5G 



noted, whilst at Bamako and Agouagon G. palpalis is naturally 

 infected in a very higli degree (1 in 30 to 1 in 50)! Observations 

 on T. gamhiense have produced a similar result in Dahomey and 

 in Casamance; 1,200 G. palpalis were examined and not one was 

 found to be infected. In Dahomey human trypanosomiasis is 

 unknown and it is not of frequent occurrence in Casamance. 



Kleine in East Africa and Bruce and his co-workers in Uganda 

 have obtained 5 per cent, positive results with considerable ease 

 from a very much smaller number of Glossina. Kinghorn and 

 Yorke have similarly found great differences in Rhodesia. Kleine 

 and Fischer obtained 2 positive results with 1,400 Hies on Lake 

 Victoria, and Taute found 4 per cent, of positive results on the 

 shores of Tanganyika. It would thus appear clear that the 

 I'eceptivity of a given species of Glossina for a given virus is not 

 constant throughout the whole area inhabited by that species. 

 Geographical distribution and receptivity of different species 

 have no necessary connection. Most writers consider, that climatic 

 factors exercise an influence on the development of the virus 

 in Glossina. Roubaud is however of opinion that this influence 

 is only indirect, prpducing in the flies physiological modifica- 

 tions in their salivary apparatus, rendering it fit or otherwise 

 for the development of the parasite. Thus in Dahomey he 

 has shown that under identical climatic conditions some species 

 will develop the virus and others will not, and this limitation 

 of receptivity on the part of the insect naturally limits the 

 distribution of the disease. The migrations of the flies, which 

 Eoubaud has shown to be very important, especially at the 

 commencement of the winter season, produce constant inter- 

 changes of Glossina between zones of receptivity and zones of 

 non-receptivity. The virus thus transported by infected flies 

 outside their normal zone will be diffused in the neighbouring- 

 zone and in the author's opinion this explains the existence of 

 sporadic cases of trypanosomiasis in certain regions where, in 

 spite of the great numbers of Glossina and the abundance of 

 game, the existence of the virus is restricted to human beings 

 and domesticated animals.. In these areas, which the author calls 

 zones of dift'usion, he considers that the infected flies come from 

 surrounding infected districts. Within these zones not only is the 

 absolute number of cases small, but the cases themselves are fre- 

 quently observed to be less severe, and these conditions Eoubaud 

 thinks may possibly lead to the disinfection of the flies in course of 

 time, in the same way as has been observed in the laboratory. 

 It thus, results that the mere presence of large numbers of 

 Glossina in any given district does not a. priori condemn the area 

 as uninhabitable for man or domesticated animals, and the 

 introduction of the poison into a tsetse-belt where it did not 

 previously exist is not necessarily a source of danger; that is to 

 say, the tsetse in that zone may not be receptive, and so long as 

 the local biological conditions remain undisturbed, these flies 

 will remain uuinfected. 



Every attempt at the suppression of Glossina must be governed 

 by a proper consideration of these facts ; that is to say, it is 

 against the belts of receptivity that action should be directed, 

 and, in order that the work of cutting down brushwood and 



