Blastocystis Hominis: Should We Treat It? (updated 2023)

Treatment options for blastocystis hominis

What Is Blastocystis Hominis?

Blastocystis hominis is a parasite that inhabits the gastrointestinal tract of humans and many animals. It has a worldwide distribution, and, is often identified as the most common parasite reported in human fecal samples (5–15% of individuals in developed countries and 50–100% of individuals in developing countries).

The parasite has been known since the early 1900s but only in the last decade has the biology and pathogenicity of this parasite undergone more intensive studies and its pathogenic potential is still controversial. For many years, it has been suggested that Blastocystis is a commensal organism living in the human intestine. 

This is partly because while there are many case reports, and epidemiological and microbiological studies support a pathogenic role of Blastocystis in causing intestinal inflammation and urticarial symptoms, there are many reports on asymptomatic colonisation by Blastocystis.

It is made more challenging because:

a total of 17 subtypes of Blastocystis Hominis have been identified so far – and perhaps not all of them are pathogenic.

How Do We Get Infected?

The difference in colonisation rates around the world can be partly explained by poor hygiene practices and consumption of contaminated water or food in developing countries. The fecal–oral route is considered to be the main mode of transmission.

Blastocystis spp. have been observed in Scottish and Malaysian sewage samples and ready-to-eat package salads in Italy. The description of two suspected waterborne outbreaks of blastocystosis, and the chlorine insensitivity of Blastocystis spp. cysts implicate chlorinated drinking water as a potentially significant transmission route (source).

Interestingly in one paper Blastocystis subtypes detected in this cohort were found to be less prevalent in Bacteroides enterotype samples. What this means is that our overall microbiome may make us more or less susceptible to picking up blastocystis in the first place. (source)


Signs & Symptoms Of Blastocystis

The common symptoms including chronic and acute diarrhea, abdominal pain, nausea, vomiting, anorexia, weight loss, and dermatological symptoms (e.g., rash, itching, and urticaria). (source)

Recent epidemiological data demonstrate the association of Blastocystis with a variety of disorders, such as diarrhea, abdominal pain, fatigue, constipation, flatulence, chronic gastrointestinal illnesses (irritable bowel syndrome), and skin rash or urticaria. 

results support the hypothesis that Blastocystis might be linked to the pathophysiology of IBS-C and intestinal flora imbalance

For those with stool results at hand, it was shown that proteases secreted from Blastocystis isolates can degrade human secretory immunoglobulin A (SIgA), so, low levels of SIgA in a stool test alongside Blastocystis Hominis may get you thinking about certain nutritional interventions (see below).

There have been several case reports suggesting that Blastocystis is related to urticaria. The amoeboid forms of Blastocystis ST3 were found in a case of acute urticaria and the authors suggested that cutaneous symptoms may be caused by disruptions to the immune homeostasis as the host produces an inflammatory response.

One study showed a possible link between Blastocystis and IBS where there was an infection rate of 46% in IBS patients and only 7% in the control group was shown

Blastocystis spp. have been observed in ulcers in the cecum, transverse colon, and rectum of an immunocompetent individual. They can also obstruct the appendiceal lumen (source)

Blastocystis And Colorectal Cancer

Recently, there have been several findings that showed intestinal colonisation of Blastocystis hominis (Blastocystis) as a risk factor to the worsening of colorectal cancer (CRC). However, studies have shown controversial results in the pathogenicity of Blastocystis.

A recent systematic review published in 2022 (source) concluded that blastocystis is a commonly identified microorganism in colorectal cancer patients. These studies have provided supportive data that Blastocystis hominis could exacerbate existing colorectal cancer via alteration in host immune response and increased oxidative damage.

Subtype 1 and subtype 3 were predominantly seen.

The researchers stated that future studies investigating the connection between colorectal cancer and Blastocystis should attempt to determine the various stages of colorectal cancer that are most likely to be associated with Blastocystis, and its relationship with other gut bacteria.

Blastocystis Hominis And Ulcerative Colitis

Several studies have shown that a change in microbiota plays an important role in the pathogenesis of inflammatory bowel disease (IBD). Furthermore, with the emergence in recent studies of differences according to the subtype of IBD and whether the disease is active or in remission, there has started to be research into the relationship between IBD and several microorganisms. Blastocystis hominis is primary among these organisms.

One group of researchers aimed to determine the role of Blastocystis hominis in the acute flare-up of ulcerative colitis (UC).

They found Blastocystis hominis positivity was determined in 37 (34%) patients with ulcerative colitis. Of the patients, 17 (32.6%) were in the acute flare-up phase, and 20 (32.2%) were in remission. In 11 (64.7%) of the B. hominis positive patients, the disease severity was determined as mild-moderate.

The researchers concluded that the results of the showed that while there was no difference between the active and remission phases in respect of Blastocystis presence, there was milder involvement in those with the parasite. (source)

Blastocystis Hominis And Schizophrenia

Previously, the protozoan isolated from individuals with irritable bowel syndrome (IBS) showed altered phenotypic features suggesting that it can be triggered to become pathogenic. Previous studies reported altered gut microbiota and high prevalence of Blastocystis sp. in schizophrenia patients. However, the phenotypic characteristics of Blastocystis sp. isolated from individuals with schizophrenia have yet to be described.

Researchers found that 12 out of 50 (24%) schizophrenia and 5 out of 100 (5%) non-schizophrenia individuals were detected Blastocystis sp. positive using both in vitro culture and PCR method with no significant association to age and gender.

