ABSTRACT
Blastocystis is an anaerobic protozoan with global importance because of infecting a variety of hosts and having high prevalence in many countries. Blastocystis isolates display remarkable genetic differences, and many subtypes (STs) have currently been defined based on polymorphism in SSU rRNA coding gene. Each 25 subtype may have different characteristics such as pathogenicity, host specificity, and structural variations. Most current research on Blastocystis has focused on these differences and molecular epidemiology. This review aimed to provide a summary of Blastocystis subtype distribution in Türkiye. Regarding human samples, 16 manuscripts were found in the literature, which presented 783 Blastocystis isolates from 9 cities in Türkiye. The most common subtype was ST3 (47.9%), the others were ST1 30 (17.5%), ST2 (14.7%), ST4 (4%), and ST5-ST7 (15.9%). There were few studies on animal hosts and environmental samples. The faecal samples from rats, farm, and pet animals were examined for Blastocystis subtypes and ST1, ST3, ST4-ST7, ST10, and ST12-ST14 were reported. In addition, two studies reported Blastocystis ST1 and ST3 subtypes in environmental water samples. In conclusion, the review of available literature showed that a systematic understanding of the subtype distribution of 35 Blastocystis in Türkiye is still lacking. Most of the studies were performed in a limited number of cities, animal hosts, and environmental samples, therefore, more studies from different provinces are needed in forthcoming research. The majority studies were performed in a limited number of provinces, animal species and very few environmental samples, so in the future; there is a need of novel studies that evaluate more samples from different provinces.
Keywords: Blastocystis, subtypes, Türkiye
INTRODUCTION
Blastocystis is an intestinal, anaerobic protozoan that infects humans and many other species including primates, canids, swine, rodents, birds, reptiles, and cockroaches (1). It has recently been classified in stramenopiles (Heteroconta) but differs from others because of colonizing the human intestine and not having a flagellated form. It has a global distribution with an independency of climate and many epidemiological studies reported Blastocystis as the most common protozoan in human faecal samples (2). Blastocystis has a frequency rate of up to 100% in underdeveloped countries, because of poor hygiene habits and socio-economic factors (3). The studies in Türkiye revealed a prevalence of Blastocystis varying between 1.4% and 23.5% (4). These studies are highly different from each other in terms of diagnostic methods and also the study population.
Although it has been a long-time since it was revealed that Blastocystis infects humans, genetic diversity, life cycle, treatment, and pathogenesis of Blastocystis have been subjected to considerable discussions in the literature (5-7). It was defined as a pathogen, opportunistic pathogen, or non-pathogenic microorganism. The main problem encountered here was that Blastocystis was found in both healthy individuals and symptomatic groups (8-10). Blastocystis infections are asymptomatic in a high rate of infected individuals and gastrointestinal symptoms are rarely observed (2,5). It was suggested that Blastocystis may be an indicator of healthy intestinal flora (11,12). In addition to non-specific gastrointestinal symptoms such as diarrhoea, abdominal pain, and bloating, dermatological symptoms can be seen in symptomatic Blastocystis infections (2,10,13).
Blastocystis has high polymorphic variations in its genome. Following the molecular phylogenetic analysis of Blastocystis small subunit ribosomal RNA gene (SSU rRNA), many genotypes or subtypes (STs) have been defined in isolates from humans and non-human hosts (14). Currently, the number of proposed subtypes reached at least to 31; however, some of them are not validated yet and a strong argument is raised (15,16). Recently, many reviews reported Blastocystis subtypes in different countries, but not from Türkiye. In this paper, we reviewed the available studies on Blastocystis subtypes in humans, animal hosts, and environmental samples.
Blastocystis Subtypes in Human Samples
Türkiye is a transcontinental country that bridges Europe and Asia. It is considered to have suitable conditions for a high prevalence and genetic diversity of Blastocystis such as moderate climate, inadequate sanitation in some cities, and migration from other countries. The first available record on Blastocystis was published in 1969, a case report (17). There was a large paucity of Blastocystis research or papers until the 1990s. Afterwards, studies about the frequencies in renal transplant patients and the treatment of Blastocystis were published (18,19). Because of unfamiliar findings on Blastocystis pathogenicity, most studies have focused on the clinical properties of Blastocystis-infected people. Research on the subject has been mostly limited to the comparisons of patient and control groups before the identification of Blastocystis subtypes. The possible clinical consequences of Blastocystis infection were studied in different patient groups such as iron deficient, cancer, haemodialysis, irritable bowel syndrome, and inflammatory bowel disease patients (20-23).
