214 Arthropods as Essential Hosts of Pathogenic Protozoa 
doubtful that this flagellate is a parasite of the invertebrate “host” 
in the sense claimed by Prowazek and other investigators. 
Tsetse-flies and Nagana —One of the greatest factors in retarding 
the development of certain regions of Africa has been the presence 
of a small fly, little larger than the common house-fly. This is the 
tsetse-fly, Glossina morsitans (fig. 165) renowned on account of the 
supposed virulence of its bite for cattle, horses and other domestic 
mammals. 
The technical characteristics of the tsetse-flies, or Glossinas, and 
their several species, will be found in a later chapter. We need 
emphasize only that they are blood-sucking Muscidse and that, 
unlike the mosquitoes, the sexes resemble each other closely in struc¬ 
ture of the mouth-parts, and in feeding habits. 
In 1894, Colonel David Bruce discovered that the fly was not in 
itself poisonous but that the deadly effect of its bite was due to the 
fact that it transmitted a highly pathogenic blood parasite, Trypano¬ 
soma brucei. This trypanosome Bruce had discovered in the blood 
of South African cattle suffering from a highly fatal disease known as 
“nagana”. On inoculating the blood of infected cattle into horses 
and dogs he produced the disease and found the blood teeming with 
the causative organism. In the course of his work he established 
beyond question that the “nagana” and the tsetse-fly disease were 
identical. 
Tsetse-flies of the species Glossina morsitans, which fed upon 
diseased animals, were found capable of giving rise to the disease 
in healthy animals up 'to forty-eight hours after feeding. Wild 
tsetse-flies taken from an infected region to a region where they did 
not normally occur were able to transmit the disease to healthy 
animals. It was found that many of the wild animals in the tsetse- 
fly regions harbored Trypanosoma brucei in their blood, though they 
showed no evidence of disease. As in the case of natives of malarial 
districts, these animals acted as reservoirs of the parasite. Non- 
immune animals subjected to the attacks of the insect earner, quickly 
succumbed to the disease. 
A question of prime importance is as to whether the insect serves 
as an essential host of the pathogenic protozoan or whether it is a 
mere mechanical earner. Bruce inclined to the latter view. He was 
unable to find living trypanosomes in the intestines or excrements 
of the fly or to produce the disease on the many occasions when he 
