H. L. Duke 
423 
PART II. 
VIII. THE TSETSE FLY AS A TRANSMITTER OF HUMAN TRYPANOSOMES. 
There are two ways in which a human trypanosome may be transmitted to 
a man by fly: * 
(1) By the direct method. 
(2) By cyclical transmission. 
(1) Direct or “mechanical" transmission of T. gamhiense by G. palpalis 
has been proved to occur. It is known, both from field observation and 
laboratory experiment, that an interruption before it has finished its feed 
makes G. palpalis doubly eager to resume and complete its meal. In the case 
of man there is, so far as is known, no other biting fly whose mouthparts are 
adapted to the transmission of trypanosomes which can compare with the 
Glossinae as a disseminator of the disease by the direct method. 
Conditions ideal for direct transmission of trypanosomes existed in the 
closely packed canoes of Victoria Nyanza. 
(2) Cyclical transmission. Here the trypanosome undergoes a cycle of 
development in the fly. During the earlier stages of this cycle the insect is 
incapable of conveying the parasite to susceptible hosts upon which it feeds. 
A condition is finally produced when the parasite is established in the salivary 
glands of the fly which is henceforward infective, as far as known, until its 
death. 
There is a large amount of evidence to show that in only a small percentage 
of Glossina palpalis is the trypanosome capable of undergoing cyclical develop¬ 
ment. The highest percentage of positive flies that we have obtained in any 
laboratory feeding experiment was 20-6(9). Miss Robertson(10) gives 3-4 as the 
average percentage of positive flies given with the Uganda strains of what is 
generally presumed to be T. gamhiense in laboratory bred flies. Frequency 
of the infecting feedings appears to make no difference in the resulting per¬ 
centage of positive flies. In the above percentage calculations any fly showing 
multiplication of trypanosomes in its gut is classed as positive. A large pro¬ 
portion of these, however, died before the invasion of the salivary glands with 
infecting forms had occurred. 
In the course of the investigations of the Commissions of the Royal Society 
a large number of experiments have been carried out to determine the per¬ 
centage of wild flies carrying the polymorphic trypanosome, which appears 
from laboratory evidence identical with the organism which caused the 
epidemic. Experiments conducted with wild flies caught near Entebbe in 
1903, where the natives were considered to be infected with trypanosomes in 
the proportion of 1 in every 3 or 4, revealed a percentage of 0-3 positive flies; 
0-1 per cent, of positive flies resulted from experiments performed in May, 1911, 
with wild flies from Damba Island, in the Sesse group. 
