TRYPANOSOMA RHODESIENSE 549 



infected. In no case did an infection result. In an earlier experiment 

 Taute fed infected G. morsitans on animals and on himself. He was 

 immune, but all the animals acquired the disease. These experiments, 

 at any rate, prove that man is not easily inoculated with T. brucei, though 

 they do not conclusively prove that he never can be. Kleine (1923) 

 further maintains that the only means of distinguishing T. brucei 

 from T. rJiodesiense in naturally infected flies and animals is to test the 

 susceptibility of human beings, as was actually done in the experiment 

 mentioned above. If this view is correct, it becomes practically impossible 

 to distinguish them. To the writer, however, it seems that the evidence 

 at present available is in favour of the identity of the human and animal 

 strains. The animal strain (T. brucei) is not readily inoculable to man, 

 but once having gained a footing there, it is more easily passed to other 

 men. In attempting to isolate T. equiperdum from horses, Watson only 

 succeeded in inoculating the trypanosome to a laboratory animal after 

 many hundred failures. Directly this was accomplished, the blood of this 

 animal readily infected other laboratory animals, and even horses, so that 

 the strain was easily maintained. 



It seems probable that T. gamhiense also originated from T. brucei of 

 animals, it may be centuries ago, and that having passed from man to 

 man through many passages, has become modified morphologically 

 (disappearance of posterior nuclear forms) and as regards its virulence 

 for laboratory animals. The human strain of T. brucei, on the other 

 hand, represents the animal strain which has only recently infected man, 

 and, having been subjected to few passages, still maintains its morpho- 

 logical characters and virulence. T. gambiense is sufficiently distinct to be 

 regarded as a species, but T. rhodesiense is merely a strain of T. brucei 

 in man. 



Duke (1921, 1923a) has expressed a similar view, but suggests that 

 T. gambiense and T. brucei are still more nearly related. He points out 

 that his previous investigations (1912c) of the trypanosomes occurring 

 in the Sesse Islands of Victoria Nyanza before the population was removed 

 revealed only T. gambiense in man and a similar trypanosome in the 

 sitatunga. He reinvestigated the subject over ten years after the islands 

 were depopulated. He finds that G. palpalis, no longer having human 

 beings to feed upon, nourishes itself on the sitatunga, which have increased 

 considerably in numbers. The trypanosome now isolated from these 

 animals is of the T. brucei type, and Duke believes that the trypano- 

 somes of the T. gambiense type, which originally were handed on in a 

 mechanical manner from man to man by the flies, have, since the de- 

 population, been handed from sitatunga to sitatunga by the same flies, 

 which have been driven to feed on them exclusively, with a consequent 



