To the Editor: I read with great interest the recent updated review of amoebiasis by van Hal and colleagues1 and their previous letter2 describing three cases of locally acquired amoebiasis due to Entamoeba histolytica in Australian men who have sex with men (MSM). These articles should alert clinicians to the emergence of invasive amoebiasis and the possibility of person-to-person transmission of E. histolytica through oral–anal or oral–genital sex among MSM in developed countries. The same phenomenon has been reported in Taiwan3 and Japan.4

The prevalence or incidence of intestinal amoebiasis among people at risk may have been underestimated in the past, as microscopy of stool specimens has lower sensitivity and specificity than E. histolytica antigen detection methods for diagnosing the disease.1,5 Cases of amoebiasis may evade detection using the diagnostic algorithm proposed by van Hal and colleagues,1 which suggests using microscopy of stool specimens to detect E. histolytica complex followed by confirmation with specific antigen detection methods or molecular methods. To increase diagnostic sensitivity and specificity, I suggest revising the diagnostic algorithm for intestinal amoebiasis in developed countries to include more accurate first-line detection methods. For example, specific antigen detection methods or polymerase chain reactions, as proposed by Tanyuksel and Petri,5 could be incorporated.

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.

In reply: We agree with Hung that molecular and antigen testing methods are more sensitive for Entamoeba histolytica detection than microscopy and that reliance on microscopy alone would result in under-detection. Our algorithm1 was presented the way it was for several reasons. Firstly, both molecular and antigen testing are significantly more expensive than microscopy. Secondly, as these tests can currently only detect a single pathogen, they would not replace microscopy. Most patients, especially men who have sex with men (MSM), have multiple intestinal parasites, so the more specific methods would remain an adjunct in parasite detection.2 Thirdly, the positive predictive value of any test is dependent on the prevalence of the disease. The prevalence of E. histolytica in Australia, based on current data, is less than 1% in high-risk populations, including MSM. Thus, at present, molecular and antigen tests would be more likely to give false positive than true positive results. However, we agree that our algorithm could be modified as suggested if prevalence rates were between 5% and 10%. Finally, as seen in the MSM population in Taiwan, this is not a static situation, and ongoing local surveillance is required.3

We agree with Prociv that, before the introduction of molecular techniques, E. histolytica prevalence would have been overestimated. We also agree that specific host factors and/or undefined parasitic virulence factors can lead to invasive disease. However, given the extensive molecular work that has been undertaken, we believe it unlikely that invasive strains of E. dispar will be discovered.4 Furthermore, recent studies show that mixed infections are common.2,5

In symptomatic patients, empirical amoebicidal therapy is warranted. However, to ensure that alternative diagnoses (eg, inflammatory bowel disease) that require different treatment are not overlooked, all attempts to accurately speciate Entamoeba complex should be undertaken. We acknowledge that speciation using the polymerase chain reaction is beyond the means of most laboratories, but this is not the case for enzyme immunoassay testing of stool samples, which is rapid, sensitive and relatively cheap.

For asymptomatic patients who are carriers of E. histolytica cysts, the World Health Organization recommends treatment.5 However, in areas of low prevalence such as Australia, Entamoeba cysts are more likely to be non-pathogenic E. dispar or E. moshkovskii species than E. histolytica.3 Thus, in Australia, treatment would be unnecessary in a high proportion of patients. Furthermore, therapy requires a luminal agent (paramomycin), which is difficult to obtain. The most practical solution is to either give no treatment or to treat only those patients who have tested positive for E. histolytica.