NAG ANA OR TSE-TSE FLY DISEASE 619 



some, a poison natural to the insect which was the pathogenic 

 agent. But if such a fly was allowed to bite a dog suffering 

 from the disease and then to bite a healthy dog, the latter 

 contracted the malady and abundant trypanosomes were found 

 in its blood. Again, threads dipped in the blood of an infected 

 animal and allowed to dry caused the disease in healthy animals 

 up to, but rarely beyond, twenty-four hours after being dried ; if, 

 however, the blood were kept moist, then it retained its infective- 

 ness up to between four and seven days ; up to forty-six hours 

 living trypanosomes could be seen in the tube of the fly's proboscis. 

 This corresponds roughly with what was found regarding the 

 limits of the infectiveness of the fly, in that twenty-four hours 

 after it has been fed on an infected animal its bite is usually in- 

 nocuous. 1 Further, Bruce showed that infection did not occur by 

 any food or water partaken of by an animal while going through a 

 fly belt, for he took horses through such a region without allowing 

 them to eat or drink, and found that they still contracted the 

 infection, if during their few hours' journey through the belt 

 they had been bitten by the tse-tse fly. Finally, he showed 

 that if flies were taken from an infected area to a healthy one 

 a few miles off and allowed at once to bite infected animals, the 

 latter contracted nagana. 



By those experiments it was thus determined that nagana 

 could be transmitted by the blood of the infected animal, that 

 is, without the agency of the fly ; that the latter had no inherent 

 power to produce the disease ; that it could, however, by 

 successively biting infected and healthy animals transmit the 

 disease to the latter; and that specimens of the insect caught in 

 infected areas harboured the parasite and were thus infective. 

 The question remained as to how the flies might become infected 

 in nature. It had been observed that in districts where the 

 tse-tse fly lived the prevalence of the disease in imported animals 

 was related to the presence in the locality of wild herbivora. 

 ISniee now found that, if considerable amounts of the blood 

 of the latter were taken to another locality and injected into 

 dogs, these in a proportion of cases contracted nagana, and from 

 this he deduced that the wild animals harboured the parasites 

 in small numbers in their blood and thus kept up the 

 possibility of infection. A further fact was that other blood- 

 sucking flies besides the tse-tse appeared incapable of acting as 

 carriers of infection. Bruce's work as a whole pointed to the 



1 This observation probably only applies to infection so far as this may 

 lr merely mechanical. There is evidence that a cyclic development 

 mvurs iii glossina. and that thus after an interval its bite is again infective. 



