Report of the Expert Consultation on Amoebiasis

Amoebiasis

A WHO/Pan American Health Organization/UNESCO Expert Consultation on Amoebiasis was held on 28- 29 January 1997 in Mexico City, Mexico. Below is presented a summary of the major conclusions, recommendations and research needs identified by the participants.

Introduction

Amoebiasis is currently defined as infection with the protozoan parasite Entamoeba histolytica. Normally resident in the large bowel, amoebae occasionally penetrate the intestinal mucosa and may disseminate to other organs. The factors that trigger invasion are unknown. E. histolytica is responsible for up to 100 000 deaths per annum, placing it second only to malaria in mortality due to protozoan parasites. It has long been known that many people apparently infected with E. histolytica never develop symptoms and spontaneously clear the infection. This was interpreted by many workers as indicating a parasite of variable virulence. However, in 1925 Emile Brumpt suggested an alternative explanation, that there were in fact 2 species, one capable of causing invasive disease and one that never causes disease, which he called E. dispar. Brumpt's hypothesis was dismissed by other workers. In the 1970s, data started to accumulate that gave support to Brumpt's hypothesis of the existence of 2 distinct organisms within what was being called E. histolytica. Biochemical, immunological, and genetic data continued to accumulate and in 1993 a formal redescription of E. histolytica was published, separating it from E. dispar.

E. histolytica can cause invasive intestinal and extra-intestinal disease, contrary to E. dispar. The confirmation of these 2 distinct species of Entamoeba is perhaps the major recent accomplishment in the field of amoebiasis research. In addition, proteins associated with virulence have been identified, including a lectin that mediates adherence to epithelial cells, a pore-forming peptide that lyses host cells, and secreted proteases that degrade host tissues. All of these virulence proteins, as well as other unique antigens present on the parasite surface are potential targets for anti-amoebic vaccines. Biochemical studies have identified the bacterial-like fermentation enzymes, which are the target of metronidazole, the most potent anti-amoebic drug in tissues, and suggest new targets for anti-amoebic drugs. This consultation was called to evaluate the implications of this recent work.

Conclusions

• E. histolytica was previously defined as a unicellular eukaryote with the following morphology: trophozoites with a single nucleus, 20-40 µm in diameter; cysts are 10-16 µm in diameter, with 4 nuclei when mature, 1 nucleus when immature with glycogen in a vacuole and often with chromatoid bodies. The nucleus is vesicular, spherical, with a membrane lined with small chromatin granules and with a small central spherical karyosome.
• Biochemical, immunological and genetic data now indicate that there are 2 species with the above morphological characteristics - E. histolytica and E. dispar - previously known as pathogenic and non-pathogenic E. histolytica, respectively. Only E. histolytica is capable of causing invasive disease. In the future, the name E. histolytica should only be used in this sense and will be used as such in the rest of this document.
• When diagnosis is made by light microscopy, the cysts of the 2 species (10-16 µm in diameter) are indistinguishable and should be reported as E. histolytica/E. dispar.
• Trophozoites with ingested red blood cells in fresh stool or other specimens and trophozoites in tissue biopsies are both strongly correlated with the presence of E. histolytica and invasive disease.
• In symptomatic individuals, the presence of high titres of specific antibody is also strongly correlated with invasive amoebiasis.
• Faecal antigen detection tests are commercially available. Currently, only one test specifically identifies E. histolytica, but other tests based on this and other technologies are under development.
• The WHO definition of amoebiasis is reaffirmed as infection with E. histolytica (in its new sense) with or without clinical manifestations.

Recommendations

• Optimally, E. histolytica should be specifically identified and, if present, treated.
• If only E. dispar is identified, treatment is unnecessary. If the infected person has gastrointestinal symptoms, other causes should be sought.
• Species identification based on culture can never exclude the presence of E. histolytica. (Note)
• In asymptomatic individuals, treatment is not appropriate when E. histolytica/E. dispar has been detected but E. histolytica has not been specifically identified, unless there are reasons to suspect infection with E. histolytica, including high specific antibody titres, a history of close contact with a case of invasive amoebiasis, or an outbreak of amoebiasis.
• If E. histolytica/E. dispar has been detected in symptomatic patients, it should not be assumed that E. histolytica is the cause of the symptoms and other explanations should also be considered.
• These recommendations are appropriate for managing all individuals, including male homosexuals, travellers returning from endemic areas, pregnant women, and individuals infected with HIV.
• Antiamoebic drugs are of 2 classes: tissue amoebicides (such as 5-nitroimidazoles) and luminal amoebicides (such as diloxanide furoate and paromomycin). Invasive disease should be treated with a tissue amoebicide followed by a luminal amoebicide. Tissue amoebicides are not appropriate for treatment of asymptomatic individuals, unless other evidence for invasive amoebiasis exists.
• Chemoprophylaxis is never appropriate.

