Report of the Expert Consultation on Amoebiasis

(Exerpted with permission from the WHO WEEKLY EPIDEMIOLOGICAL RECORD, No. 14, 4 APRIL 1997)


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.


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.



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:

Additional information is available upon request addressed to the Schistosomiasis and Intestinal Parasites Unit, Division of Control of Tropical Diseases (CTD/SIP), WHO, 1211 Geneva 27, Switzerland.

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.

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