Tropical Parasitology

: 2015  |  Volume : 5  |  Issue : 1  |  Page : 3--5

Laboratory diagnosis of Blastocystis spp

Christen Rune Stensvold 
 Department of Microbiology and Infection Control, Laboratory of Parasitology, Statens Serum Institut, Artillerivej 5, DK 2300 Copenhagen S, Denmark

Correspondence Address:
Christen Rune Stensvold
Department of Microbiology and Infection Control, Laboratory of Parasitology, Statens Serum Institut, Artillerivej 5, DK 2300 Copenhagen S

How to cite this article:
Stensvold CR. Laboratory diagnosis of Blastocystis spp.Trop Parasitol 2015;5:3-5

How to cite this URL:
Stensvold CR. Laboratory diagnosis of Blastocystis spp. Trop Parasitol [serial online] 2015 [cited 2020 Aug 5 ];5:3-5
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Blastocystis is one of at least two stramenopiles organisms parasitic to humans. A strict anaerobe, it is a parasite of the colon, transmitted by feces, and colonizing humans and a large variety of nonhuman hosts, ranging from nonhuman primates, over birds, to insects, including cockroaches. [1] Colonizing probably more than 1 billion people globally, its clinical and public health significance remains obscure, primarily because it is common in both healthy individuals and patients suffering from intestinal symptoms, including common manifestations such as diarrhea and irritable bowel syndrome; [2],[3] hence, it is currently not possible to distinguish between colonization and infection. Drugs or drug combinations reported to enable the eradication of Blastocystis fail to do so consistently, impeding studies aiming to explore the effect of treatment. [4],[5] Blastocystis exhibits extreme genetic diversity, currently comprising seventeen subtypes (ST1-ST17), nine of which have been identified in humans, and one of the current working hypotheses is that differences in clinical outcome may rely on differences in subtypes.

Accurate diagnosis is critical to clinicoepidemiological studies addressing the role of Blastocystis in health and disease. Several methods have been used to diagnose the parasite. Surveys reporting on intestinal parasites and their prevalence often use classical parasitological techniques, including microscopy of fecal concentrates. These methods are applicable to a wide variety of intestinal parasites and typically known as the "Ova and Cysts" (or-misleadingly-"Ova and Parasites") methods. Blastocystis is often seen listed in data collections arising from studies employing these methods, but consensus is emerging that such methods are impaired by significant lack of sensitivity, which is why data on Blastocystis obtained by the Ova and Cysts method is not particularly informative. Indeed, this predicament may have significantly hampered our ability to develop a more detailed understanding of the role of Blastocystis in health and disease. [6]

The insensitivity of microscopy of fecal concentrates likely stems from disintegration of Blastocystis during technical processing, but it should also be kept in mind that there is no consensus regarding Blastocystis morphology. Many stages have been accounted for, although it appears that there are no benchmark criteria for the identification of the vacuolar, granular, avacuolar, mutlivacuolar, amoeboid, and cyst stages, and it is possible that some of the stages are artifacts arising from, e.g., exposure to oxygen. To many, the morphology of Blastocystis may be seen as a continuum of these different "stages," and the morphological classification of Blastocystis remains a bit of a mystery. Surprisingly, the cyst stage, which is probably responsible for transmission, is very rarely reported of and may require special procedures for accurate detection. [7]

In their article, Elghareeb et al. [17] compare different methods for detecting Blastocystis in 1200 fecal samples from diarrheic patients in Qualyobia, Egypt, demonstrating a vast range in diagnostic sensitivity. Hence, 22.8% (n = 274) of the samples were positive for Blastocystis by xenic in vitro culture (XIVC), while 12.3%, 10.0%, 6.0%, and 3.5% were positive by trichrome staining of fixed fecal smears, formol ether concentration, iodine-stained smear, and direct smear, respectively. Interestingly, XIVC was more sensitive than any of the traditional parasitological techniques. Intriguingly, the authors include a table of the different morphological stages detected by the five methods, and here it appears that culture was the only method enabling detection of all four morphological stages reported of in the study, including the cyst stage. It is interesting that the authors report on cysts in culture since it might be expected that cysts would excyst when incubated in culture under anaerobic conditions at 37°C. Approaches to differentiating morphological stages by diagnostic methods as seen here should be encouraged, but clear definitions on the criteria used for morphological classification are warranted.

