498 



Mechanical Trcuismission of Sleeping Sickness hi/ the Tsetse Fly. 



By Colonel Sir David Bruce, C.B., F.E.S., Army Medical Service ; Captains. 

 A. E. Hamerton, D.S.O., and H. E. Bateman, Eoyal Army Medical 

 Corps ; and Captain F. P. Mackie, Indian Medical Service. (Sleeping 

 Sickness Commission of the Eoyal Society, Uganda, 1908-10.) 



(Eeceived June 22,— Bead June 30, 1910.) 



Up to the beginning of 1909 it was believed that the spread of trypano- 

 sonie diseases, such as Sleeping Sickness and Nagana, was effected by the 

 mechanical transmission of the parasite by the tsetse tiy. The proboscis was- 

 supposed to be contaminated by being dipped in the infected blood, and some 

 of the trypanosomes were pictured as remaining for some time within the- 

 tube and capable of being injected into a fresh animal at the next feed of 

 the Hy. Successful experiments were described which seemed to prove that 

 the tsetse fly was capable of remaining infective for 48 hours, but not longer ; 

 and it was thought that any given Sleeping Sickness area would be free from 

 danger a few days after the infected population had lieen removed from it. 



Dr. Kleine, however, at the end of 1908, sliowed that the tsetse fly 

 remains infective for a much longer period, and that a period of non-infectivity 

 of 20 days or more elapses before this power of passing on the parasite is. 

 gained. In other words, that the Trypanosoma gamhicnse undergoes some 

 process of develo])ment in the liy before it is al)le to infect a fresh animal. It 

 was evident, then, that the mechanical theory liad to be modified. 



It was now held that, in addition to the mechanical method, a mode 

 involving a developmental phase within the fly must be reckoned with. But 

 it was still considered likely that the mechanical was by far the more 

 common mode of infection, and that for every case due to a developmentally- 

 infeoted fly, a hundred would be due to recent contamination. At the same 

 time, it was quite evident, from an examination of the old feeding experi- 

 ments after 8, 12, 24, and 48 liours, that the successful results recorded miglit 

 liave to be credited to late-infectivity rather than to mechanical trans- 

 mission. The experiments lasted so long' that there was enough time for tlie 

 unsuspected late development of the parasite in the ily to liave caused the 

 infection. 



Tlie following experiments were therefore made to put this matter to the 

 jiroof. Tlio flies, after having had their infective feed, were, as a rule, not 

 allowed to feed on a healthy animal for more than 12 days, in tliis way 



