THE HAEMOFLAGELLATES 197 



particular kind of host, are as yet somewhat scanty. In one or 

 two cases, however, the parasites are known to be fairly common. 

 Trypanosoma lewisi, for example, occurs in a considerable percentage 

 of sewer-rats throughout the world, having accompanied these 

 rodents in their ubiquitous migrations ; the proportion of hosts 

 infected varies usually from 10 to 40 per cent. 



In considering the occurrence of Trypanosomes in Mammals 

 careful distinction must be drawn between true or natural hosts 

 and strange or casual ones. In the former case, by reason of the 

 long-existing association between host and parasite, a condition of 

 mutual toleration has been reached, which, in ordinary circum- 

 stances, enables a proper balance to be maintained on both sides. 

 On the other hand, when a Trypanosome gains an entry into 

 animals which have never been previously liable, by their dis- 

 tribution, to its invasion, and which are consequently unaccustomed 

 and unadapted to the organism, it usually produces markedly 

 harmful effects. Such a state of affairs has resulted, for example, 

 from the inarch of civilisation into the " hinterlands " of the 

 various Colonies, where man, together with the numerous domestic 

 animals which accompany him, lias been brought into proximity to 

 big game, etc., and what is equally important, into the zone of the 

 blood-sucking insects which prey upon the same. 



Very many of the common domestic Mammals can be success- 

 fully infected (either in an accidental way or else artificially) with 

 different "pathogenic" Trypanosomes, to which they succumb 

 more or less readily ; they cannot be regarded, however, as natural 

 hosts of those Trypanosomes. In considering disease-causing forms, 

 the more narrowly the original source of the parasite concerned is 

 defined, the closer do we get to the true Vertebrate host or hosts. 

 In the case of the Nagana parasite, it has been shown that such are 

 almost certainly to be found among buffaloes and various Antilo- 

 pidae (e.g. the gnu, "koodoo," etc.), while, again, the native host of 

 T. equinum, of Mai de Caderas in South America, is most probably 

 the capybara. It may be said undoubtedly, with regard to the 

 many lethal Trypanosomes now known, that there is, in each case, 

 some indigenous wild animal tolerant of that particular form, 

 which serves as a latent source of supply to strange Mammals 

 coming into the vicinity. 



Transmission. In the transmission of the parasites from one 

 Vertebrate individual to another, a blood-sucking Invertebrate is 

 almost invariably concerned. 1 In the case of all Trypanosomes of 



1 Trypanosoma equipenlum, the cause of Douriue or horse-syphilis, is conveyed by 

 the act of coitus ; and it is quite uncertain whether this parasite is ever transmitted 

 naturally by an insect. Moreover, Koch has recently brought forward evidence 

 (29, Schluss-Bericht) which, he thinks, tends to show that the human parasite 

 (T. yambiensr} can also be transmitted by sexual intercourse. 