Among the Blastocystis subtypes, ST1 to ST9 and ST12 have been isolated from human samples (14). The common subtypes in humans are ST1 to ST4, in addition, ST9 has not been reported other than in human samples (16). The studies on Blastocystis subtypes in different provinces of Türkiye mostly revealed similar findings in terms of subtype distribution; however, some differences were reported in terms of the number of subtypes. More recently, ST1-7 has been found in human samples in Türkiye. Most of them revealed the predominance of ST3, followed by either ST2 or ST1. In general, Europeans are mostly colonized by ST1–ST4, in contrast, ST4 is rarely found in other territories including Africa, America, and the Middle East (24). The studies from Türkiye used two methods to identify Blastocystis subtypes: Partial sequencing of SSU rDNA coding gene also known as barcoding and sequence-tagged-sites polymerase chain reaction (PCR)
(STS PCR). The sequencing of amplified PCR products allows to determine allele differences and intra-subtype genetic variation of Blastocystis. In addition, with this method the comparison of other sequences from Genbank or other sources is possible. The detailed data on Blastocystis subtypes in human samples is presented in Table 1. Blastocystis subtypes from the following cities: Adana, Ankara, Aydın, Erzincan, Eskişehir, İstanbul, İzmir, Manisa, and Muğla. Most studies have been conducted in the Aegean region of Anatolia. Currently, we have no data for human samples in eastern Türkiye. The data from other regions are also very limited, for example, there are only two studies from south of Türkiye, the Mediterranean Region.
Blastocystis Subtypes in Animal Samples
Although relatively most research has been carried out on humans, a very limited number of studies exist that detected Blastocystis subtypes in animal samples. The animal hosts of Blastocystis, which are previously studied in Türkiye for Blastocystis subtypes, are livestock animals, pet animals, and laboratory rats (38-43). The details of these studies are given in Table 2. Host specificity and zoonotic potential of Blastocystis have been shown in these studies. The prevalence of Blastocystis in animal samples is an important topic for humans, because of the zoonotic potential of Blastocystis. It was reported that identified STs in humans are also found in domestic and wild animals (44). Many subtypes including ST1-ST7, ST10, ST12, and ST14 have been reported in cattle samples with higher frequencies of ST10 and ST14 (45,46). Similar to the studies in Türkiye , ST10 is the predominant Blastocystis subtype in cattle samples in several countries including the USA (47), China (48,49), the UK, Thailand (50), and Lebanon (51). A study in Türkiye reported that all laboratory animals were infected with ST4, this finding was also consistent with the others that showed the predominance of ST4 in rodents (39,52). A meta-analysis of Blastocystis ST found in the cats showed that ST4 (29.5%) was the most common, which was followed by ST10 (22.5%), ST1 (19.8%), and, ST3 (17.6%) (53). In accordance with this data, a study in Türkiye reported that all Blastocystis isolates in stray cats were ST4 (38). Finally, birds can be infected with zoonotic subtypes including ST1, ST2, and ST4-ST8, among them ST6 and ST7 are the most common (14,54). The predominant subtypes were ST6 and ST7 in chicken faecal samples in Diyarbakır (42). However, it should be noted that the study groups in animal studies in Türkiye were limited to specific animal groups mostly cattle, sheep, and chicken. Blastocystis subtypes in Türkiye are presented in Figure 1.
Blastocystis Subtypes in Environmental Samples
The previous studies reported the presence of Blastocystis in water samples, including rivers, recreational waters, and lakes (55,56). Lack of appropriate filtration or piped water supplies may be a significant risk factor for the spread of Blastocystis infection (57,58). In addition, the consumption of raw or untreated water sources in the environment may be associated with the infection (59,60). Currently, two studies in Türkiye investigated Blastocystis subtypes in environmental samples. The first study investigated the Blastocystis subtypes in different water samples from Middle Black Sea Region (Sinop, Amasya, and Samsun). They defined subtypes in ten samples and found an equal distribution of ST1 and ST3 (61). The subtypes in this study were as follows: Sinop (ST1, one seawater sample), Samsun (ST1 and ST3, two and one river samples), and Amasya (ST1 and ST3, two and four river samples). Another study in the same region included only river samples and subtyped four Blastocystis isolates. They reported that three were ST1 and one was ST3 (62).
CONCLUSION
In different provinces, Blastocystis subtypes were investigated in limited numbers and sample sizes. More data is required to have better information on the Blastocystis molecular epidemiology in Türkiye. In addition, there is a need for studies dealing with Blastocystis subtypes in animal samples, especially wild animal hosts.
* Ethics
Peer-review: Internally peer-reviewed.
* Authorship Contributions
Concept: E.M., H.E., S.E., Design: E.M., H.E., S.E., Data Collection or Processing: E.M., Analysis or Interpretation: E.M., H.E., S.E., Literature Search: E.M., H.E., S.E., Writing: E.M.
Conflict of Interest: No conflict of interest was declared by the authors.
Financial Disclosure: The authors declared that this study received no financial support.
