426 
Tsetse Flies and Trypanosomiasis 
Under such conditions we have every reason to expect that strains of 
trypanosomes may sometimes develop in nature through repeated “mechani¬ 
cal” transmission, comparable in their animal reactions to strains developed 
in laboratories. 
In this case we have something more than mere possibility or plausibility 
to go upon—it is a definite probability that search will reveal the existence of 
such strains in nature. Moreover the development of such a strain might well 
be signalled by some “outbreak” or “epidemic” of trypanosomiasis in a 
vertebrate host ordinarily resistant to the normal strain of the parasite, such, 
for example, as the epidemics of virulent human trypanosomiasis which swept 
the Nile basin. Here would be a distinctly plausible explanation for various 
circumstances—for the recovery of the infected islanders—for the curiously 
prompt and complete disappearance of the virulent type of the disease—for 
the sustained virulence of the virulent type, and absence of evidence suggesting 
any gradual change in it—for absence of any irrefutable evidence that the 
virulent type can be transmitted from game or domestic animals to man—for 
the almost complete absence, in fact, of any positive evidence that a “cy¬ 
clically” infected fly ever transmits true “sleeping sickness” to man. 
We must recognise the probability that strains of fly-borne trypanosomes 
of acknowledged virulence may sometimes be developed naturally, and cannot 
disregard the possibility that such a strain actually developed to cause the 
great epidemic of sleeping sickness. It is curious, if not significant, that we 
can find no evidence in negation of what at first appeared a highly improbable 
hypothesis. 
The crew of a native canoe illustrates well the importance of direct trans¬ 
mission in the Uganda epidemic. In these canoes any number from two to 
twenty and more natives might work for hours in close juxtaposition. 
When a canoe is pushed off from a fly infested shore (and the canoe landings 
on Victoria Nyanza were notoriously densely infested), or if it passes close in 
to such a shore, a small—and sometimes a large—swarm of flies will come off 
and follow it to open water. The flies will not come off to a passing canoe more 
than one hundred yards or so from shore, neither will they freely leave a canoe 
that they have followed to a greater distance from shore, but will accompany 
it for hours. Under these conditions more may feed on the crew than if the 
same crew remained on the beach, and it is the commonest sight under these 
conditions to see one paddler after another dislodge a biting fly, which, in the 
course of a few minutes, may have bitten several men. 
Conditions for “mechanical transmission” are absolutely ideal—as favour¬ 
able in proportion to the number of flies involved as could be reproduced in the 
laboratory. It is proved by laboratory experiments that this species of flv 
can transmit this species of trypanosome with a certain facility under such 
conditions; and it is not only possible or plausible, but virtually certain that 
mechanical transmission in canoes must have occurred with frequency and 
regularity during the early stages of the epidemic. 
