is the most common parasite found in human stool specimens [12
] with prevalence reported to range from 1.5% and 10% in developed countries and from 30% and 50% in developing countries [1
]. The parasite was encountered 28.5 times more than Giardia lamblia
in a study conducted in the United States in 2000 [14
]. In another study on the epidemiology of Blastocystis spp.
conducted in the United States in 2006, 16,374 stool specimens were analyzed, and prevalence of Blastocystis spp.
was found at a ratio of 18% [15
]. In our study, the Blastocystis spp.
ratios in patients with intestinal complaints and in the control group were 5.74% and 3.12%, respectively. This ratio is close to the ratio reported in the developed countries and also is in parallel with ratios (ranges from 1.4% to 44.3%) reported from Turkey to date. The fact that the criterion for Blastocystis spp.
positivity has been accepted as the presence of five or more parasites in the microscopic field at X40 magnification in some of the studies, whereas consideration of positivity regardless of the number in others may explain the wide range of prevalence reported in the literature.
Although the specific pathogenicity of Blastocystis spp.
is still undetermined, the intestinal inflammation induced by this parasite was reported to yield specific inflammatory changes in the gut wall. In this regard, in an experimental animal model study in mice, vacuolar form of the parasite was reported to cause invasion to the lamina propria, submucosal and muscular layers of the large intestine leading mixed inflammatory cell infiltration and active colitis in infected mice [16
]. Although there are case reports on the pathogenicity of Blastocystis spp.
in humans, the number of those studies is inadequate [17
]. Nonetheless, proteases of Blastocystis spp.
were considered a virulence factor that is responsible for protein degredation and have possible pathogenic role in host immune evasion [19
Clinical manifestation of Blastocystis infection was reported to include asymptomatic, acute symptomatic and chronic symptomatic cases in the past studies based on use of different methologies and study populations. In a study in 2010, B. hominis
was reported to be more frequent in patients with intestinal symptoms than in the asymptomatic patients (5.67% vs. 3.43%, respectively) [20
]. In our study, albeit not statistically significant, frequency of blastocystis infection was also higher in symptomatic than asymptomatic patients (5.74% vs 3.12%). Researchers report that Blastocystis spp.
is the most common parasite encountered in symptomatic patients based on two main reasons. Firstly, clinicians are reluctant to treat this parasite due to its low pathogenicity and self-restricting symptomatology; and secondly, Blastocystis spp.
that are resistant to other antiparasitic medications have the ability to colonize easily into empty intestinal niches after the treatment of other pathogenic protozoa with conventional medications [7
Although no clear correlation between diarrhea and Blastocystis spp. could be established by means of the controlled studies conducted to date, its relation with chronic bowel diseases of unknown origin such as IBD and IBS is currently being investigated due to the increasing prevalence of this parasite in developed countries.
Being a functional bowel disorder associated with abdominal pain or discomfort and defecation disorders, IBS is generally considered a psychosomatic disease, etiology of which has not been clarified yet [22
]. IBS prevalence in developing countries varies between 35%-43% and it is a frequent disease in the clinical practice with 12% of the general medical visits and 25-50% of gastroenterology consultations resulting with the positive diagnosis [23
]. Although IBS is a functional bowel disease, studies on the association between Blastocystis spp.
and IBS revealed that parasite infection was evident in almost half of the patients with IBS [11
]. Accordingly, Yakoob et al. reported that 52% of IBS patients and 24% of the control group had Blastocystis spp.
]. In other related studies, B. hominis
was found in 17.3% of symptomatic patients with IBS and in 6% of cases in the asymptomatic control group, and the difference between the two groups was reported to be significant [26
]. In our study, patients with blastocystis infection and IBS demonstrated similar symptomatology in parallel with the literature. However, while there was a significant parallelism between the frequency of blastocystis infection and IBS prevalence in studies conducted in different countries, [24
]. Blastocystis spp.
was found in 51 (5.82%) of 877 symptomatic patients in our study with no statistically significant difference compared with the ratio obtained in the asymptomatic control group (3.12%) (p
The IBD sub-group (n
335) of patients with gastrointestinal complaints (n
2334) was determined to have the most frequent Blastocystis spp.
infection (8.35%) with significantly higher rates compared with the controls (3.12%; p
0.019) Likewise, 24 of 28 patients with Blastocystis spp
in the IBD group were UC patients (n
276) with significantly higher rate of infection (8.7%) in these patients compared to controls (3.12%, p
0.016). However, while showing a tendency towards higher ratio in patients with CD, there was no statistical difference between CD patients (6.78%) and control group in terms of frequency of Blastocystis spp.
UC is a chronic inflammatory disease of the intestines manifesting itself with consequent activity and remission periods. Many studies have investigated the influence of intestinal infections on the clinical course of UC. In this respect, in a prospective case-controlled study by Yamamoto-Furusho et al. conducted in Mexico on the correlation of UC disease with parasite infections, Blastocystis spp.
was reported as the most common parasite, at a rate of 10%, which is very similar to prevalence of parasite in our study (8.7%). They also reported that presence of Blastocystis spp
., Entamoeba histolytica
and Entamoeba coli
was correlated with relapsing illness [27
]. In another study, six UC patients with refractory symptoms were found to have B. hominis
in their stool examinations with complete recovery by 14-day metronidazole treatment [28
]. In a study by Mylonaki et al. emphasizing the importance of routine microscopic stool analysis in IBD exacerbation, Blastocystis spp.
was amongst the detected enteric infectious agents [29
]. These results suggested the correlation of Blastocystis spp
. with IBD and strengthened the hypothesis that the parasite may have a role in the exacerbation of diseases [28
]. In contrast, Nagler et al. reported that Blastocystis spp.
was not a significant pathogen in IBD exacerbations. However, they examined only 12 patients retrospectively, so the lack of Blastocystis spp.
might be associated with the inadequate number of patients examined [30
In our study, a significant rate of Blastocystis spp.
infection was detected in patients with gastrointestinal symptoms via common laboratory methods. However, there are studies reporting that the sensitivity of the standard methods used in clinical practice are lower than that of molecular methods and culture [31
]. While it has long been accepted that B. hominis
is the single blastocyst specie infecting humans, genetic analysis conducted recently demonstrated that nine different types of blastocyst may infect humans. Besides, antigenic heterogeneity of Blastocystis spp.
supports the likelihood of virulent and avirulent species of this parasite [32
]. In this regard, starting from the hypothesis of Boorom, more virulent and easily transmissible Blastocystis spp.
may be the cause of the increase in the prevalence of IBDs in the 1990s in Europe [33
] which indicates use of more sensitive methods such as polymerase chain reaction and culture in detection of more virulent types of this parasite.
The major limitation of the present study is the case–control design increasing the likelihood of confounding variables and bias (sampling and observation and recall bias). Nevertheless, a convenience sample (sampled in the same way as the cases) was selected from population attending the same outpatient department to overcome sampling bias and to enable the controls to represent the same population as the cases. Besides, the likelihood of retrospective recall bias seems to be minimized by using data recorded, for other purposes, before the outcome had occurred and therefore before the study had started. We didn’t assess the potential viral causes of the gastrointestinal complaints and this is the limitation of our study. Another limitation important in terms of clinical relevance of our findings is the neglect of including ulcerative colitis activation criteria in the diagnostic evaluation of patients in the study group. For this reason, albeit its clinical importance, we are unable to draw a conclusion regarding the causative role of Blastocystosis infection among UC patients admitting with exacerbation of the disease.