Out of the 15 sequenced isolates, ST3 was the most prevalent subtype (66.7%) followed by ST1 (20%) and ST6 (13.3%).

The isolates from schizophrenia individuals demonstrated significant slower growth rate and larger range of cell diameter (3.3-140 µm).

We detected higher amoebic forms and metronidazole resistance among schizophrenia isolates with variation in cell surface glycoprotein where 98% of cells from schizophrenia showed consistent medium to high binding affinity to Concavalin A staining compared to non-schizophrenia isolates that demonstrated only 76% high lectin binding affinity.

The researchers concluded that their findings demonstrated Blastocystis isolated from schizophrenia individuals showed variation in phenotype specifically in morphology and drug resistance. The findings indicate that the gut environment (schizophrenia and non-schizophrenia) and treatment of schizophrenia could have influenced the phenotype of Blastocystis.

Why So Much Confusion? (it’s not just about the subtype)

The pathogenicity of Blastocysis probably depends on the duration (acute or chronic) and intensity of infection, host genetic factors, or Blastocystis subtypes. (source)

To date, 22 subtypes (ST1–ST22) of Blastocystis spp. have been identified, based on the sequence analysis of the small subunit ribosomal RNA gene, with 10 (ST1 to ST9 and ST12) reported in humans of which ST3 is the most commonly detected subtype. (source)

Interestingly, Blastocystis hominis subtypes 3 and 4 were inversely correlated with Akkermansia, suggesting differential associations of subtypes with host health. (source)

Our Microbiome Influences Blastocystis

In a fascinating paper published this year (2023) researchers studied the association and interaction of Blastocystis hominis and the gut microbiome. (source)

They investigated the gut microbiome of Blastocystis-free and Blastocystis sub-type 3-infected individuals who are symptomatic and asymptomatic.

They tested if the expression of phenotype and pathogenic characteristics of Blastocystis sub-type 3 was influenced by the alteration of its accompanying microbiota. Blastocystis sub-type 3 infection alters bacterial composition. Its presence in asymptomatic individuals showed a significant effect on microbial richness compared to symptomatic ones.

The findings suggested that colonisation of Blastocystis sub-type 3 could contribute to the alteration of microbial functions. For the first time, these researchers demonstrated the influence of bacteria on Blastocystis pathogenicity. When Blastocystis hominis, isolated from a symptomatic individual, was co-cultured with bacterial suspension of Blastocystis from an asymptomatic individual, the parasite demonstrated increased growth and reduced potential pathogenic expressions.

This study also reveals that Blastocystis hominis infection could influence microbial functions without much effect on the microbiota diversity itself.

The fact that the gut microbiome may be able to alter the characteristics of the parasite, may explain the contradictory findings related to the parasite’s pathogenic role.

The study provides evidence that asymptomatic Blastocystis in a human gut can be triggered to show pathogenic characteristics when influenced by the intestinal microbiota.

To Treat Or Not To Treat Blastocystis?

Blastocystis hominis was positively associated with high intestinal bacterial genus richness

When Blastocystis is detected by microscopy or PCR in diagnostic examinations of faecal samples from humans with suspected disease, it is often not possible to determine if the finding represents acute infection or intestinal colonization.

These are two important points to acknowledge – we shouldn’t treat Blastocystis just because we find it on testing, and there may actually be benefit from being colonised with Blastocystis – namely a higher bacterial genus richness – something we are all striving for!

One study concluded that:

“The associations between Blastocystishominis and the bacterial microbiota found in this study could imply a link between Blastocystis and a healthy microbiota as well as with diets high in vegetables.”

Treatment failure is common and eradication of Blastocystis hominis can be difficult to achieve. It is unknown whether this is due to poor drug efficacy, a failure of the host defence in eliminating a pathogen despite an appropriate drug effect, or if persistent Blastocystis hominis findings in symptomatic disease signify that this organism constitutes a normal and stable part of the gut microbiota.

Treating Blastocystis Hominis

Furthermore, some strains develop resistance against currently recommended drugs, such as metronidazole; therefore, the use of natural remedies or special diets has many positive aspects that may address this problem.

The literature has revealed that garlic, ginger, some medical plants, and many spices contain the most effective organic compounds for parasite eradication. They work by inhibiting parasitic enzymes and nucleic acids, as well as by inhibiting protein synthesis.

Probiotics such as saccharomyces boulardii may also be beneficial when treatment is required.

Natural herbs, vegetables, or spices as an alternative for blastocystosis hominis treatment not only reduce drug resistance, but also their side effects and the cost of treatment, especially in developing countries. 

it is known that the composition of the intestinal bacterial populations modulates the progression of protozoan infection and the outcome of parasitic disease.

Again this final quote takes us back to point that perhaps we should look to optimise gut health via supporting a healthy mucosal immune system via a colourful, fibre rich diet. By supplementing probiotics such as saccharomyces boulardii rather than instantly thinking we need to ‘kill, kill, kill’ and take high dose antimicrobials.


Since a gut microbiota with high diversity is generally considered healthy, Blastocystis hominis might be seen as an indicator of gastrointestinal health, an opinion also voiced by other researchers.

We shouldn’t simply treat blastocystis hominis because it is detected in a stool test. It needs to be put in the context of the other markers/results within the test results, as well as the clients symptoms. There is also more than one way to manage a parasitic infection like Blastocystis hominis and this includes supporting the health of our immune system, which might include supplementing with probiotics such as Saccharomyces Boulardii, but certainly includes appropriate amounts of exercise, managing our stress, optimising our sleep habits and evening routine, and eating a diverse, nutritional and fibre rich diet.


Share this post