GENUS: TRYPANOSOMA 449 



ment two main groups of organisms. Some have suggested that it 

 indicates a tendency towards the production of male and female indi- 

 viduals, but, at present, there is no evidence to support this view. 



3. PATHOGENICITY. — Trypanosomes are often grouped as patho- 

 genic and non-pathogenic forms. The former are those which give rise 

 to disease in man or domestic animals, and they do so, not because these 

 are their natural hosts, but because the man or animal is susceptible to 

 inoculation with a trypanosome, which in its natural host is non-patho- 

 genic. As a general statement, it is safe to regard all trypanosomes as 

 non-pathogenic to their natural hosts. The vast majority resemble 

 T. lewisi of the rat, which, under ordinary circumstances, cannot infect 

 any other host than the rat. A small number of trypanosomes are, 

 however, inoculable into man, the domestic animals, and experimental 

 laboratory animals, and in these unnatural hosts they often produce 

 serious symptoms of disease. To their natural hosts, which in many 

 cases are the big game of Africa, they are apparently harmless. The 

 various pathogenic trypanosomes, however, vary in the effect they 

 produce on laboratory animals, and these variations are of some assistance 

 in the identification of the species. T. brucei, for instance, is readily 

 inoculated into the rat, mouse, and guinea-pig, whereas, with T. pecorum, 

 these animals cannot be infected. The human strain of T. brucei 

 {T. rJiodesiense) inoculated from man into the rat rapidly produces a 

 very heavy infection which quickly kills the animal, whereas T. gambiense 

 under these circumstances may fail to infect the rat, though it usually 

 does so, leading to a chronic type of infection characterized by the 

 presence of a small number of trypanosomes in the blood at any time, 

 the animals surviving for even a year or more. By passage from rat 

 to rat the virulence of such a strain may be increased, till finally the 

 infection may become as intense and as rapidly fatal as that produced 

 by T. brucei. 



The animals most frequently employed for these tests are rats, mice, 

 guinea-pigs, dogs, monkeys, goats, and even the larger domestic animals, 

 such as donkeys, horses, mules, and cattle. 



It sometimes happens that in the inoculated animals there appear 

 particular forms of trypanosome which were not present in the original 

 host. Thus, in man, T. brucei (T. rhodesiense) closely resembles T. gam- 

 biense, but in the inoculated rat there appear a certain number of 

 posterior-nuclear forms which enable the trypanosomes to be distinguished 

 from T. gambiense (Plate V. a and b, p. 456). 



In these experiments it is of importance to note the period of incuba- 

 tion before trypanosomes appear in the blood of the inoculated animals, 

 the intensity of the infection produced, and the duration of the infection. 



I. " 29 



