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EVOLUTION OF ANIMALS 



Part V 



Trypanosomes 



The trypanosomes are blood parasites in all classes of vertebrates, but so 

 far as known are pathogenic only in man and domestic animals where, in an 

 evolutionary time sense, they have but recently developed. They are trans- 

 mitted from one vertebrate to another by blood-sucking invertebrates — those 

 of fishes, salamanders, frogs and reptiles by leeches — those of land vertebrates 

 by ticks and insects. Within these carriers they go through a cycle of several 

 days' development without which they cannot be transmitted into their second 

 host (Fig. 21.10). 



The trypanosome of the rat (Trypanosoma lewisi) is nonpathogenic and 

 common in our native wild rats, often so abundant that the blood literally 



Fig. 21.10. Trypanosoma gambiense among the red cells of human blood. These 

 microscopic blood parasites are the cause of trypanosomiasis, the sleeping sickness 

 of tropical West Africa. They pass one period of their life history in the tsetse flies 

 that are essential for their distribution. Aside from that they are parasites in the 

 blood of man and certain of the wild game animals of Africa. (Courtesy, General 

 Biological Supply House, Chicago, 111.) 



twinkles from their motions. Yet the rats thus infected show no signs of harm. 

 The rat becomes infected by licking its skin and thus gathers the feces of in- 

 fected rat fleas. After an incubadon time of two weeks the parasites appear in 

 the blood as typical trypanosomes and multiply enormously. Fleas suck up the 

 trypanosomes with every meal of an infected rat's blood. In the lining cells 

 of the flea's stomach, they muldply by repeated divisions and transform into 

 the mature trypanosomes then ejected upon the rat's skin in the feces of the 

 fleas. 



This life history shows important characteristics of such parasites; their 

 great capacity to multiply, and their ability to change form and adjust them- 

 selves to environments in which they thrive and are carried about and dis- 

 tributed. This life history also displays the ability of a host animal to become 



