Associations
Reported associations, grouped by condition, with the strength of the evidence stated plainly. Association is not causation: several of these findings come from small cohorts, single cases or cell cultures.
The most cited work here found that Hashimoto's patients carrying Blastocystis had raised serum IL-17, and that eradicating the organism was followed by significant falls in TSH, anti-TPO antibodies and IL-17. A separate case report described normalised thyroid antibodies over four years after metronidazole treatment. These are small studies — one interventional cohort and one case — so they suggest a testable hypothesis rather than settled practice. In rheumatoid arthritis, protozoal carriage including Blastocystis correlated with the gut-permeability marker IFABP2 and with TNF-α, and there is an older case of Blastocystis recovered from synovial fluid.
Lee MG, Rawlins SC, Didier M, DeCeulaer K · Annals of the Rheumatic Diseases
Case report of Blastocystis found in synovial fluid causing infective arthritis in an immunosuppressed rheumatoid arthritis patient.
Open in PubMed · PMID 2322029El-Zawawy HT, Farag HF, Tolba MM, Abdalsamea HA · Therapeutic Advances in Endocrinology and Metabolism
Hashimoto's patients carrying Blastocystis had higher serum IL-17; after eradication, TSH, anti-TPO antibodies and IL-17 all fell significantly.
Open in PubMed · PMID 32128107Rajič B, Arapović J, Raguž K, et al. · Journal of Infection in Developing Countries
Case report: metronidazole eradication resolved urticaria and angioedema and thyroid antibodies normalised over four years of follow-up.
Open in PubMed · PMID 26230132Guzmán-Guzmán IP, Nogueda-Torres B, Zaragoza-García O, et al. · Frontiers in Medicine
In rheumatoid arthritis patients, protozoal infection including Blastocystis correlated with higher gut-permeability marker IFABP2 and higher TNF-α.
DOI 10.3389/fmed.2022.846934Blastocystis is repeatedly found more often in IBS patients than in controls, and carriage has been associated with fewer protective stool bacteria such as Bifidobacterium. The counter-evidence matters: one faecal microbiota comparison found the IBS-typical shift towards Firmicutes in both carriers and non-carriers, so Blastocystis may be a marker of an already altered gut rather than its cause.
Nagel R, Traub RJ, Allcock RJN, et al. · Microbiome
IBS patients showed more Firmicutes and fewer Bacteroidetes regardless of Blastocystis status — carriage alone did not explain the phylum-level shift.
DOI 10.1186/s40168-016-0191-0Nourrisson C, Scanzi J, Pereira B, et al. · PLoS ONE
Blastocystis carriage was associated with reduced protective bacteria (including Bifidobacterium) in the stool of IBS patients compared with controls.
DOI 10.1371/journal.pone.0111868Case-control data report Blastocystis significantly more often in inflammatory bowel disease overall and in ulcerative colitis in particular, and one Turkish study examined it as a possible trigger of colitis flares. Findings are not unanimous — some cohorts with active ulcerative colitis show low protist prevalence, plausibly because active inflammation and its treatment are hostile to colonisation.
Kök M, Çekin Y, Çekin AH, Uyar S, Harmandar F, Şahintürk Y · Turkish Journal of Gastroenterology
Examines Blastocystis as a possible trigger of ulcerative colitis flares, likely acting through altered gut microbiota and mucosal inflammation.
DOI 10.5152/tjg.2018.18498Case-control study (see DOI record for full author list) · BMC Gastroenterology
Case-control data found Blastocystis significantly more frequent in inflammatory bowel disease overall (OR 2.82) and in ulcerative colitis specifically (OR 2.95).
DOI 10.1186/1471-230X-12-122This evidence is laboratory-based, not epidemiological. Antigen from subtype 3 increased proliferation of HCT116 colorectal cancer cells by roughly a third, and a later study found that Blastocystis exposure reduced the efficacy of the chemotherapy drug 5-fluorouracil against cancer cells. No study shows that Blastocystis causes colorectal cancer in people — and large metagenomic work actually finds the organism less common in colorectal cancer cohorts.
Kumarasamy V, Kuppusamy UR, Samudi C, Kumar S · Parasitology Research
Subtype 3 antigen increased proliferation of HCT116 colorectal cancer cells by roughly a third in vitro, more than the other subtypes tested.
Open in PubMed · PMID 23933809Kumarasamy V, Kuppusamy UR, Pailoor J, Kumar SG · Experimental Parasitology
Exposure to Blastocystis reduced the effectiveness of the chemotherapy drug 5-fluorouracil against colorectal cancer cells in laboratory models.
DOI 10.1016/j.exppara.2023.108564A case-control dermatology study found Blastocystis — especially subtype 3 and the amoeboid form — significantly more common in urticaria patients than controls, and case reports describe hives resolving after eradication. A review concludes the link is plausible but unproven.
Rajič B, Arapović J, Raguž K, et al. · Journal of Infection in Developing Countries
Case report: metronidazole eradication resolved urticaria and angioedema and thyroid antibodies normalised over four years of follow-up.
Open in PubMed · PMID 26230132Zuelfakkar NM, Abdel Hameed DM, Hassanin OM · Clinical and Experimental Dermatology
Blastocystis — especially subtype 3 and the amoeboid form — was significantly more common in patients with urticaria than in controls (P<0.001).
DOI 10.1111/j.1365-2230.2011.04127.xLepczyńska M, Chen W, Dzika E · International Journal of Dermatology
Review concluding that a link between Blastocystis and chronic urticaria is biologically plausible but not yet confirmed, and may be subtype-dependent.
DOI 10.1111/ijd.13064Any honest reading has to include this. A 2024 study of 56,989 people across 32 countries linked Blastocystis carriage to plant-rich diets, lower BMI and more favourable cardiometabolic markers. Comparative metagenomics across 1,689 subjects found no positive association with disease, and carriers had higher microbial diversity with more Prevotella and Faecalibacterium. Detecting Blastocystis is therefore not, on its own, a diagnosis.
Deng L, Lee JWJ, Tan KSW · Parasites & Vectors
In patients with diarrhoea, ST7 carriage was linked to lower bacterial diversity and enrichment of Enterobacteriaceae and Escherichia–Shigella.
DOI 10.1186/s13071-022-05435-zNagel R, Traub RJ, Allcock RJN, et al. · Microbiome
IBS patients showed more Firmicutes and fewer Bacteroidetes regardless of Blastocystis status — carriage alone did not explain the phylum-level shift.
DOI 10.1186/s40168-016-0191-0Nourrisson C, Scanzi J, Pereira B, et al. · PLoS ONE
Blastocystis carriage was associated with reduced protective bacteria (including Bifidobacterium) in the stool of IBS patients compared with controls.
DOI 10.1371/journal.pone.0111868Piperni E, Nguyen LH, Manghi P, et al. · Cell
In a very large multi-country metagenomic study, carrying Blastocystis was associated with plant-rich diets, lower BMI and better cardiometabolic markers.
Open in PubMed · PMID 38981480Beghini F, Pasolli E, Truong TD, Putignani L, Cacciò SM, Segata N · ISME Journal
Across 1,689 subjects there was no positive association with disease; Blastocystis was less common in colorectal cancer and Crohn's-associated dysbiosis.
DOI 10.1038/ismej.2017.139Stensvold CR, Sørland BA, Berg RPKD, et al. · Microorganisms
Blastocystis carriers had significantly higher microbial diversity and more eubiosis-associated genera such as Prevotella and Faecalibacterium.
Open in PubMed · PMID 35208781