In return cases, or prolonged untreated cases of entamœbic dysentery, a generation of smaller trophozoites is associated with, or replaces the larger ones. In stools they are frequently refractile and consequently stain slowly intra vitam. These trophozoites are the “smaller, senile, or pre-cyst generation” of Darling. This pre-cyst generation is characterized by the presence of blocks of crystalloidal substance in the cytoplasm, and by the possession of a prominent, densely stainable karyosome. Darling believes this generation to be the same as that described by Elmassian as Entamœba minuta.[22]
Walker,[23] Darling,[24] Wenyon[25] and others believe that Entamœba histolytica, which was only seen by Schaudinn in a single case, that of a Chinaman, is really E. tetragena. Darling states that if the published illustrations of E. histolytica and of E. tetragena are collected from the literature and compared, it will be seen that the writers have been calling E. histolytica the large trophozoites seen in dysenteric stools. These large trophozoites frequently display no karyosome, but they can be demonstrated as E. tetragena by animal inoculation, or by the history of the case. On the other hand, the illustrations of E. tetragena show that the authors have been dealing with the small generation or reduced forms (“E. minuta”), which are the direct descendants of the large trophozoites. If kittens are inoculated rectally with dysenteric material containing large trophozoites, the strain may be carried in successive kittens for four to six transfers. If, on the other hand, kittens are inoculated rectally with small trophozoites of the pre-cyst generation, the transmission cannot be carried through more than one or two kittens. Wenyon has succeeded in maintaining E. tetragena in kittens for several generations.
In some of the preparations from the last remove, pathological forms of the trophozoites may be seen. These show abnormal forms of budding, especially peripherally, such as have been described by Schaudinn and by Craig as characteristic of E. histolytica. Schaudinn’s small peripheral, exogenous buds and cysts are thus explained. Craig has latterly changed his views.
Further, Darling states that tetragena cysts fed by the mouth to kittens produce bowel lesions in which trophozoites having the characters of E. tetragena, E. histolytica and E. nipponica (Koidzumi) occur.
In view of the work of recent observers, the peculiar exogenous encystment which Schaudinn made characteristic of Entamœba histolytica has been shown to be due to degenerative changes in senile races of the amœba. E. histolytica and E. tetragena are one and the same species, and its trophozoite is subject to variation. According to some observers the histolytica type of nucleus—described by Schaudinn as being poor in chromatin and not easily seen in the fresh state—occurs frequently in patients with severe symptoms of dysentery; on the other hand, the tetragena type of nucleus—round and easily seen in the fresh state—may occur in cases presenting slight dysenteric symptoms. Intermediate types of nuclei are seen. The name of this species, the principal pathogenic amœba of man, must then be E. histolytica by priority. The cystic stages of E. histolytica are those first recorded by Viereck and formerly described as E. tetragena. The geographical distribution of E. histolytica is wide.
Noc’s Entamœba (1909).
A species of Entamœba was cultivated by Noc[26] in 1909 from cysts derived from liver abscesses, from dysenteric stools and from the water supply of Saigon, Cochin China. He cultivated it in association with bacteria. It is pathogenic. It has been considered allied to E. histolytica, and shows internal segmentation or schizogony. It exhibits polymorphism. This amœba has been found by Greig and Wells (1911) in cases of dysentery in India. It is an important organism and requires further investigation.
Certain other Entamœbæ[27] have been described at various times from the intestinal tract of man. Probably most, if not all, of these are not good species and in some cases much more information is needed.
Entamœba tropicalis (Lesage, 1908). This parasite is said to be non-pathogenic, and to occur in the intestine of man in the tropics. It has a general resemblance to E. coli, but forms small cysts (6 µ to 10 µ in diameter). The nucleus of the cyst is said to break up into a variable number of daughter nuclei, from three to thirteen having been noted. Lesage states that it is culturable in symbiosis with bacteria. It is probably a variety of E. coli, if not a cultural amœba.
Entamœba hominis (Walker, 1908) has a diameter of 6 µ to 15 µ. A contractile vacuole is present. Encystment is total, and small cysts are formed. It is culturable. The original strain, now lost, was obtained from an autopsy in Boston Hospital. This organism is probably a cultural amœba.