Discussion

Observations on the life cycle and transmission of Dientamoeba fragilis. Here we discuss the life cycle and mode of transmission of Dientamoeba fragilis that is not well understood. Historically, transmission was believed to occur through Enterobius vermicularis ova or directly via trophozoites in stool. However, recently evidence for fecal–oral transmission of cysts has emerged. Since cultured trophozoites are vulnerable to extremes of pH, we suspect cysts are the main mode of transmission. Cysts of D. fragilis are readily produced by laboratory rodents after oral infection, however their detection in human clinical samples is rare. Additionally, D. fragilis was detected in pigs, cats, dogs, budgerigars, rats, goats, cattle, and non-human primates. This highlights the potential for zoonotic transmission, probably through the cyst stage as E. vermicularis is a human specific helminth. Zoonosis is unlikely to be the sole source of human infections. Epidemiological analysis of the case prevalence for different age groups reveals dual peaks for children and adults at parental ages which is a common indication of human-to-human transmission. The specificity of current real time PCR diagnostic assays for the detection of D. fragilis are unreliable. Cross reactivity with Pentatrichomonas hominis and Tritrichomonas foetus was observed when testing the assays with DNA from cultures and such assays are not directly transferable from use on human to animal specimens. Further one PCR test has a significant problem with false positive results on human samples. We suggest that the current estimates of D. fragilis prevalence are overestimated, and the identification of novel animal hosts without the support of additional (sequence) data is unreliable. We provide further characterisation of D. fragilis cysts by electron microscopy.