Causes of Dientamoeba fragilis
Dientamoeba fragilis was considered as non-pathogenic for a long time and was then neglected by the scientific research community. As such, the complete life cycle of this parasite has not been elucidated yet so the real causes of these infections remain unknown. However, in recent years, many hypotheses have been suggested in order to find how this parasite is transmitted to human.
One of the main problems in finding the real mode of transmission of this parasite is the lack of enough evidences in order to confirm the actual occurrence of a cyst stage, which is a dormant stage allowing the parasite to survive to the harsh conditions prevailing outside the human or animal body. In fact, there is no known actual cyst stage for any of the protozoan parasites from the group called trichomonads in which Dientamoeba fragilis belongs, which are usually transmitted by direct contact. For example, it is notably the case of the related parasite Trichomonas vaginalis, which is transmitted by direct sexual contacts. However, the suggested high prevalence of infections with Dientamoeba fragilis, a known intestinal parasite, could not be correlated with a transmission by direct contact only.
In fact, intestinal protozoan parasites are usually transmitted by accidental ingestion via the so-called fecal-oral route involving the contact between the mouth and something (food, drink, hand or object) contaminated with faeces of an infected human or animal. This mode of transmission is usually linked with the production of cysts in intestinal protozoan parasites allowing them to survive outside the body. This stage is mandatory, as the active form of the parasite would not be able to survive for a period long enough to allow a good transmission rate outside the human body. It is especially true in the case of Dientamoeba fragilis, as the species name fragilis refers to the fragility of its active form that dies very quickly outside the human or animal body. Until recently, there was no known identified cyst stage for this specific parasite. However, a recent report has suggested that Dientamoeba fragilis has indeed a cyst stage excreted by rodents and it is a confirmation that this parasite is indeed able to be transmitted successfully by the fecal-oral route in this specific animal. Furthermore, very recently, another researchers’ group has found some evidences of cysts and precystic forms of this parasite in clinical samples derived from human faeces. However, even though it gave a hint of the actual presence of a cyst stage, this finding has yet to be confirmed independently by other groups.
Another interesting hypothesis to explain the transmission of this parasite is the Trojan Horse trick. This particular hypothesis stipulates that Dientamoeba fragilis is susceptible to be transmitted at the same time than some helminth eggs, in which it can be protected from the harsh environment. This hypothesis is supported by the fact that Dientamoeba fragilis is frequently found concomitantly with pinworms (Enterobius vermicularis) when testing people with gastrointestinal symptoms. It has then been suggested that the active form of Dientamoeba fragilis hides itself within the pinworm eggs in order to get protected from the harsh environment prevailing outside the human or the animal body. Another related parasite, the turkey parasite Histomonas meleagridis, is transmitted to its host inside the eggs of another parasite, the nematode Heterakis gallinarum, which reinforced the belief in this hypothesis for the transmission of Dientamoeba fragilis. Furthermore, DNA from Dientamoeba fragilis has indeed been found in some pinworms. However, this hypothesis has not been confirmed yet by solid scientific proofs, as many studies trying to confirm it gave contradictory results and that there is not always an infection with pinworms associated with Dientamoeba fragilis.
It is interesting to mention that some studies had also tried to find an animal that can serve as an intermediate host in the life cycle of this parasite allowing the further infection of humans. However, even though this parasite can be found in the intestinal tract of pigs and gorillas, a direct link between such an animal reservoir and transmission to human has not been proven yet.
Finally, as more studies will be performed on Dientamoeba fragilis, we will eventually be able to elucidate its complete life cycle. When this will be done, the exact causes underlying the infection by this parasite will be unveiled. Note that it is possible that they are linked to one of the mentioned hypothesis or to a combination of them depending on the circumstances. As such, the contradictory results obtained when scientists tried to confirm the Trojan Horse hypothesis might be explained by the fact that this mode of transmission could be involved in some cases, but not in others.