Blastocystis hominis'' n. sp. et formes voisines |journal=Bulletin of the Exotic Pathology Society |volume=5 |pages=725–30 |year=1912}}</ref> This was then contradicted by Zierdt,<ref name="Pérez-Cordón et al 2007">{{cite journal|first1=Gregorio|last1=Pérez-Cordón|first2=María J.|last2=Rosales|first3=María del Mar|last3=Gavira|first4=Renzo A.|last4=Valdez|first5=Franklin|last5=Vargas|first6=Ofelia|last6=Córdova|url=http://www.scielo.org.pe/scielo.php?script=sci_arttext&pid=S1727-99332007000300021|title=Finding of ''Blastocystis'' sp. in bivalves of the genus ''Donax''|journal=Revista Peruana de Biología|date=December 2007|volume=14|issue=2|pages=301–302|doi=10.15381/rpb.v14i2.1824 |issn=1727-9933}}</ref> who reclassified it under [[subphylum]] [[Apicomplexa|Sporozoa]] (and later in [[Sarcodina]]), based on some distinctive [[protist]]an features{{which|date=October 2017}} of the ''Blastocystis'' cell. Its sensitivity to antiprotozoal drugs and its inability to grow on [[fungus|fungal]] media further indicated that it was a protozoan.
Major revisions had been made to its classification. An analysis of gene sequences was performed in 1996, which placed it into the group [[Stramenopiles]].<ref>{{cite journal |vauthors=Stechmann A, Hamblin K, Pérez-Brocal V |title=Organelles in Blastocystis that Blur the Distinction between Mitochondria and Hydrogenosomes |journal=Current Biology |volume=18 |issue=8 |pages=580–5 |date=April 2008 |pmid=18403202 |pmc=2428068 |doi=10.1016/j.cub.2008.03.037|bibcode=2008CBio...18..580S |display-authors=etal}}</ref><ref name="SILBERMAN_1996">{{cite journal |vauthors=Silberman JD, Sogin ML, Leipe DD, Clark CG |title=Human parasite finds taxonomic home |journal=Nature |volume=380 |issue=6573 |page=398 |year=1996 |pmid=8602239 |doi=10.1038/380398a0|bibcode=1996Natur.380..398S |s2cid=4356573 |doi-access=free }}</ref>
Other Stramenopiles include brown algae, mildew, diatoms, the organism that caused the [[Great Famine (Ireland)|Great Famine of Ireland]], and the organism responsible for [[
==Signs and symptoms==
{{See also|Blastocystosis}}
Most published studies have reported that between 50% and 80% of individuals mono-infected with ''Blastocystis'' will show symptoms.<ref name="YOSHIZAWA_2004">{{cite journal |vauthors=Yoshikawa H, Wu Z, Kimata I |title=Polymerase chain reaction-based genotype classification among human Blastocystis hominis populations isolated from different countries |journal=Parasitol. Res. |volume=92 |issue=1 |pages=22–9 |date=January 2004 |pmid=14598169 |doi=10.1007/s00436-003-0995-2 |s2cid=2994758 |display-authors=etal}}</ref><ref name="AMIN_2002">_{{cite journal |author=Amin OM |title=Seasonal prevalence of intestinal parasites in the United States during 2000 |journal=Am. J. Trop. Med. Hyg. |volume=66 |issue=6 |pages=799–803 |date=June 2002 |pmid=12224595 |doi= 10.4269/ajtmh.2002.66.799|doi-access=free }}</ref> Factors influencing presentation of symptoms have been listed as the patient's age, with younger patients less likely to show symptoms, as well as genetic changes that influence the production of cytokines.<ref name="DOGRUMAN_2009">{{cite journal |vauthors=Dogruman-Al F, Kustimur S, Yoshikawa H |title=Blastocystis subtypes in irritable bowel syndrome and inflammatory bowel disease in Ankara, Turkey |journal=Mem. Inst. Oswaldo Cruz |volume=104 |issue=5 |pages=724–7 |date=August 2009 |pmid=19820833 |doi= 10.1590/S0074-02762009000500011|display-authors=etal|doi-access=free |hdl=1807/57897 |hdl-access=free }}</ref> Some studies have suggested that pathogenicity may be linked to specific subtypes of ''Blastocystis''<ref name="TAN_2006">{{cite journal |vauthors=Tan TC, Suresh KG |title=Predominance of amoeboid forms of Blastocystis hominis in isolates from symptomatic patients |journal=Parasitol. Res. |volume=98 |issue=3 |pages=189–93 |date=February 2006 |pmid=16323025 |doi=10.1007/s00436-005-0033-7 |s2cid=11103669 }}</ref> and experimental infection of animals has reported varying degrees of illness depending on the subtype used.