Amoebiasis: current status in Australia

To the Editor: van Hal and colleagues deserve congratulations for their lucid, concise and timely review of the complex problem of human amoebic infection and its diagnosis.1 Not surprisingly, however, their article raises more questions than it answers.

To me, the gist of their message was as follows: what was in the past diagnosed as Entamoeba histolytica infection, based on the microscopic identification of organisms from faeces, culture or histological sections, actually may have been caused by other species, viz. E. dispar (a recently described non-pathogen) or E. moshkovskii (known for a long time from sewage samples, but only recently found to infect humans). Because these species are all identical morphologically, they can be distinguished reliably only by sophisticated molecular techniques.

To complicate matters further, despite E. dispar having been virtually defined as the “non-invasive form” of Entamoeba, most true E. histolytica infections are still asymptomatic.2 Not only the parasite, but also individual host factors (perhaps including genetics), determine pathogenicity. Thus, not all people infected with the same pathogenic strain will manifest symptoms or signs of invasive disease. Given their biology and evolution, it is conceivable that, eventually, invasive strains of even E. dispar will be discovered! Furthermore — and this seems not to have been investigated yet — mixed infections involving different species and strains of these parasites almost certainly occur (not to mention the “traditional” non-pathogenic amoebae, which frequently do occur in mixed infections).

The authors advocate treatment of even asymptomatic E. histolytica infections, but how would these be detected outside epidemiological surveys or healthy population screening programs? Given the difficulty and expense of specifically identifying the infective organism even in symptomatic cases, and the relative cheapness of treatment, surely it would be sufficient simply to treat on the basis of clinical presentation plus the identification of E. histolytica-like parasites, with or without objective evidence of histopathology. Anything more could be justified only within the context of a well funded and carefully designed research program and/or epidemiological study.