In 1903 Schaudinn reported the existence of two species of amoebae, one harmless and named Entamoeba coli, the other pathogenic and named E. histolytica. In 1907 Viereck described a pathogenic amoeba which, by reason of its four nuclei in the encysted stage, he called E. tetragena.
As the result of the work of Hartmann, Whitmore, Darling, Wenyon and the recent conclusive findings of Walker we now hold the view that Schaudinn was working with E. tetragena and not with a separate species, so that by the law of priority we must drop the name E. tetragena and accept E. histolytica.
Geographical Distribution.—Amoebic dysentery seems to be especially prevalent in Indo-China, China and the Philippines, as well as in parts of India. It is also very common in Egypt and Northern Africa. In South America, especially Brazil, it is common, as is also true of the West Indies and Central America. It is an important disease in the Southern States of the United States, as well as in Italy and other parts of Southern Europe. On the whole it is probable that it exists in greater or less degree in most of the tropical and subtropical parts of the world.
Etiology and Epidemiology
Etiology.—For a long time the authorities in Manila held that it was impracticable to differentiate between a pathogenic and nonpathogenic species, taking the view that the principal factor in the production of dysentery was that of symbiosis between amoebae and suitable bacteria, it having been thought that they observed in cultures of amoebae evidences of both symbiosis and antagonism on the part of amoebae to certain species of bacteria. They furthermore were convinced that pathogenic amoebae could be cultured on a medium of about 1/10th the strength of ordinary nutrient bouillon or agar and that dysentery could be produced by such cultural amoebae. Such views had an important bearing on epidemiology as it was thought that where amoebae could be cultured from green vegetables, fruit, or water supply there was positive evidence of the possibility of infection with amoebic dysentery from such a source.
The above views are no longer entertained and, due to Walker, working in Manila with experiments on man, we now know that cultural amoebae are without effect in the production of dysentery and that there are certainly two well known species of amoebae having man for a host, the one pathogenic, Entamoeba histolytica, and the other a harmless commensal, Entamoeba coli. Two other species, Endolimax nana and Iodamoeba bütschlii, have recently been described but are generally considered nonpathogenic.
Some authorities prefer the generic names Löschia and Endamoeba to Entamoeba.
Schaudinn, in 1903, described the pathogenic amoeba, which he named E. histolytica, as follows: 1. Distinct, highly refractile and tenacious ectoplasm. He considered this tough external portion of the cytoplasm as the explanation of the ability of the pathogenic amoeba to bore its way into the intestinal submucosa. 2. Eccentric nucleus which was indistinct by reason of little chromatin. 3. Reproduction by peripheral budding in which small aggregations of chromatin reached the periphery of the cytoplasm and, enclosed in a resistant capsule, broke off from the parent amoeba and constituted the infecting stage.