Genus Entamoeba

The nucleus is vesicular, with a comparatively small endosome located at or near its center, with or without periendosomal granules around the endosome, and with a varying number of granules around the periphery of the nucleus. Cysts are formed; they contain 1 to 8 nuclei and may or may not contain chromatoid bodies (rod-like bodies which stain with hematoxylin and which are absorbed and disappear as the cysts mature). This genus occurs in both vertebrates and invertebrates.

The name of this genus was the subject of one of the most famous taxonomic controversies in protozoology (Dobell, 1938; Kirby, 1945). The genus Endamoeba was established by Leidy in 1879 for an amoeba of the cockroach, Endamoebae blattae. In 1895 and in ignorance of this name, Casagrandi and Barbagallo introduced the name Entamoeba for the human amoeba, E. coll. The nuclei of these two forms are very different, that of the cockroach species lacking a central endosome. Since the appearance of the nucleus is the most important differentiating character between genera in this family, it is obvious to a protozoologist that these forms belong in different genera. However, the first syllable of their names is derived from the same Greek root. Hence the International Commission of Zoological Nomenclature was asked to decide whether the name that had been given second should be changed to something else (i.e., whether the two names were homonyms). The Commission, which had no protozoologists among its members, went beyond this, and decided that Entamoeba was a synonym of Entamoeba and that both protozoa belonged to the same genus. Altho the latter is obviously not true, this dictum was accepted by many protozoologists. Finally, after much agitation and several long, involved papers by various authorities, the International Commission finally reversed itself, and Entamoeba is now universally recognized as the correct name for the species occurring in vertebrates.

The nomenclature and taxonomy of the species of Entamoeba are about as confused as it is possible to make them. Some of the problems are explained below, but puzzle addicts are referred to the cited papers for the details.

Members of the genus found in domestic animals and man can be divided into 4 groups on the basis of trophozoite and cyst morphology. A fifth group includes species about which insufficient morphological information is available to determine which of the other groups they belong in. Most of the species within each group are morphologically indistinguishable; they are differentiated on the basis of size, hosts, pathogenicity, etc. Criteria of this type are given different weights by different taxonomists, and this fact combined with a lack of cross-transmission studies for many species accounts for some of the confusion. More is due to the fact that the original descriptions of some of the species were so poor as to make it difficult or impossible to be sure what forms the authors were dealing with.

The groups of Entamoeba will be described first, and then the individual species.

1. Histolytica Group. The nucleus has a small, central endosome, a ring of small peripheral granules and a few scattered chromatin granules between them. The cysts have 4 nuclei when mature, and their chromatoid bodies are rods with rounded ends. Glycogen vacuoles, when present in the cyst, are usually diffuse and illdefined.

Entamoeba histolytica of man, other primates, the dog, cat and rarely the pig.
Entamoeba hartmanni of man and presumably also of the other hosts of E. histolytica.
Entamoeba equi of the horse.
Entamoeba analis of the duck.
Entamoeba moshkovskii of sewage.

2. Coli Group. The nucleus has a somewhat larger, eccentric endosome than that of the histolytica group and has a ring of coarse peripheral granules and some scattered chromatin granules between them. The cysts have 8 nuclei when mature, and their chromatoid bodies are splinter-like. Glycogen vacuoles, when present in the cyst, may be fairly well defined.

Entamoeba coli of man, other primates, the dog and possibly the pig.
Entamoeba wenyoni of the goat.
Entamoeba muris of mice, rats, hamsters, and other rodents.
Entamoeba caviae of the guinea pig.
Entamoeba cuniculi of rabbits.
Entamoeba gallinarum of the chicken, turkey, guinea fowl, duck, and goose.

3. Bovis Group. The endosome of the nucleus varies in size; it may be as small as that of the histolytica group, but is generally larger than that of the coli group. The ring of peripheral granules in the nucleus may be fine or coarse, evenly or irregularly distributed. Periendosomal granules may be present. The cysts have 1 nucleus when mature, and their chromatoid bodies are either rods with rounded ends or less often splinter-like. Glycogen granules, when present in the cyst, are usually fairly well defined.

Entamoeba bovis of cattle.
Entamoeba ovis of sheep and goats.
Entamoeba dilimani of goats.
Entamoeba suis of the pig and perhaps man.
Entamoeba bubalis of the carabao.
Entamoeba chattoni of monkeys and probably man.

4. Gingivalis Group. The nucleus has a small, central endosome and a ring of small peripheral granules. There are no cysts. Members of this group are found in the mouth.

Entamoeba gingivalis of man, other primates, the dog and cat.
Entamoeba equibuccalis of the horse.
Entamoeba suigingivalis of the pig.

