1910.] 



Trypanosoma lewisi hy the Rat-flea. 



283 



positive experiment with Ceratophyllus fasciatus, and two positive experiments 

 with Ctenopldhalnms agi/rtcs. The fleas were in every case taken from infected 

 wild rats, without any knowledge of when they first had an opportunity 

 of feeding on infected blood. From such experiments it is useless to specu- 

 late on the method of infection. Nuttall, in the same paper, records a positive 

 result with the rat-louse Hconatopiaun s-plnulosus, and concludes — " Since 

 three distinct kinds of blood-sucking insects are capable of transmitting 

 T. letvisi, it appears doubtful that this flagellate is a parasite of the 

 invertebrate host in the sense claimed by Prowazek and other investigators." 

 From the standpoint of transmission, however, the most important considera- 

 tion in the general trypanosome problem is the distinction between " direct " 

 and " cyclical " infection, which we laid down and defined above ; and from 

 the experiments which form the subject of the present communication, 

 we conclude that the rat-flea Cerutophyllus fasciatus transmits T. leivisi from 

 infected to non-infected healthy rats by the " cyclical " method, and that 

 transmission by the " direct " method has not taken place. The importance, 

 both from the scientific and the practical standpoint, of this " cyclical " method 

 of infection cannot be over-estimated. 



The length of the multiplication-period in the rat is probably less 

 dependent on external conditions, and appears to be fairly constant. From 

 these experiments, 12 days may be taken as the average length of the period 

 from the time of inoculation by the flea until the multiplication ceases of the 

 trypanosomes in the rat's blood. On the other hand. Experiment D defines 

 the incubation-period in the flea within narrow limits : at the most from 

 December 7 to 15, i.e. eight days, at the least from December 8 to 14, 

 i.e. six days. From some of our other experiments, however, it would appear 

 that in some cases the incubation-period is much longer, and that the cycle 

 takes longer to complete in some fleas, or under some circumstances which are 

 perhaps related to the conditions of the trypanosomes when taken up by the flea. 



With regard to the cycle of development which the trypanosomes undergo 

 in the flea, we hope to return to this in a future communication, but in the 

 meantime we may point out from the observations recorded above of fleas 

 dissected, that multiplication probably starts in the rectum of the flea, where 

 masses of Crithidia-\i\iQ forms are seen attached to the walls of the gut 

 between the large rectal glands so-called. That these forms are stages in the 

 development of T. lexvisi in the flea is supported by the fact that in 50 fleas 

 from the breeding cages dissected and examined, no trace of any flagellates 

 was seen, whereas of 24 fleas from the same source, dissected after they had 

 fed upon rats infected with T. kwUi, flagellates were found in 15, in 8 of 

 which CritMdia-YikQ forms were found in the rectum. 



z 2 



