32G DR. W. YORKE ON 



enormously in numbers, and with this increase in game there has 

 been a corresponding increase in the number of tsetse fly. At 

 Nawalia, in the Luangwa Yalley, where we were stationed, 

 Glossina tnorsitans was present in enormous numbers, and natives 

 sent out to collect the flies had no difficulty in capturing large 

 numbers within a short distance of the laboratory. Nawalia is 

 the site of an old Government station which was closed a few 

 years ago on account of Sleeping Sickness. The magistrate who 

 was stationed there in 1905 told me that he only occasionally saw 

 tsetse flies in this district at that time. 



Again, it has been suggested that the big game might be only 

 one of the reservoirs of the disease, and that the infection might 

 equally well be conveyed by the small vermin. It must be re- 

 membered, however, that the small vermin axe to a considerable 

 extent nocturnal in their habits, and although Glossina morsitans 

 does occasionally bite at night, especially when the moon is full, 

 yet nobody who has lived in " fly " areas can have any doubt but 

 that this is exceptional, and that for practical purposes the fly feeds 

 in the daytime only. Dr. Kinghorn and I examined a large 

 number of small vermin— rats, mice, wild rabbits, etc. — without 

 finding a single instance of natural infection. Furthermore, it 

 might be remarked that there is no evidence to show that the 

 small vermin are tolerant of the human trypanosome as are the 

 big game. In those which we infected experimentally the disease 

 ran an acute course and the animals died. If this be the case 

 with the majoiity of the small vermin, they cannot have the same 

 sio-nificance as reservoirs of the virus as have the big game, which 

 can probably harbour the parasite for long periods of time with- 

 out exhibiting signs of disease. 



We return, therefore, to the original position. The big game 

 is the natural reservoir of the infection, and the role of the tsetse 

 fly, Glossina morsitans, is to transfer the virus from the big game 

 to man and his flocks and herds. At the present state of our 

 knowledge we are unable to attack successfully the tsetse fly, nor, 

 unfortunately, is there any prospect of our being able to do so in 

 the near future. Whether anything would be gained in this 

 direction by slaughtering the big game is still a moot point : there- 

 fore I will not consider this side of the question, but advocate 

 the advisability of attempting to drive back the game from 

 inhabited regions solely because the game are the reservoir of 

 the infection. 



It has been asserted that the power to slaughter all game 

 animals in an infected district is unsound in principle, because 

 the game, when harried, would betake itself to places difficult of 

 access to man, or scatter in small herds, or in pairs, or singly, 

 over wide areas, and that should this occur, it is highly probable 

 that it might be followed by tsetse, thus spreading the danger of 

 infection to wide areas now free from game and fly. 



To such criticism as this it is not difficult to reply. If the 

 game when harassed betook itself to places difficult of access to 