<ref name="HUSSEIN_2008">{{cite journal |vauthors=Hussein EM, Hussein AM, Eida MM, Atwa MM |title=Pathophysiological variability of different genotypes of human Blastocystis hominis Egyptian isolates in experimentally infected rats |journal=Parasitol. Res. |volume=102 |issue=5 |pages=853–60 |date=April 2008 |pmid=18193282 |doi=10.1007/s00436-007-0833-z |s2cid=2766424 }}</ref> While some subtypes appear to be less likely to result in symptomatic infection, those subtypes are also found in symptomatic individuals who have no other infection found.<ref name="DOGRUMAN_2009" /> Symptoms associated with the infection are diarrhea, constipation, nausea, abdominal cramps, bloating, excessive gas, and anal itching.<ref name=cdc>{{cite web|url=https://www.cdc.gov/ncidod/dpd/parasites/blastocystishominis/factsht_blastocystis_hominis.htm |title=Division of Parasitic Diseases - Blastocystis hominis Infection Fact Sheet |publisher=Cdc.gov |date=2008-06-06 |access-date=2010-04-04}}</ref> Most cases of the infection appear to become diagnosed as [[irritable bowel syndrome]] (IBS), according to studies from Denmark,<ref name="STENSVOLD_2009B">{{cite journal |vauthors=Stensvold CR, Lewis HC, Hammerum AM |s2cid=38864528 |title=Blastocystis: unravelling potential risk factors and clinical significance of a common but neglected parasite |journal=Epidemiol. Infect. |volume=137 |issue=11 |pages=1655–63 |date=November 2009 |pmid=19393117 |doi=10.1017/S0950268809002672 |display-authors=etal|doi-access=free }}</ref> Pakistan, the United Kingdom, and Italy.<ref name="Blastocystosis prevalence" /> The timescale of infection with the parasite can range from weeks to years.<ref name=pmid2298869>{{cite journal |vauthors=Doyle PW, Helgason MM, Mathias RG, Proctor EM |title=Epidemiology and pathogenicity of Blastocystis hominis |journal=Journal of Clinical Microbiology |volume=28 |issue=1 |pages=116–21 |date=January 1990 |pmid=2298869 |pmc=269548 |doi=10.1128/JCM.28.1.116-121.1990 }}</ref> In the early 2000s, Egyptian physicians identified 84 patients with diarrhea and enteritis apparently caused by Blastocystis hominis. After three days of [[nitazoxanide]] treatment, symptoms cleared and no fecal organisms were detectable in 36 (86%) of 42 treated patients and in 16 (38%) of 42 people who received placebo (''P'' < .0001). The investigators concluded that either ''B. hominis'' is pathogenic and can often be effectively treated with nitazoxanide, or that nitazoxanide (a drug approved by the FDA for the treatment of [[giardia]] and [[cryptosporidia]]) eradicated an unidentifiable organism.<ref name="pmid16234044">{{cite journal | vauthors = Rossignol JF, Kabil SM, Said M, Samir H, Younis AM | title = Effect of nitazoxanide in persistent diarrhea and enteritis associated with Blastocystis hominis | journal = Clin. Gastroenterol. Hepatol. | volume = 3 | issue = 10 | pages = 987–91 | year = 2005 | pmid = 16234044 | doi = 10.1016/S1542-3565(05)00427-1| doi-access = free }}</ref>
==Taxonomy==
