XVIII] TRANSMISSION 347 



is more or less tapering. The undulating membrane is well 

 developed, being larger than that of T. nanum, and there is no 

 free flagellum. The oval trophonucleus is situated about the 

 middle of the body and the small spherical kinetonucleus is at 

 the posterior extremity. The cytoplasm is generally homo- 

 geneous and not very granular. 



Mode of infection. The results of Kinghorn and Yorke in 

 the Luangwa Valley have shewn that Glossina morsitans is 

 probably the most important agent for the spread of this 

 infection. By collecting large numbers of these tsetse-flies 

 and feeding them on healthy monkeys, out of 3202 flies at 

 least two were found to be infected with T. pecorum. 



Under experimental conditions, Glossina palpalis can also 

 serve as the intermediate host for this species and animals may 

 be infected by the bites of palpalis that have previously ingested 

 blood containing T. pecorum. The mode of transmission is 

 indirect, that is to say the trypanosome undergoes a cyclical 

 development within the alimentary canal of the fly, but this 

 development is so extremely slow that it is evident that G. 

 palpalis is not the usual intermediate host of this parasite. It 

 is also significant that wild palpalis has never been found 

 naturally infected with pecorum. The stages in the develop- 

 ment of T. pecorum in the alimentary canal of G. palpalis have 

 been observed by Fraser and Duke, and also by Miss Robertson. 



Although these two species of Glossina have thus been proved 

 capable of transmitting this infection, there is considerable 

 evidence to shew that the disease can exist in the absence of 

 tsetse. Thus Bruce and other members of the Sleeping Sick- 

 ness Commission saw an outbreak of T. pecorum infection 

 under the following circumstances. The cattle belonging to 

 this Commission grazed at the foot of Mpumu Hill, half to the 

 east and half to the west. Tabanus and Hcematopota were 

 occasionally seen but always in very small numbers. In 

 September, 1909, swarms of Tabanus secedens Walk, suddenly 

 appeared to the west of the hill and a month later to the east. 

 Soon afterwards the cattle, which had been healthy for a year, 

 shewed signs of T. pecorum infection, first those which grazed 

 to the west, then those which grazed to the east. It should be 



