46 SOME xMINUTE ANIMAL PARASITES 



trypanosomes have become habituated to their hosts 

 for extensive periods of time, and it is suggested 

 that gradually the infected animals acquire the 

 power of permanently resisting the deadly action 

 of the parasites upon them, and of converting the 

 parasite from a lethal or fatal one into a practically 

 innocuous one. The phenomenon of a vertebrate 

 literally swarming with trypanosomes and yet appar- 

 ently in good health and performing all its normal 

 activities is then witnessed. Such a non-pathogenic 

 trypanosome is found in the rats of most parts of the 

 world. This parasite, T. lewisi, is an organism con- 

 cerning whose method of transmission there has 

 been more controversy than has been experienced 

 over any other trypanosome. Balancing the evi- 

 dence, it is obvious that the common rat-flea, 

 Ceratophyllus fasciatus, is the most usual transmitting 

 agent, but in addition, rat-lice and rat-fleas other 

 than Ceratophyllus, are concerned occasionally in 

 the spread of T. lewisi from rat to rat. T. lewisi has 

 recently been shown to pass one stage of its life 

 within the gut epithelium of C. fasciatus. 



There are many other mammalian trypanosomes 

 that cannot be discussed in detail here. Nor are 

 trypanosomes restricted to mammals, for reptiles, 

 amphibia, and fishes all are parasitized by them. 

 The trypanosomes of fishes seem to be widely spread 

 among the various genera, but to be present in 

 relatively small numbers in the blood of their hosts. 

 They are probably spread from fish to fish by various 

 leeches. The agents of the transmission of many 

 reptilian trypanosomes from host to host are ticks. 



