214 Arthropods as Essential Hosts of Pathogenic Protozoa 



doubtftil 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 Muscidffi 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 hrucei. 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 Trypanosof-na 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 carrier, 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 carrier. Bruce inclined to the latter view. He was 

 unable to flnd living trypanosomes in the intestines or excrements 

 of the fly or to produce the disease on the many occasions when he 



