Nagana (Trypanosoma brucei) 237 



Experiments with rats. — From the foregoing it will be noted that 

 the rat always succumbs from the acute form of the trypanosomiasis 

 when inoculated with the blood strain of the parasite. When, however, 

 it is injected with cultures grown in vitro the disease may be drawn 

 out to the subacute and in some cases to the chronic variety of the 

 disease. This is apparently due to the generation and age of the cul- 

 ture, temperature at which it was incubated and the number of try- 

 panosomes introduced into the animal. 



Rats inoculated with generation one of Trypanosoma brucei show 

 variable results primarily due to the age of the culture. The possibility 

 of the original blood survival being present is not taken into considera- 

 tion although this may be actually the case. They have been observed 

 but proven to be avirulent. 



Infection can usually be produced when cultures of that generation 

 are injected that are 6 to 11 days old. Cultures older than that are 

 non-infective. 



The period of incubation, as well as the duration of the disease 

 varies. The former lies between four to eight days and the latter may 

 be from 8 to 14 days. Whereas, if generation two, seven days old, 

 which is the time of its maximum growth, is injected the period of 

 incubation is three to seven days and the time of death is seven to ten 

 days. The variations in the period of incubation and in the duration of 

 the disease that have been noted when rats are injected with generation 

 two are even more irregular when inoculations with subsequent genera- 

 tions are made. The higher the generation the greater the variations. 

 Thus, the average period of incubation in rats receiving generation two 

 is 4.6 days and death occurs in an average of 8.9 days. 



Rats inoculated with subsequent generations gradually show a 

 longer period of incubation and the time of death is deferred. With 

 generation 40, the former was 12 to 13 days and the latter was 21 days, 

 although with generations 50 to 57 and 61 the organism seems to have 

 become temporarily exalted. The rats receiving the culture of these 

 generations showed periods of incubation varying from five to nine 

 days. In the rat receiving a seven day culture of generation 89 the 

 parasites were present in the blood in six days and the animal did not 

 die until the 94th day. Again, in the case of generation 154 the period 

 of incubation was eight to ten days and. death did not occur until on the 

 126th day. With generation 190 the rat became infected on the sixth 

 to seventh day and died on the 78th day. 



This variation seemed especially more pronounced if the cultures 

 were incubated 14 or even 21 days before inoculations were made. This 

 is nicely shown with generation 85. The period of incubation when a 

 seven day old culture was used was six days, death occurred on the 

 41st day. If the age of the culture is 14 days the trypanosomes appear 

 in the blood of the rat at about the same time (seven days) but the 

 animal did not die until on the 84th day. And if a 21 day old culture 

 of the same generation was used the period of incubation was 8 to 11 

 days with death coming on the 51st day. In one exceptional case, that 

 of a 21 day old culture of generation 93, the rat became infected be- 



