46 ZOOLOGY FOR MEDICAL STUDENTS chap. 



trypanosomes to be sucked up in the infected blood and injected 

 bodily into the blood of the new individual to produce infection 

 therein. 



All the detailed knowledge of Trypanosomes is of comparatively 

 recent date. Many species have been described but our knowledge of 

 most of these is very incomplete. In the following list we will confine 

 ourselves to species which are of special interest and importance either 

 practical or scientific. 



T. brucii is of practical interest as the cause of Tsetse fly disease of 

 domesticated animals in various parts of Africa, and of special scientific 

 interest as being the species of Trypanosoma in which the method of 

 transmission was first discovered. The disease ('' Nagana ") has long been 

 known as occurring over large districts in Africa and as being invariably 

 fatal to Horses, Asses and Dogs, and usually so to imported Cattle. These 

 districts were known as " fly belts," the disease being associated with 

 the bite of a particular species of fly {Glossina morsiians — " Tsetse "). 

 It was suggested by David Livingstone that the disease was due to a 

 living microbe injected into the bite, but the actual proof of this was 

 first given by the experiments of David Bruce, which showed that any 

 individual fly was powerless to cause the disease unless it had previously 

 bitten a diseased animal. This clearly pointed to the poison being not 

 something inherent to the fly but rather something which the fly merely 

 transported from an animal already diseased. The fact that the infectivity 

 of the fly appeared from these early experiments to last only for a period 

 of about 48 hours after it had bitten the diseased animal pointed to the 

 " poison " being really some living organism which was able to survive 

 about the fly's proboscis for a period not longer than 48 hours. This 

 suspicion led to the examination of the blood of diseased animals and 

 in it Bruce duly discovered the trypanosome which we now know as 

 T. brucii. 



Subsequent research has fully borne out Bruce's discovery, with 

 however an important amplification namely that a fly which has lost 

 its primary or direct infectivity becomes again infective after a period 

 of about 18 days, and apparently now remains so for the rest of its life. 

 In other words there occurs here in addition to the direct infection 

 first observed a cyclical infection which in all probability is the 

 important one. 



T. evansi causes a disease (" Surra "), somewhat similar to Nagana, 

 affecting especially Horses and Camels. It probably is transmitted 

 also by a biting fly — possibly a Horse-fly (Tabanus) or Stable-fly 



