The Animal Parasites of Man. Max Braun

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      Fig. 8.—Entamœba histolytica. a, trophozoite (tetragena type) containing red blood corpuscles, × 1,300; b and c, two isolated nuclei showing different appearances of karyosome, centriole and nuclear membrane, × 2,600. (After Hartmann.)

      (Entamœba tetragena, Viereck, 1907.)

      This amœba must now be considered to be a part of the lifecycle of Entamœba histolytica, in fact a very important part of that cycle, especially in its tetranucleate cystic stages.

      This organism, the so-called Entamœba tetragena, may occur in the human intestine in cases of amœbic dysentery, especially in mild or chronic cases. It was discovered by Viereck in 1907 in patients suffering from dysentery contracted in Africa. Soon afterwards an independent description was published by Hartmann, who called the amœba E. africana. It was also studied by Bensen and Werner. Recently (1912–13) much work has been published on this amœba by Darling and others; in this way its relationship to Schaudinn’s E. histolytica has been made known.

In general morphology it somewhat resembles Entamœba coli, and its discoverer at first mistook it for a variety of that species. According to Hartmann, a distinct ectoplasm is only clearly visible when a pseudopodium is protruded (fig. 5). The granular endoplasm may contain ingested red blood corpuscles (fig. 6). The large, round nucleus is visible in the fresh state (fig. 8, a). So-called chromidial masses (? crystalloidal substances) may occur in the cytoplasm.

      Fig. 9.—Entamœba histolytica (tetragena form). a, emission of chromatin from nucleus; b, nuclear division; c, degenerating form with two nuclei; d, e, f, cysts containing one, two and four nuclei respectively, and showing chromidial blocks. × 2,000. (After Hartmann.)

      Some investigators, as Hartmann,19 lay stress on the internal structure of the nucleus (fig. 8, b, c), best seen in preparations fixed wet and stained with iron-hæmatoxylin. The nucleus is limited by a well-marked nuclear membrane, on the inside of which granules or nodules of chromatin may occur. There is a karyosome, which, in successfully stained specimens, shows, at times, a central dot called a centriole. (The nucleus of Entamœba coli does not contain such a centriole.) However, the structure of the nucleus varies at different periods during the life-cycle.

      The diameter of the trophozoites or vegetative forms (fig. 8, a) is variously given as from 20 µ to 40 µ. Multiplication proceeds by binary fission and also by schizogony into four merozoites.20

Reproduction takes place by endogenous encystment (fig. 9, d-f), which is preceded by nuclear division into two, reduction and then autogamy. The interpretation of the latter phenomenon as autogamy is disputed by some authors. The round cysts, which may measure 12 µ to 15 µ in diameter, contain four nuclei, together with darkly staining masses of various shapes, the so-called “chromidial blocks” (fig. 9, f). The cyst-wall of E. histolytica (tetragena) is thinner than that of E. coli, and the diameter of the cyst is rather less. E. histolytica has not yet been cultivated.

      Infection in man occurs by way of the mouth by the ingestion of cysts. A patient showing acute symptoms of dysentery is not usually infective, for he is merely harbouring the large trophozoites, which, by experiment, have been shown not to be infective to animals (kittens) when administered by the mouth. The stools of recovered patients may still contain cysts, and they may thus act as cyst-carriers or reservoirs of disease by infecting water and soil. The stools of such cyst-carriers are often solid, and so cysts of E. histolytica (tetragena) are easily overlooked. Mathis (1913)21 points out that healthy carriers of E. histolytica may be found; 8 per cent. of the natives of Tonkin examined by him were healthy carriers of cysts.

      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 Wenyon25 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 СКАЧАТЬ