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A tenth group was reported in China in 2007,<ref name="pmid17912552">{{cite journal |vauthors=Li LH, Zhang XP, Lv S |title=Cross-sectional surveys and subtype classification of human Blastocystis isolates from four epidemiological settings in China |journal=Parasitology Research |volume=102 |issue=1 |pages=83–90 |date=December 2007 |pmid=17912552 |doi=10.1007/s00436-007-0727-0|s2cid=21447943 |display-authors=etal}}</ref> but a full analysis of its relationships has not yet been performed and it is not yet clear whether it is a group within a described subtype or a new subtype. A definite tenth subtype was then found in a variety of other mammals, including primates, but it has not as yet been found in humans.<ref name="STENSVOLD_2009">{{cite journal |vauthors=Stensvold CR, Alfellani MA, Nørskov-Lauritsen S |title=Subtype distribution of Blastocystis isolates from synanthropic and zoo animals and identification of a new subtype |journal=International Journal for Parasitology |volume=39 |issue=4 |pages=473–9 |date=March 2009 |pmid=18755193 |doi=10.1016/j.ijpara.2008.07.006|s2cid=30802689 |display-authors=etal|url=https://researchonline.lshtm.ac.uk/7112/1/7112.pdf }}</ref>
There are now at least 13 genetically distinct small subunit [[ribosomal RNA]] lineages.<ref name="Parkar_2010">{{cite journal |vauthors=Parkar U, Traub RJ, Vitali S |title=Molecular characterization of ''Blastocystis'' isolates from zoo animals and their animal-keepers. |journal=Veterinary Parasitology |volume=169 |issue=1–2 |pages=8–17 |year=2010 |pmid=20089360 |doi=10.1016/j.vetpar.2009.12.032|display-authors=etal|url=http://researchrepository.murdoch.edu.au/id/eprint/3809/ }}</ref> These additional subtypes were found in a variety of mammalian hosts (including elephants and giraffes) and it is very likely that more subtypes will be found as more hosts are surveyed.
==Epidemiology==
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=== Transmission ===
Fecal-oral transmission is the most accepted pathway, and recent studies have shown that transmission involves only the cyst form of the parasite.<ref>{{cite journal |vauthors=Yoshikawa H, Yoshida K, Nakajima A, Yamanari K, Iwatani S, Kimata I |title=Fecal-oral transmission of the cyst form of Blastocystis hominis in rats |journal=Parasitology Research |volume=94 |issue=6 |pages=391–6 |date=December 2004 |pmid=15480786 |doi=10.1007/s00436-004-1230-5|s2cid=25408611 }}</ref> The extent to which
=== Reservoir ===
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;Vacuolar form
The vacuolar form is the typical cell form of ''Blastocystis'' seen in culture and is often used for the identification of the organism. These vacuolar forms vary greatly in size, with [[diameter]]s ranging between 2
[[File:Four common forms of Blastocystis hominis Valzn.jpg|frame|Four common forms of ''Blastocystis hominis''. Clockwise from top left: vacuolar, granular, amoeboid, and cyst forms.]]
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;Amoeboid form
The other form that exists is the amoeboid form.
;Cyst form
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==Life cycle==
The supposed life cycle begins with ingestion of the cyst form. After ingestion, the cyst develops into other forms which may in turn re-develop into cyst forms. Through human faeces, the cyst forms enter the external environment and are transmitted to humans and other animals via the [[Fecal–oral route|faecal–oral route]], repeating the entire cycle.
[[File:Blastocystis life cycle Valzn.png|thumb|center|550px|Life cycle of ''Blastocystis'' proposed by Tan<ref name="TAN_2004">{{cite journal |author=Tan KS |title=Blastocystis in humans and animals: new insights using modern methodologies |journal=Veterinary Parasitology |volume=126 |issue=1–2 |pages=121–44 |date=December 2004 |pmid=15567582 |doi=10.1016/j.vetpar.2004.09.017}}</ref>]]
==Obtaining and culturing ''Blastocystis''==
The [[American Type Culture Collection|ATCC]] maintains a collection of ''Blastocystis'' isolates.
==See also==
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