656 TRYPANOSOMIASIS 



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 originated 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 trypano- 

 somes 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, how- 

 ever, the blood were kept moist, then it retained its infectiveness 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 corresponded 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 was usually innocuous. 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 in- 

 fected area to a healthy one a few miles off and allowed 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. Bruce 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. Bruce's work as a whole pointed to the trypanosome as 

 the cause of nagana, and this has since been finally established by the 

 origination of the disease by artificial cultures of the organism. 



The Tr. brucei (Fig. 188), according to Laveran, measures in the horse 

 from 28 to 33 ^ long and from 1*5 to 2'5 /x, broad ; in the rat and dog it is 

 somewhat shorter. It is motile, but its activity is less than that of Tr. 

 lewisi. When stained it presents the usual appearances ; its posterior 



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

 merely mechanical. There is evidence that a cyclic development occurs iu 

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



