Dientamoeba fragilis cysts and pre-cysts in historic slide collections and a review of cyst formation among the parabasalia

Abstract Dientamoeba fragilis transmission is a basic aspect of this intestinal parasite’s biology that is poorly understood. Early historical reports reflecting the absence of a cyst are often cited as a central argument in debates supporting the lack of a D. fragilis cyst. While D. fragilis cysts have been described since Dobell’s original description, their existence is not universally accepted. Here, Dobell’s, Wenyon’s and Hoare’s collection of historical faecal smears stored at the Natural History Museum (London), dating back to the 1890s and the early 1900s, was examined for forms consistent with modern descriptions of D. fragilis cysts, and an example was found in 1 slide. Such rare forms were also detected during examination of stained faecal smears archived in the parasite reference laboratory collection at the United States Centers for Disease Control and Prevention. We discuss these observations in the context of literature describing cyst formation in parabasalids. Additionally, we report some incidental findings from past immunofluorescence experiments on cultured D. fragilis , which suggest differential staining that appears to correlate with life cycle stages. Considering published literature on the subject of D. fragilis cysts and the broader picture of cyst formation across diverse members of Parabasalia, we recommended that future investigations on D. fragilis transmission consider mounting evidence for the role of a true cyst despite its rarity in human faecal specimens. The factors leading to cyst formation and further characteristics of this life cycle stage require further study.