PATHOGENIC SPECIES 



97 



animal has recovered from an attack by one strain of trypano- 

 some it is rendered immune and will not succumb to second 

 attacks of the same strain, though it is still susceptible to others. 

 As remarked before there are at least three species of trypano- 

 somes which are known to cause disease in man, two in Africa 

 and one in South America. The African species, causing sleep- 

 ing sickness, are the most deadly but the South American species 

 is frequently fatal, especially to children, and often renders a life 

 worse than useless. The Gambian trypanosome, Trypanosoma 



Fig. 20. Trypanosoma gambiense in rat blood, showing long, intermediate and 

 short forms all in one microscopic field. X about 1200. Drawn from microphoto- 

 graph by Minchin. 



gambiense, is the cause of the commoner and more widespread 

 form of sleeping sickness, while the Rhodesian species, T. rho- 

 desiense, is the cause of. the recently established East African 

 form of the disease. The most salient distinguishing character- 

 istic between these two species of trypanosomes is the posterior 

 situation of the nucleus of the Rhodesian parasite in a certain 

 per cent of individuals when they are developed in rats and some 

 other animals (Fig. 19). This is a feature never observed in the 

 Gambian trypanosome. Both species vary a great deal in form, 



