Trends in Parasitology
Volume 30, Issue 3, March 2014, Pages 136-140
Journal home page for Trends in Parasitology

Review
Transmission of Dientamoeba fragilis: pinworm or cysts?

https://doi.org/10.1016/j.pt.2014.01.005Get rights and content

Highlights

  • At least two different modes of transmission of Dientamoeba fragilis have been proposed.

  • Vector-borne transmission by pinworm and direct transmission by a cyst have been suggested.

  • Based on current evidence it is not possible to prove the mode(s) of transmission.

  • Suggestions for experimental work to close the gap of knowledge are given.

Recently, conflicting evidence has been published on the mode of transmission of the trichomonad Dientamoeba fragilis. Detection of D. fragilis DNA inside Enterobius vermicularis eggs agrees with the prediction of Dobell in 1940 that the eggs of a nematode act as a vector for transmission. However, the identification of a cyst stage of D. fragilis in the stool of rodents infected with a human isolate has also been reported, and this implies a life cycle similar to those of most other intestinal protistan parasites. Herein we discuss the recent data, identify gaps in the experimental evidence, and propose a method for determining which view of the life cycle of this organism is correct.

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

References (40)

  • E.H. Johnson

    Emerging from obscurity: biological, clinical, and diagnostic aspects of Dientamoeba fragilis

    Clin. Microbiol. Rev.

    (2004)
  • J. Ögren

    Dientamoeba fragilis DNA detection in Enterobius vermicularis eggs

    Pathog. Dis.

    (2013)
  • C. Dobell

    The Amoebae Living in Man. A Zoological Monograph

    (1919)
  • C. Dobell

    Researches on the intestinal protozoa of monkeys and man. X. The life history of Dientamoeba fragilis: observations, experiments, and speculations

    Parasitology

    (1940)
  • R.B. Burrows et al.

    Enterobius vermicularis as a probable vector of Dientamoeba fragilis

    Am. J. Trop. Med. Hyg.

    (1956)
  • G. Ockert

    Epidemiology of Dientamoeba fragilis Jepps and Dobell, 1918. 2. Attempt at species transfer with Enterobius eggs

    J. Hyg. Epidemiol. Microbiol. Immunol.

    (1972)
  • J. Yang et al.

    Dientamoeba fragilis: a review with notes on its epidemiology, pathogenicity, mode of transmission, and diagnosis

    Am. J. Trop. Med. Hyg.

    (1977)
  • G. Ockert

    Epidemiology of Dientamoeba fragilis Jepps and Dobell 1918. 1. Spread of the species in child collectives

    J. Hyg. Epidemiol. Microbiol. Immunol.

    (1972)
  • M. Portús et al.

    Contribution to the knowledge of intestinal protozoa infestation in the hospital population of Barcelona (author's translation)

    Med. Clin. (Barc.)

    (1981)
  • U. Preiss

    Dientamoeba fragilis infection, a cause of gastrointestinal symptoms in childhood

    Klin. Padiatr.

    (1990)
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