Volume 187 - Issue 6
Amoebiasis: current status in Australia
Authors: Sebastiaan J van Hal, Damien J Stark, Debbie Marriott and Jock L Harkness
Med J Aust 2007; 187 (6): 372-374.
Published online: 17 September 2007
Published online: 17 September 2007
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.
References
- van Hal SJ, Stark DJ, Fotedar R, et al. Amoebiasis: current status in Australia. Med J Aust 2007; 186: 412-416. 0_BABGEGDG
- Stark D, Fotedar R, van Hal S, et al. Prevalence of enteric protozoa in human immunodeficiency virus (HIV)-positive and HIV-negative men who have sex with men from Sydney, Australia. Am J Trop Med Hyg 2007; 76: 549-552. 0_BABJIFBG
- Hung CC, Deng HY, Hsiao WH, et al. Invasive amebiasis as an emerging parasitic disease in patients with human immunodeficiency virus type 1 infection in Taiwan. Arch Intern Med 2005; 165: 409-415. 0_i1091833
- Ackers JP, Mirelman D. Progress in research on Entamoeba histolytica pathogenesis. Curr Opin Microbiol 2006; 9: 367-373. 0_i1091835
- World Health Organization/Pan American Health Organization/UNESCO report of a consultation of experts on amebiasis. Wkly Epidemiol Rec 1997; 72: 97-99. 0_i1091837