322 DU. W. YORKE ON 



causing Sleeping Sickness in other portions of Tropical Africa. 

 The name Trypanosoma rhoclesiense was given to this new- 

 parasite. 



Since these discoveries many cases of Sleeping Sickness have 

 been found in Rhodesia and JSTyasaland, and in 191 1 the Chartered 

 Company decided to have the matter thoroughly investigated ; it 

 was with this object that Dr. Kinghorn and I were sent to North 

 Eastern Rhodesia. 



The first problem that we had to solve was to ascertain the 

 vector responsible for the spread of Sleeping Sickness in a country 

 where Glossina palpalis does not exist. Now although Glossina 

 jyaljxdis has not been found either in Nyasaland or in the 

 Luangwa Valley of Rhodesia, yet Glossina 7norsitans, the tsetse 

 fly which is known to cause " fly " disease in domestic stock, is 

 present in enormous numbers, and it was soon proved by Dr. 

 Kinghorn and myself that it is this fly which is i^esponsible for the 

 spread of human trypanosomiasis. This discovery is one of great 

 practical significauce, for whereas the former fly, Glossina paljmlis, 

 is limited in its distribution to water-courses, and is never found 

 far from the banks of certain rivers or the lake shores, the latter, 

 Glossina morsitans, is ubiquitous, its distribution being quite 

 independent of water. Hence, it is at once obvious that it is 

 impossible to attempt to deal with Sleeping Sickness in Rhodesia 

 and Nyasaland by any such simple method as removing the 

 native population back from the water-courses and lake shores — 

 a procedure which was attended with such remarkable results in 

 Uganda. The problem of preventing the spread of Sleeping 

 Sickness in these countries, which a few yeai-s ago were thought 

 to be in no danger, is one of infinitely gi^eater difficulty than was 

 that which had to be faced in Uganda. 



A large number of wild Glossina inorsitans was examined 

 in the Luangwa Valley in order to ascertain the proportion 

 capable of infecting man with trypanosomiasis. This information 

 is important, as it afi"ords an approximate idea of the potential 

 danger of the district. We found that 1 in 500 wild Glossina 

 inorsitans was infective in nature. This is an astonishingly 

 large proportion, and it is at once evident that some host other 

 than man must be infected with the human trypanosome in order 

 to account for the large number of natui'ally infective Glossina 

 morsitans. "With the object of ascertaining what was the chief 

 vertebrate reservoir of the virus, we examined a large number of 

 the wild fauna of Africa. In all, we examined 250 wild animals 

 (including elephant, rhinoceros, hippopotamus, lion, buffalo, 14 

 different kinds of antelope, caracal, galago, squirrel, genet, hunting 

 dog, giant rat, and wild rabbit), 256 monkeys, 35 domestic stock, 

 142 wild rats, and 15 wild mice, making a total of 698. The 

 results were striking, a large proportion of the antelope being 

 found to be infected with the parasites which cause Sleeping 

 Sickness in man and trypanosomiasis in domestic stock. As a 

 conservative estimate, the percentage of big game infected with 



