276 Trypanosomiasis 
take place directly from vertebrate to vertebrate through the contact of 
abraded or even healthy mucous membranes and skin. Although much 
has been written regarding a supposed sexual phase in the life-cycle of 
trypanosomes, we are still in the dark as to its occurrence; the little 
evidence there is points, however, to its possible occurrence in the 
invertebrate hosts which serve as vectors, so that, at any rate pro¬ 
visionally, these may be regarded as the definitive hosts of the parasite. 
Infection may be brought about under experimental conditions by the 
inoculation of blood containing trypanosomes, or by applying blood to 
abraded or intact mucous membrane and skin. In nature, the infection 
of dogs by feeding on animals dead of Surra (due to T. evansi) has 
frequently been observed, and similar results have been obtained 
experimentally. Dourine in horses is commonly communicated in 
nature in the act of coitus, and Koch suspected that the trypanosome 
of sleeping sickness (T. gambiense) might be similarly communicable 
in man, a supposition which has since been strengthened by laboratory 
experience. In one instance, there is evidence that non-biting insects,— 
Musca dornestica, —may serve as vectors; I refer to the equine disease 
“ murrina ” at Panama, due to T. hippicum, which is not apparently 
transmitted unless the animals show wounds of the skin upon which 
flies may alight and thus convey the parasites directly from host to 
host. In this case, also, it is possible that infection may take place by 
coitus. 
Judging from earlier evidence which proved that different species of 
trypanosomes could be readily communicated from animal to animal by 
inoculation, in some cases by the transference of minute quantities of 
blood, it was generally supposed that the parasites, under natural 
conditions, were communicated by biting flies in a purely mechanical 
manner. Experimental evidence on this point appeared, moreover, to 
bear out this supposition, for in a number of instances trypanosomes 
were transmitted from diseased to healthy animals by removing a fly 
from an infected animal upon which it was feeding and, soon after, 
transferring it to a healthy animal upon which it was allowed to 
complete its meal. For a long time the classical experiments of Bruce, 
in Zululand, were accepted as evidence of the mechanical transference of 
T. hrucei by Glossina morsitans, he having found that infected flies 
captured in nature did not infect healthy horses if the flies were 
prevented from feeding upon them for 24 hours or more after they had 
been captured. Although some authors, notably in India, still regard 
mechanical transference by biting flies (Stomoxys, Tabanus, Gidicidae) 
