Trends in Parasitology
ReviewTransmission of Dientamoeba fragilis: pinworm or cysts?
Section snippets
Dientamoeba: basic information is elusive despite its ubiquity
Dientamoeba fragilis (see Glossary) is an intestinal trichomonad parasite that has lost its microtubular cytoskeleton and flagella, leading to an amoeboid lifestyle [1]. Its life cycle has remained a mystery since its description 95 years ago because only a fragile trophozoite stage and no cyst stage has been described, unlike most other intestinal protists where a cyst is essential for transmission of the infection. Three recent publications address the major gap in the D. fragilis life cycle,
History and Histomonas
Because it is an intestinal parasite, one might assume that, like most other intestinal protozoa, D. fragilis requires a cyst stage to survive in the external environment. However, until very recently, although there have been a few inconclusive reports of pseudocysts, precysts, or cysts of D. fragilis (see references in [1]), it has been generally accepted that no cyst form exists for this parasite. Indeed, Clifford Dobell said, ‘although a prolonged and very careful search has been made for
The link to Enterobius
Dobell believed that the vector for Dientamoeba could be Trichuris or Ascaris eggs but, for many years now, Enterobius vermicularis (pinworm) eggs have been the leading candidate as the vector for D. fragilis transmission. This is consistent with the continued presence of E. vermicularis, especially in children, in many countries where D. fragilis infection remains common whereas other nematodes are increasingly rare or absent. Moreover, pinworm and D. fragilis infections can be
How solid is the evidence for egg transmission of Histomonas?
The whole construct of nematode egg transmission of D. fragilis rests on the parallels with Histomonas; it is therefore essential to know how solid the evidence is for the requirement of H. gallinae in Histomonas transmission. For many years, experimental infection of birds with Histomonas has employed, among other methods, oral administration of eggs or other stages of H. gallinae containing Histomonas [19]. The interaction between the two organisms has been investigated at the morphological
Cysts of Dientamoeba?
If Histomonas produces cysts, why should this not also be true of Dientamoeba? Is there any evidence for cysts in this parasite? As mentioned above, there have been sporadic reports over the years of cyst-like structures but nothing definitive. However, apparently bona fide D. fragilis cysts with thick walls have been reported recently [2], and the authors propose these to be the missing link in transmission of D. fragilis between hosts. This discovery comes as a great surprise to many in the
The evidence
In the egg studies, E. vermicularis eggs of human origin from adhesive tape samples, swabs, or female adult worms were surface-sterilised using hypochlorite 3, 4 or extensively washed [4] before DNA extraction and PCR. Notably, DNA was extracted from the last buffer solution used to wash the eggs, and this was shown by PCR to be negative for D. fragilis in every [3] or almost every [4] case. DNA was extracted from individual [3] or pooled [4] eggs, and D. fragilis was detected by PCR and
Concluding remarks: closing the loop
To make or break the link between the cyst and D. fragilis there is a variety of options; for instance, it should be possible to stain the cysts specifically by fluorescent in situ hybridisation using Dientamoeba-specific oligonucleotide probes that hybridise to the ribosomal RNA. With suitable controls, this approach could give unambiguous results. The fact that there is a thick cyst wall should not be an insurmountable barrier because this approach has been successful for Giardia,
Acknowledgements
We would like to thank Drs Bobbi S. Pritt and Marianne Lebbad for helpful discussions and comments on this manuscript.
Glossary
- Adhesive tape test
- also known as transparent adhesive test, cellophane tape test, or Scotch tape test, this is the gold standard diagnostic test for detecting pinworm (Enterobius vermicularis) infection. The tape is pressed against the anus and perianal area of the patient causing pinworm eggs to stick to the tape surface; this allows detection (and collection) by simple light microscopy.
- Amoeboid
- cells of no fixed shape where movement involves protrusion of cytoplasm of the cell to form
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Cited by (34)
Differentiation of Blastocystis and parasitic archamoebids encountered in untreated wastewater samples by amplicon-based next-generation sequencing
2020, Parasite Epidemiology and ControlCitation Excerpt :This might explain why we did not detect Dientamoeba fragilis, which is a parasite that is very common in Sweden and the rest of Scandinavia. Whether D. fragilis produces cysts is a matter of current debate (Clark et al., 2014; Munasinghe et al., 2013), but if such cysts existed, DNA from such cysts would likely be present in the extracted DNA. However, we also know that the sensitivity of our platform in terms of detecting flagellate DNA is low, so Giardia and Dientamoeba are detected only when tested samples are highly positive as evidenced by e.g. real-time PCR (unpublished observations).
Molecular epidemiology of Dientamoeba fragilis
2018, Acta TropicaCitation Excerpt :Second, DNA of D. fragilis has been detected in DNA extracted from E. vermicularis eggs of human origin from adhesive tape samples, swabs, or female adult worms (Ögren et al., 2013; Röser et al., 2013b). Third, there is an epidemiological association between Enterobius and Dientamoeba: in fact, the parasites show a similar age distribution, and coinfection occurs at a higher than expected level (Clark et al., 2014). Finally, but importantly, there is a parallelism with a closely related organism, Histomonas meleagridis, a parasite of poultry.
Other Entamoeba, Amebas, and Intestinal Flagellates
2018, Principles and Practice of Pediatric Infectious DiseasesOccurrence and removal efficiency of parasitic protozoa in Swedish wastewater treatment plants
2017, Science of the Total EnvironmentCitation Excerpt :A few studies have however reported pseudocysts and cyst-like states (Munasinghe et al., 2013; Stark et al., 2014), which could indicate the potential for D. fragilis to disseminate via wastewater. For now, the general scientific consensus is that no cyst forms exist and the currently leading hypothesis is that D. fragilis is transmitted via pinworm ova (Clark et al., 2014; Ögren et al., 2013). One study has detected D. fragilis in wastewater (Stark et al., 2012), although the authors were unable to establish if the detected specimen was viable or infectious.
Protozoa
2017, Infectious Diseases, 2-Volume Set