People who have worked with Blastocystis cultures know how easy Blastocystis organisms are to pick up by microscopy of a few microliters of positive culture, and hence specificities lower than 100% are unexpected. Surprisingly, Younis et al. indicates the specificity of culture relative to trichrome staining as only 88%. Contrary to evaluating cultures for the presence of Blastocystis, examining permanently stained preparations of fecal smears requires skill and experience. Parasites such as Blastocystis and Dientamoeba may be difficult to pick up in giemsa-stained fecal smears even by highly experienced parasitologists; for instance, they may be confused with leukocytes or even epithelial cells. Compared with giemsa, trichrome staining is more commonly used for Blastocystis; yet, in the absence of molecular confirmation, the diagnostic performances of the various methods evaluated in the study by Younis may be difficult to appreciate.

Meanwhile, state-of-the-art diagnosis is real-time polymerase chain reaction (PCR), such as the assays proposed by Poirier et al. and Stensvold et al., [8],[9] not only due to high sensitivity, but also since DNAs positive for Blastocystis by real-time PCR can be used for molecular characterization using barcoding, [10],[11] which has recently been facilitated and standardized thanks to the "Blastocystis Subtype (18S) and Sequence Typing (MLST) Databases" available at [12] It was recently shown that the sensitivity of culture compared with DNA-based methods is in the range of 52-79%. [8],[9]

Little is known about factors potentially influencing the sensitivity of culture as a diagnostic method, including the time lag from sampling to culture, culture medium used, whether cysts are excysting during XIVC, and whether culture amplifies all subtypes equally well. Jones' medium has been used by many, probably because it is easy and inexpensive to manufacture, but many different types of media would work with Blastocystis. [13] Svensson [14] pointed out that Blastocystis is likely to grow avidly in any starch-containing medium, provided that the starch is split (typically by starch splitting bacteria). Jones' medium does not contain starch, and so using a starch-containing medium may enable a higher sensitivity of culture. Of note, long-term culture may favor one subtype over another in cases of mixed subtype infections, [15] but experience is limited.

Although it is still unclear whether intensity of colonization is linked to existence of symptoms in carriers of Blastocystis, one aspect of Blastocystis diagnosis that is very rarely touched upon, if ever, is the clinical and epidemiological value of a qualitative result, that is, a result simply indicating whether a sample is positive or negative. Using real-time PCR, El Safadi et al. recently showed a Blastocystis prevalence of 100% in a larger group of children in Senegal. [16] Having DNAs available for further analysis, they were able to map the distribution of subtypes. For surveys, it might be interesting to know the proportion of individuals colonized, but it might be even more important to know what subtypes are present, and so a positive result should preferably be expanded by subtype analysis. In cases where posttreatment tests are carried out to test whether Blastocystis has been eradicated or not, obtaining a qualitative result is obviously useful, but also here, adding subtype-even allele-information would be valuable to learn whether any strain detected post-treatment is identical to the one detected prior to treatment. These thoughts should also accompany decisions on whether or not to implement other, non-DNA based, qualitative methods such as antigen detecting ELISA systems.

Despite rapid advances in the field of sequencing, the use of DNA-based diagnostic methods are still relative laborious and expensive, requiring quite advanced technological infrastructures. Subtle, easy-to-use, and low-cost subtype-differentiating methods should be developed to enable screening of large batches of fecal samples along the lines of fecal dipstick tests or tests exploiting synthetic biology technology.


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