Blastocystis sp.

Blastocystis is a cosmopolitan, enteric protozoan parasite, with a prevalence that is generally higher in developing countries. Different genotypes of Blastocystis exist in nature and recent observations indicate that man is indeed a host of many zoonotic genotypes. This has led to conflicting observations on the pathogenic role of the protozoan, due to the existence of both pathogenic and non-pathogenic genotypes. Numerous genotypes found in animals can be transmitted to man and vice versa, and therefore animals represent the reservoir for human infection. Therefore, when this parasite is found in human specimens, the laboratory report should indicate Blastocystis sp. and not Blastocystis hominis.

Size: range 4 to 200 μm (usual range, 4-15 μm).

Morphology: Polymorphic protozoan with four morphological variants:

-     Vacuolar form: spherical, with a large vacuole centrally located that takes up about 90% of the cell. The cytoplasm and its contents are squeezed to a thin peripheral rim. Nuclei and inclusion bodies are visible in the thickest portion of the cytoplasm. The vacuole may be empty or contain thin or wool-like material. This is the form most frequently found in stool specimens.

-     Granular form: similar to the vacuolar form, but with granulations in the cytoplasm or, more frequently, in the central vacuole.

-     Ameboid form: rarely reported due to the heterogeneous descriptions of its morphology. This form is occasionally seen in vitro cultures, but has also been reported in dysenteric stool specimens.

-     Cystic form: only recently described. Due to its small size (2-5 μm), it may be overlooked during microscopic examination; it is a rare finding in cultures too. The cyst is round or oval, with a double wall. The cytoplasm contains 1 to 4 nuclei.

Different ways of reproduction for Blastocystis have been hypothesized and so far binary fission seems the most likely. When cysts are ingested, they excyst in the large intestine and take on either the vacuolar, granular or the ameboid form. The latter encyst during their passage through the large intestine and are eliminated in feces.

For microscopic examination of Blastocystis, fresh fecal samples should be diluted, if necessary, in physiological saline rather than water. This is because water causes the instantaneous lysis of the vacuolar, granular and amoeboid forms of Blastocystis, but not of the cystic forms. Microscopic diagnosis of Blastocystis in fresh specimens is relatively straight forward, except for a possible confusion with yeast or drops of fat. Ideally, samples are first subjected to gradient density concentration (on Ficoll), especially for investigation of cystic forms, but this complex method is possible only in research laboratories and is not routinely used.

Temporary staining is not very helpful in the identification of Blastocystis. The concentration technique using formalin-ether or formalin-ethyl acetate is not sensitive enough for detecting the parasite. The most sensitive method is trichrome staining, by which the large vacuole is stained green or grayish. Giemsa-stained specimens are more difficult to interpret: an inexperienced microscopist might confuse vacuolar forms of Blastocystis with strongly vacuolated forms of Dientamoeba fragilis or with other protozoa, and vice versa.

There is not enough evidence so far to completely exclude a pathogenic role for Blastocystis sp. For this reason, it would be cautious to consider Blastocystis sp. an emerging pathogen. Laboratory reports should therefore indicate this protozoan as Blastocystis sp., specifying whether five or more parasites are seen in a high power field (100x objective). The pathogenicity of only one among the different types of Blastocystis sp. could explain the divergent data currently available. Therefore, future clinical studies should take into consideration the heterogeneity of Blastocystis sp.


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