562 TRYPANOSOMIASIS 



possible that both phenomena occur, that not every infection 

 results in multiplication of the parasite in the body of the 

 victim, and that in certain cases where multiplication does 

 occur a resistance is developed which enables the body to kill 

 the parasites. The occurrence of the mononuclear reaction is 

 here significant ; it has been suggested that, when this resist- 

 ance is weak, the organism gains entrance to the spinal canal, 

 and that then sleeping sickness results. 



The whole of the recent work on the disease is of the highest 

 interest and importance. Short of the prolonged subculture of 

 the parasite and the reproduction of the disease by such cultures, 

 the strongest evidence may be said to exist that the Tr. 

 ugandense is the cause of sleeping sickness. 



Other Pathogenic Trypanosomata. It is beyond the scope 

 of this work to deal at length with the other diseases of animals 

 caused by trypanosomes. The chief of these have been 

 mentioned in the opening paragraph, but it may be said that 

 many others have been described in various species of mammals, 

 birds, and fishes, and that these are spread either by flies or by 

 leeches. The most interesting of those mentioned is Dourine, 

 a condition resembling in many ways nagana. It, however, 

 presents this peculiarity, that infection does not take place by 

 an intermediate host, but apparently directly through coitus, as 

 it occurs only in stallions and in mares covered by these. 



In several of the trypanosomal infections of animals it 

 appears as if, as in the case of Tr. Lewisi, the animal suffers 

 little inconvenience from the presence of the parasite in its 

 blood, and the view has even been put forward that with all 

 pathogenic trypanosomes there exists a host which carries the 

 organism without being affected by its presence more, for 

 example, than is the rat by Tr. Lewisi. Though no opinion 

 can be expressed on this point, it is necessary to bear the fact 

 in mind that either natural or acquired immunity can exist 

 against such protozoa. Not only is this important from the 

 point of view of the investigation of the conditions under which 

 such tolerance arises, but also from the bearing which the 

 existence of this tolerance may have on the spread in nature of 

 the parasites to a susceptible species from immune animals which 

 still harbour trypanosomes in their blood. We are, however, 

 as yet quite ignorant of many of the processes at work in the 

 body during a trypanosomal infection, and of the causes of the 

 symptoms and other morbid effects. 