Research needs

During the last few years with very limited funding considerable advances have been made in the understanding of E. histolytica infection. The opportunity now exists to exploit this knowledge to make a major impact on the morbidity and mortality due to E. histolytica. Significantly increased resources should be devoted to research on amoebiasis; the priority areas that should be supported are:

• Improved methods for the specific diagnosis of E. histolytica infection using technologies appropriate for developing countries are urgently needed.
• Accurate prevalence data for E. histolytica are not available and obtaining them should be a high priority.
• Molecular epidemiological studies are now possible and should be vigorously pursued to determine whether some subgroups of E. histolytica are more likely to cause invasive disease than others.
• Data on the prevalence of asymptomatic carriers of E. histolytica, their likelihood of progression to invasive disease, and the frequency of transmission to naive individuals are inadequate. More information is crucial for planning rational control strategies and must be obtained.
• Knowledge of the host and parasite factors responsible for the invasiveness of E. histolytica is essential for understanding how this organism causes disease.
• Fundamental studies on the immunology of human amoebiasis are essential for evaluating the feasibility of developing an E. histolytica vaccine.

The members of the expert committee were as follows (Photo):

Dr. John P. Ackers (Co-chair), Department of Medical Parasitology, London School of Hygiene and Tropical Medicine, London, England

Dr. C. Graham Clark (Rapporteur), Department of Medical Parasitology, London School of Hygiene and Tropical Medicine, London, England

Dr. Louis S. Diamond, Laboratory of Parasitic Diseases, National Institutes of Health, Bethesda, MD, USA

Dra. Martha Espinosa Cantellano, Departamento de Patología Experimental, CINVESTAV-IPN, México D.F. México

Dr. Terry F.H.G. Jackson, Medical Research Council (Durban), South African Medical Research Council, Congella, Kwazulu-Natal, South Africa

Dr. Adolfo Martínez-Palomo (Co-chair), Departamento de Patología Experimental, CINVESTAV-IPN, México D.F., México

Dr. David Mirelman, Departments of Membrane Research and Biophysics, The Weizmann Institute of Science, Rehovot, Israel

Dr. Onofre Muñoz Hernández, Direccion de Prestaciones Médicas, Centro Medico Nacional Siglo XXI, México D.F. México

Dr. Ruy Pérez Tamayo, Departamento de Medicina Experimental, Facultad de Medicina, UNAM, México D.F. Mexico

Dr. William A. Petri, Jr., Department of Medicine, University of Virginia, Charlottesville, VA USA

Dr. Sharon L. Reed, Department of Infectious Diseases, UCSD Medical Center, San Diego, CA USA

Dr. Guillermo M. Ruíz Palacios, Departamento de Infectología, Instituto de la Nutrición "Salvador Zubiran", México D.F. Mexico

Dr. John C. Samuelson, Department of Tropical Public Health, Harvard School of Public Health, Boston, MA USA

Dr. Josée Ignacio Santos Preciado, Departamento de Infectolgía, Hospital Infantil de México "Federic Gómez", México D.F., México

Dr. Egbert Tannich, Department of Molecular Biology, Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany

Dr. Tsutomu Takeuchi, Department of Tropical Medicine and Parasitology, School of Medicine, Keio University, Tokyo, Japan

Dra. Cecilia Ximénez García, Depto. de Medicina Experimental, Facultad de Medicina, UNAM, México D.F. México

Note: It has been brought to our attention that the meaning of this recommendation is not entirely clear. This was included because of the observation that not all faecal samples positive by microscopy for E. histolytica/E.dispar are positive in culture and that in experimental mixed cultures one species usually outgrows the other. Therefore it is assumed that in some mixed E. histolytica/E.dispar infections E. histolytica will not grow in culture, or will be outgrown by E. dispar, and thus its presence will not be detected by methods using culture derived materials, for example isoenzyme analysis. So even if you get a culture of organisms from faeces and they are typed as E.dispar, one cannot exclude that E.histolytica was present in the original infection also but did not grow.