NAGANA OR TSE-TSE FLY DISEASE 553 



in 1894 that the blood of animals suffering from nagana 

 swarmed with a trypanosome now known as the Tr. Brucei, 

 and in 1895 he was instructed by the Governor of Natal to 

 undertake the investigation which led him to work out the true 

 etiology of the disease. It may be said that this research 

 forms the starting-point of the important work done during 

 the last decade with regard to infections by trypanosomes. 

 In his earlier work Bruce found that the parasite was present 

 in the blood of every animal suffering from nagana and absent 

 from the blood of healthy animals in the affected districts; 

 further, that the fever which marks the onset of the disease was 

 accompanied by the appearance of the trypanosome in the 

 blood; and finally, that the transference of the smallest 

 quantity of blood from an affected to a healthy animal origin- 

 ated the disease. He then proceeded to investigate the part 

 played by the tse-tse fly in the condition. He found that if 

 flies taken from the fly belt were transported to a place where 

 nagana did not occur, kept for a few days, and then allowed to 

 bite susceptible animals, the latter did not contract the disease 

 this result showing that it was not, as had been supposed by 

 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 24 hours after being dried ; if, however, 

 the blood were kept moist, then it retained its infectiveness up 

 to between 4 and 7 days; up to 46 hours living trypano- 

 somes 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 a fly, in that 24 hours after it has 

 been fed on an infected animal its bite is usually innocuous. 

 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 



