Parasites assigned to the genus Entamoeba are single celled eukaryotes and
parasitise all classes of vertebrates, a few invertebrates and possibly other unicellular eukaryotes also. All species have a simple life cycle that usually consists of an infective cyst stage and a multiplying trophozoite stage. Transmission of the infection occurs via ingestion of cysts in faecally contaminated food or water. Humans can be host to at least six species of Entamoeba in addition to several amoebae belonging to other genera. However, only one species of Entamoeba infecting humans is known to cause disease. Entamoeba histolytica is the third leading cause of morbidity and mortality due to parasitic disease in humans (after malaria and schistosomiasis) and is estimated to be responsible for between 50,000 and 100,000 deaths every year. Infection can lead to amoebic dysentery, resulting from trophozoites invading the intestinal wall, and amoebic liver abscess and other extraintestinal lesions, resulting from spread of trophozoites from the intestine via the bloodstream. Differentiation among species of Entamoeba is clearly important for accurate diagnosis of individuals at risk and to prevent unnecessary or inappropriate chemotherapy.
There are very few morphological criteria on which Entamoeba species identification can be based. Classically, descriptions of new Entamoeba
species have relied on :
Traditionally, species are grouped into those that produce cysts
with one, four or eight nuclei. A few species are known not to produce cysts.
In many publications Entamoeba histolytica is cited as infecting one tenth of the world population, or 500 million people. In recent years a new understanding of this organism has lead to the recognition that there are in fact two species within what has previously been known as E. histolytica. Of these two organisms, E. histolytica is the cause of all invasive disease while the other, E. dispar, is not capable of invading tissue. These organisms were previously known as 'pathogenic E. histolytica' and 'nonpathogenic E. histolytica,' respectively, and these names will be encountered in the literature. The relative prevalence of these two species is not yet fully known but it is clear that in most parts of the world E. dispar is easily the more common of the two. The two species are morphologically identical and differentiating between the two relies on relatively sophisticated methods: isoenzyme, antigen and/or DNA analyses. Kits for the differentiation of E. histolytica and E. dispar directly from faecal samples are available commercially from only one source but others will be available soon.
On the 28th and 29th of January 1997 a Consultation of Experts on Amoebiasis sponsored by WHO, PAHO and UNESCO took place in Mexico City the purpose of which was to make recommendations based on the recent evidence for the existence of two species within what had been called Entamoeba histolytica. The resulting document, which was unanimously approved by the participants, was published in a Weekly Epidemiological Report of the WHO. The text of this report can be found here.
Since that time it has also been found that a third morphologically identical species is more common in humans than had been thought. Several groups have reported high local prevalence of E. moshkovskii, previously thought to be primarily a free-living species and rare in humans. How much this organism contributes to misdiagnosis of E. histolytica infection is still unclear.
What is very clear is that we lack recent data on the prevalence of these species. The number of 500 million infections is probably over 90% E. dispar/E. moshkovskii as it has been estimated that only about 36 and 50 million cases of disease occur annually, of which about 70,000 die. Only one class of drugs is used to treat invasive disease, the 5-nitroimidazoles of which metronidazole (Flagyl) is the most widely available. To date we have been fortunate in that no evidence of resistance to this drug has emerged. However we may be one drug resistance mutation away from a catastrophe.