THE PRESIDENTIAL ADDRESS. 15 
by the American Army Commission, under the directorship of Reed, and 
in 1903 Leishman announced his discovery of the protozoal cause of kala- 
azar. 
These protozoal diseases are world-wide, like the bacterial, but it is 
in the warmer climates that their effect is most felt. 
The great plagues of the tropics, such as malaria, amcebic dysentery, 
kala-azar, and sleeping sickness among men, Texas fever, tsetse-fly disease, 
and others among domestic animals, are caused by minute microscopical 
animal parasites. 
Large tracts of country have been and are still rendered uninhabitable 
to white settlers by their presence. 
The opening up of Africa, for example, was rendered difficult by the 
tsetse-fly, before the advent of railways. No sooner had an expedition 
started for the interior than the fly attacked the cattle transport, and 
before long the expedition had to make its way back as best it could to 
its base on the coast. The only way to get into the country was on foot 
with native porters. 
The protozoal diseases of domestic animals have also led to enormous 
loss in all parts of the world. Texas fever, or red-water, has swept 
whole countries of their cattle. After the Boer war, South Africa was 
devastated by the introduction of East Coast fever, another protozoal 
disease of cattle closely related to Texas fever. 
How is the prevention of these diseases to be brought about! We 
find that up to the present little can be done by way of vaccination or 
inoculation or by the use of anti-sera as in the bacterial diseases. On 
studying the natural history of these protozoal parasites, however, it is 
found that many of them depend on an intermediate insect host for their 
continued existence, and it is by taking advantage of this characteristic 
that methods of prevention can be devised. 
To illustrate this, I might cite the classical examples of malaria and 
yellow fever, but, as these must be familiar to you all, I shall take instead 
the trypanosome diseases of Africa, the best known of which are sleeping 
sickness in man and nagana or tsetse-fly disease in the domestic animals. 
Nagana or Tsetse-fly Disease. 
In 1894, a year after Theobald Smith and Kilborne had published 
their famous monograph on Texas fever, a severe epidemic among native 
cattle in the north of Zululand was reported to the Natal Government. 
The disease was called nagana by the natives, and it is curious that there 