5. Insufficiently known species. This group includes Entamoeba gedoelsti of the horse and E. caudata of the dog. The nucleus of E. caudata resembles that of E. coli, while the nucleus of E. caudata resembles that of E. histolytica. The cysts of these species are unknown.

The only species of Entamoeba pathogenic for mammals is E. histolytica. The fact that it has been recorded in an average of about 18% of the people examined in various surveys thruout the world and yet only about 1/5 of them have signs or symptoms of disease has puzzled epidemiologists for many years. Two other facts contribute to the problem. One is that there are two different sizes of these amoebae, the smaller of which is not associated with disease; it has been encountered in about 1/3 of the people in these surveys. The other is that amoebic dysentery occurs mostly in the tropics. Autochthonous cases occur so seldom in western Europe that many European parasitologists believe that cases of amoebic dysentery which occur in their countries have been imported from the tropics either directly or thru contact with infected persons. These parasitologists have not had the benefit of the American experience with the disease. There is no question that indigenous cases occur in the temperate zpne of this country.

Several hypotheses have been advanced in explanation (see Hoare, 1958). The first theory, suggested about 1913 and still held by perhaps the majority of parasitologists, is based on an unwillingness to assign separate specific names to protozoa which differ only in size and pathogenicity. According to this view, the species Entamoeba histolytica is composed of a small race and a large race. The small race is not pathogenic, while the large race may or may not be. In its virulent phase the latter invades the tissues; the trophozoites of this phase are large - the "magna" form. In its commensal phase it remains in the lumen of the intestine, feeding on bacteria and saprozoically. The trophozoites of this phase are small - the "minuta” form. Under proper conditions, this non-pathogenic "minuta" form can invade the intestinal mucosa and turn into the pathogenic "magna" form. The trophozoites of the small race of E. histolytica are usually 12 to 15 u in diameter and the cysts are 5 to 9 u in diameter with a mean of 7 to 8 u. The trophozoites of the "magna" form of the large race are 20 to 30 u and those of the "minuta" form 12 to 15 u in diameter. However, the cysts of both the "magna" and "minuta" forms are the same size, 10 to 20 u in diameter, with a mean of about 12 u.

The second theory was proposed by Brumpt in 1925. He recognized 3 species. He called the small, non-pathogenic race Entamoeba hartmanni, and divided the large race into 2 species. Of these, E. dispar is non-pathogenic and occurs thruout the world, while E. dysenteriae is pathogenic, altho it may cause no apparent symptoms in carriers, and occurs only in warm and hot countries.

The third theory was formulated formally by Hoare in 1957. It calls the small, non-pathogenic race E. hartmanni, but retains the name E. histolytica for the large race of the first theory. It then divides E. histolytica into an avirulent race (corresponding to Brumpt's E. dispar) and a virulent race (corresponding to Brumpt's E. dysenteriae) which may invade the gut wall or live in the lumen without causing symptoms.

This view has a great deal to recommend it. By giving the non-pathogenic small form a separate name, it makes it easier for the physician to interpret laboratory reports and prevents faulty diagnoses and needless treatment. However, the question whether there actually are completely non-pathogenic strains of E. histolytica (sensu stricto) which cannot be induced to become pathogenic has not yet been answered satisfactorily. Concomitant bacteria, nutritional deficiencies and other factors affect the pathogenicity of the amoebae. Indeed, Phillips et al. (1955) found it impossible to infect bacteria-free guinea pigs with E. histolytica at all, altho normal guinea pigs or those infected with Escherichia coli or Aerobacter aerogenes could be readily infected and subsequently developed intestinal lesions.

Some of the amoebae reported as E. histolytica from domestic animals may well have been actually E. hartmanni, but unless they were specifically described as having small cysts, it is impossible to know which they were.

The above discussion has to do primarily with a matter of nomenclature. In addition, another species morphologically identical with E. histolytica has been found in sewage. This is E. moshkovskii It has not been found in fresh feces, but nevertheless its existence must be taken into consideration in diagnosis. It is not infective for rats, kittens, guinea pigs or frog or salamander larvae and its optimum temperature is about 24° C, altho it will grow poorly at 37° C.

Before beginning a systematic account of the species of Entamoeba, a word is in order regarding the bovis group. All of these look alike, with minor differences which may not be of taxonomic significance. Different names have been given to the forms in different hosts, but no cross-transmission studies have been attempted, and it is quite likely that when they are, some of these forms will be found to be synonyms. In this case, Entamoeba bovis will have precedence over the other names.

The name Entamoeba polecki has been used for members of the bovis group from the pig and goat, but it is a nomen nudum. Prowazek's (1912) original description and figures of it are so poor that it is impossible to know whether he was dealing with a member of the genus Entamoeba at all.

Noble and Noble (1952) and Hoare (1956) have reviewed the amoebae of domestic animals.