136 LIFE-HISTORY OF PROTOZOA 



Man seems already to have become a natural host to Trypanosoina 

 gambiense, but to be only in process of becoming so for T. hrucei {T. 

 rhodesiense). 



An important feature of parasitism is the specificity of any particular 

 parasite for its host. It is found in nature that some parasites are unable 

 to live in any other host than the one in which they naturally occur. 

 This undoubtedly depends upon the peculiar character of the body fluids 

 of these animals. Some parasites have become so specialized that they 

 cannot survive in any other fluid than the one to which they have become 

 accustomed. Very frequently, however, a particular parasite is able to 

 live in hosts which are nearly related, the fluids of which may be presumed 

 to differ only slightly from one another. Thus Plasmodiuin vivax, which 

 causes benign tertian malaria, cannot survive in any other vertebrate 

 host than man, though Mesnil and Roubaud (1920) have shown that it 

 may multiply for a short period in the chimpanzee. Other parasites are 

 much less specific, for many of the pathogenic trypanosomes can develop 

 in small rodents, which under natural conditions are never infected by 

 them. In such cases it seems probable that, quite apart from the suita- 

 bility of the fluid of a host, the rapidity with which a host can develop 

 antibodies is the determining factor as to whether a parasite can establish 

 itself or not. Instances are known in which it is only after many attempts 

 to introduce a parasite into a host that success is at last attained. An 

 instance of this is quoted below (p. 576), where Watson, attempting to isolate 

 a strain of Trypanosoma equiperdum from horses in laboratory animals, 

 only succeeded in one after inoculating over 600 animals. The infection, 

 once established, was then readily inoculated from one animal to another. 

 It is evident that here the fluids of the animal which gave a successful 

 result differed from those in which inoculation had failed, or that amongst 

 the organisms injected on the successful occasion there happened to be 

 a few which found the environment congenial and were able to resist 

 the antibodies developed. The fact that subsequent subinoculations were 

 easily carried out seems to suggest that the explanation is to be found 

 in the parasites themselves. Not infrequently an animal which has 

 acquired an infection will free itself, after which it is found to be immune 

 to further inoculations. On the other hand, it has been shown that in 

 some cases, when an infection has disappeared or has been much reduced, 

 further inoculations of the same organism may bring about a super- 

 imposed infection which may be more severe than that first produced. 

 Such an instance has been described by Ndller (1917) in the case of frogs 

 infected with Trypanosoma rotatorium. 



It may be stated as a general rule that the specificity of parasitic 

 Protozoa for their particular hosts is much more marked than is the case 



