Crithidia—with a less well-developed undulating membrane anteriorly (see fig. [49]).
Herpetomonas—including the so-called Leptomonas, with anterior free flagellum only, and no undulating membrane.
Leishmania—non-flagellate forms in mammalian blood, flagellate herpetomonad stages in culture, probably occurring naturally in Arthropods.
Genus. Trypanosoma, Gruby, 1843.
The members of the genus possess a single flagellum, which arises posteriorly, adjacent to a blepharoplast or kinetic nucleus. The flagellum forms a margin to an undulating membrane, and may or may not be continued beyond the body as a free flagellum. Many species are parasitic in vertebrate blood and in the digestive tracts of insects.
Historical.
The history of blood flagellates goes back to the year 1841, in which Valentin discovered in the blood of a brook-trout (Salmo fario L.) minute bodies, from 7 µ to 13 µ in length, with active movements and presenting marked changes in form. Valentin considered the parasite a new species of the old genus Proteus or Amœba, Ehrbg. This announcement led Gluge (1842) to publish a similar discovery he had made in frog’s blood. The latter forms were called by Mayer (1843) Amœba rotatoria, Paramœcium loricatum and P. costatum, while Gruby (1843) called them Trypanosoma sanguinis.[53] Later it was discovered that similar organisms occurred also in the blood of birds (Wedl (1850), Danilewsky) and of mammals. Gros (1845) found them in the mouse and mole, Chaussat (1850) in the house rat, Lewis (1879) in the Indian rat, Wittich (1881) in the hamster. Danilewsky (1886–89) and Chalachnikow (1888) investigated the structure and division of trypanosomes.
In the case of all these forms, there was no discussion as to a pathogenic influence on the host. Opinion, however, as to the action of trypanosomes changed when, in 1880, Evans found flagellates in the blood of horses in India that suffered from a disease endemic there called “surra,” and associated the parasites with the disease. Steel and Evans were successful in transmitting the parasites—first known as Spirochæta evansi, Steel, then as Trichomonas evansi, Crookshank, and finally as Trypanosoma evansi—to dogs, mules and horses. They recognized that the above mentioned flagellates in the blood of the experimental animals were the causal agents of the disease.
From that time there was a considerable increase in the literature, the contents of which have been summarized by Laveran and Blanchard. In 1894 Rouget discovered trypanosomes in the blood of African horses that suffer from “stallion’s disease” (dourine). In 1894 Bruce found similar forms (T. brucei) in the blood of South African mammals suffering from “nagana,” and in consequence attention was drawn to the part which the much dreaded tsetse-fly played in the transmission of “nagana.” In 1901 Elmassian discovered trypanosomes in the blood of horses that were stricken with “mal de caderas,” which is very common in the Argentine. The disease in cattle named “galziekte” (gall-sickness), occurring in the Transvaal, was also at one time attributed to a trypanosome remarkable for its great size, and like some other species, bearing the name of its discoverer (T. theileri).
The study of the species hitherto known has been carried on partly by the above mentioned authors and in part by others, e.g., Rabinowitsch and Kempner, Laveran and Mesnil, Wasiliewski, Senn. It was greatly advanced by the method of double staining (with alkaline methylene blue and eosin) introduced by Romanowsky (1891) and elaborated by Ziemann, Leishman, Giemsa and others. By this means the presence of a terminal flagellum and of an undulating membrane at the side of the flattened and extended body was demonstrated. Laveran and Mesnil (1901) discovered allied flagellates in the blood of the fish, Scardinius erythrophthalmus. These flagellates, now placed in the genus Trypanoplasma, had a second free flagellum in addition to the one bordering the undulating membrane. Trypanoplasms have since been found in both freshwater and marine fishes. The transmission of trypanoplasms of freshwater fishes is effected by leeches. Trypanoplasma varium from Cobitis is transmitted by Hemiclepsis marginata according to Léger, while the Trypanoplasmata of Cyprinus carpio and Abramis brama reach new hosts by the agency of Piscicola according to Keysselitz.