2S 
THIRTEENTH REPORT. 
one of man, and was conveyed by the fly wholly from man to man. then 
it might be possible to eradicate the disease by removing the natives 
from the infected territory for a sufficient length of time, that is until 
the infected Hies had died off. Segregation has been practiced on a large 
scale in Uganda, but the studies of Bruce and his co-workers have shown 
that the flies caught in places which had not been visited by the natives 
for two years were still infective. This fact made it quite clear that 
the trypanosome of sleeping sickness is really a parasite of some wild 
animal. In other words the wild animals serve as the natural reservoir 
for the virus. Experiments on antelopes, by Bruce and his co-workers, 
have shown that these animals can be infected by the tsetse-fly and. 
that in turn the antelope, though its blood shows no parasite recognizable 
by direct examination, nevertheless, is capable of infecting clean tsetse- 
flies. It would seem, therefore, that the antelope or some similar wild 
animal was the natural carrier of the Tr. gambicnse. A reasonable proof 
in support of this view will be furnished when the parasite is actually 
found in the free roaming wild animals. Koch's belief that the crocodile 
was such a reservoir has in nowise been confirmed. 
The fact that the tsetse-fly does transmit the disease was soon estab- 
lished but it was not clear as to just how this was effected. It was at 
first believed that the flies were mere passive carriers, and that they car- 
ried the parasite directly from the infected animal or man to man. In 
other words, they were supposed to act like the needle of a syringe, in a 
purely mechanical way. If this were so, the flies would become cleansed 
after a few hours, or at most in a day or two. The early observations of 
Bruce (1895) on the tsetse-fly (Glossina morsitans) which transmit a 
horse disease (nagana) indicated that the fly was a mechanical carrier. 
On the other hand, if the ingested trypanosomes underwent develop- 
ment and multiplication in the gut, the fly itself would become infected 
or diseased and would therefore become an active carrier. As such, it 
would be able to transmit the disease weeks and even months after the 
infective feed. 
Independent observations made by the British Sleeping Sickness Com- 
mission and by Koch (1900) revealed the presence of flagellates in thc- 
gut of a large number of captured flies. These flagellates were at once 
supposed to be developmental forms of Tr. gambiense but this was soon 
shown, by Minehin and by the author, not to be the case. Of the four 
types of flagellates present in the gut of the flies, to which I called at- 
tention, only one seemed to have a possible relation to Tr. gambiense. 
The other types were undoubtedly either natural parasites of the fly or 
were derived from ingested trypanosomes. The most common type, 
designated by me as Tr. Gragi, was believed by Koch to be of crocodile 
origin while Minehin considered it to be derived from avian trypano- 
somes. 
The question as to whether the tsetse fly could act as an active ear- 
lier was finally, settled by Kleine (1909) who demonstrated that flies 
which had fed upon blood infected with Tr. bmeei , at ter a nou-infective 
period of 20 days could transmit the disease. This important fact was 
soon confirmed by Bruce and his co-workers, as well as by other in- 
vestigators, and constitutes a distinct achievement. While it is possible 
for the fly to transmit the disease from animal to animal by interrupted 
feeding it is now certain that this is not the common, natural way. 
