The trypanosomes of birds probably develop in mosquitoes; those of fishes and turtles in leeches.
Two methods of transmission of trypanosomes must be distinguished. The one, a mechanical method, can be performed by any blood-sucking insect for a few hours after a meal of infected blood; the other, indirect or cyclical, is a developmental cycle, and can only take place in the particular definitive host—that is, in most instances, some species of glossina—to which the particular species of trypanosome has become adapted.
Fig. 234.
T. gambiense
The development of T. gambiense (Fig. 234) in G. palpalis, thanks to the researches of Kleine, Taute, Bruce and his collaborators, is now fairly completely known. Apparently the whole development takes eighteen to twenty-five days or more. Five to seven days after ingestion of infected blood the trypanosomes become scarce in the digestive tract of the fly, only to reappear in large numbers and in a variety of crithidial and other forms. Finally an invasion of the salivary glands by short, stumpy, trypaniform individuals takes place. In T. vivax (cazalboui) a completely different mode of development in glossina takes place, a method termed by Roubaud évolution par fixation directe; this occurs solely in the proboscis of Glossina palpalis. The trypanosomes assume a leptomonas shape and attach themselves by their flagellum to the labium or hypopharynx of the proboscis tube, multiplying in the salivary fluid; these flies remain infective for life. A third method of development occurs in the case of T. dimorphon—the évolution par fixation indirecte of Roubaud. In this case the trypanosomes multiply first in the digestive tract of the glossina, and then pass forwards into the proboscis.
In T. cruzi the development both in the intermediary and in the definitive host is of so different a character that Chagas has relegated it to a special genus, Schizotrypanum. In man the adult trypanosomes multiply not in the peripheral circulation but in the internal organs of the body. In the capillaries of the lungs the adult trypanosome loses its flagellum and occasionally the kinetonucleus as well, the body becomes round, and the mass breaks up into eight small individuals (merozoites). Some regard these merozoites as exhibiting a sexual differentiation. The merozoites enter the red blood-cells, and so gain the general circulation. Within the corpuscles they form into a normal trypanosome, which, when set free, either repeats the schizogonic cycle or enters into the invertebrate host, in which it develops still further.
A second type, and probably the normal method of multiplication in man, takes place within the hypertrophied endothelial cells of the lung and striped muscles, notably the cardiac muscle, but can apparently occur in any tissue.
