TRYPANOSOMES AND THEIR HOSTS 17 



But even man himself shows a gradual acquirement 

 of immunity. In West Africa Sleeping Sickness is not 

 generally so virulQnt as in the Uganda epidemic, and 

 the disease has been known in that area for a very much 

 longer time. The species of Trypanosome known as 

 T. nigeriense causes only a mild form of disease which 

 is very rarely fatal, so it would appear to be the species 

 which has been longest in contact with man. On the 

 other hand, T. rhodesiense is very much more virulent than 

 any other human Trypanosome known to us, and it 

 appears to have arisen as a sudden " mutation " from 

 the animal -inhabiting species T. brucei. It is only ten 

 years since this undesirable addition to the list of " human " 

 Trypanosomes made its presence known to us by causing 

 a disease previously unknown in that part, or at least 

 unrecognized as distinct from malaria, 



Trypanosomes are transferred from one host to another 

 by the agency of various invertebrates, which are as 

 necessary for its existence as are the animals in whose 

 blood they also live. 



In the blood they only multiply by fission, asexually 

 or vegetatively ; but in the invertebrate host they go 

 through a sexual form of reproduction. Development 

 generally commences in the alimentary canal, whence the 

 Trypanosomes find their way into the "salivary glands," 

 so that they are inoculated into the new vertebrate host 

 when the blood-sucker injects the irritating fluid secreted 

 by those glands in order to produce a free flow of blood. 

 The effect of this fluid is familiar to all who have been 

 " bitten " by mosquitoes. Such an alternation of sexual 

 with asexual methods of reproduction is called a "cycle" ; 

 the complicated life history of the malaria parasite is a 

 more familiar example. 



I think the fact that the sexual process takes place 

 in the invertebrate host points to that being the original 

 host for the Trypanosomes ; the life in the blood is as it 



3 



