48 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



came into his mind that the trypanosome of sleeping sickness might 

 similarly be transmitted by a Tsetse or other blood-sucking fly. Blood- 

 sucking flies were collected from all over the region of Africa under 

 investigation and the localities in which each species occurred were 

 plotted out on a map. When the distribution of the various species 

 of biting flies was gone into, it was found that the distribution 

 of one species Glossina palpalis a close ally of the Tsetse corre- 

 sponded practically exactly with the distribution of cases of sleeping 

 sickness. This was strong presumptive evidence of the correctness of 

 Bruce's suspicion that sleeping sickness was a kind of human " Tsetse-fly 

 disease." 



Experiment soon demonstrated the truth of this. It was found that 

 a Glossina palpalis which made a meal of infected blood was able to 

 inoculate a monkey with the disease if allowed to bite it within a period 

 of a few days. Thus, as in the case of G. morsitans with T. brucii, the 

 possibility of direct infection was proved. Just as in the case of Nagana, 

 however, further investigation showed direct infection to be of relatively 

 minor importance. After a period of 20-30 days the fly was found in 

 a certain small percentage of cases to recover its infectivity, the 

 infectivity now lasting for several months if not for the whole life 

 of the fly. This prolonged cyclical infectivity of the fly is associated 

 with the trypanosome undergoing in its alimentary canal the various 

 changes described on p. 44. Our knowledge of these changes is almost 

 entirely due to the work of Miss Muriel Robertson. 



It seems fairly clear that the cyclical transmission by G. palpalis is 

 that which is of real practical importance in spreading epidemics of 

 sleeping sickness. Direct transmission by the insect no doubt occurs 

 occasionally but probably much more rarely. It must not be forgotten 

 that T. equiperdum is conveyed by sexual contact and it is at least a 

 possibility that this may happen occasionally also in the case of sleeping 

 sickness a possibility the less to be ignored in view of experiments 

 (Hindle) which have shown that T. ga&biense is able to make its way 

 through thin skin. 



For purposes of combating the spread of sleeping sickness three 

 methods at once suggest themselves : 



(1) The segregation of sleeping-sickness patients within fly-proof 

 houses so as to prevent new flies from becoming infected. This method 

 is unfortunately made ineffective by the occurrence of T. gambiense 

 as a natural parasite of the Sitatunga antelope (Limnotragus] which 

 consequently acts as a persistent reservoir of the trypanosome. 



(2) Destruction of the wild antelopes in the neighbourhood of human 



