428 DISEASES OF THE BLOOD. 



Another variety of trypanosoma, the trypanosoma Theileri, is espe- 

 cially striking on account of its size. It is only found in oxen, and 

 exhibits a very slight degree of virulence. 



Koch divides trypanosomata into two great groups. 



The grouping is based on three important peculiarities : firstly, the 

 morphology of the parasite ; secondly, its virulence ; and, thirdly, its 

 relation to the host. 



The first group only exists in one species of animal. They have 

 become so completely accustomed to this method of life that they can- 

 not exist under other circumstances. Their virulence is slight but con- 

 stant. This group comprises the trypanosoma of rats and the try- 

 panosoma Theileri. 



The second group (to which all other trypanosomata belong) shows 

 great variation in virulence and in form. These trypanosomata are not 

 peculiar to any one species, but may affect dogs, rats, horses, etc. Their 

 morphological peculiarities also vary according to the animals in which 

 they are found. Thus, the tsetse parasites when cultivated in the bodies 

 of dogs and rats become much smaller than usual, and the centrosome 

 appears near the end ; when cultivated in horses the end appears 

 pointed, and the centrosome lies near the centre ; in the pig the para- 

 sites lose their peculiar short flagellum. Their virulence also varies 

 within wide limits. 



It has been found possible, as in the case of bacteria, to modify the 

 virulence of trypanosomata by successive passages through difierent 

 animals. By inoculating dogs with comparatively innocuous trypano- 

 somata and conveying the disease from dog to dog the virulence is 

 markedly increased. On the other hand, parasites which prove ver}^ 

 virulent for oxen become much less active for these animals after pas- 

 sages through rats and dogs. This apparently trifling discovery laid the 

 foundation for protective inoculation experiments. Parasites of the 

 second group can also exist in the bodies of almost all mammals. 



Koch is of opinion that the parasites of surra in India and of tsetse 

 disease in Africa are absolutely identical. Laveran, on the other hand, 

 states that he has protected animals against tsetse, and that they have 

 nevertheless suffered from surra. 



That this in no way disproves the identity of the two parasites is 

 shown by other experiments. 



Koch, whilst in Dar-es-Salaam, made some interesting experiments 

 for the purpose of discovering a method of protective inoculation. He 

 had found that the virulence of the ox parasites could be modified. He 

 therefore inoculated oxen first with these weakened parasites and after- 

 wards with others of high virulence. All the control animals died while 

 those treated as above remained alive. 



