150 ACTION OF DRUGS IN PROTOZOAL INFECTIONS 



was destroyed. Certain strains of pathogenic trypanosomes can be 

 handed on indefinitely from mouse to mouse by direct inoculation of 

 blood without there being any evidence that the rate of multiplication 

 by binary fission slackens in any way. In these cases the trypanosomes 

 multiply so rapidly that the host is overcome by the parasite before any 

 degree of immunity capable of checking the infection has been developed. 

 At each inoculation the trypanosomes are introduced into a new host 

 which has no immune bodies, and multiplication is continued with the 

 same result. For the development of immunity it is essential that the 

 rate of multiplication of a parasite shall not be so great as to bring about 

 destruction of the host before it has time to respond to the infection by 

 the production of sufficient antibodies to check the development of the 

 parasite. From the point of view of the parasite this is the condition 

 most favourable to its survival, and it appears to be the one which obtains 

 in most, if not all, natural infections. 



A parasite may acquire the power of resisting the antibodies in the 

 serum. Jacoby (1909a) obtained a strain of Trypanosoma brucei which 

 was resistant to human serum, which normally will cause the disappear- 

 ance of the trypanosomes from the blood of mice. By repeatedly injecting 

 small quantities of normal human serum into an infected mouse and con- 

 tinuing the process in subinoculated mice, a strain of trypanosomes was 

 eventually secured which, as regards its development in mice, was un- 

 influenced by as large a dose (2 c.c.) of human serum as the mouse could 

 tolerate. Leboeuf (1911) in a similar manner obtained races of T. brucei 

 which were resistant to the serum of baboons. 



ACTION OF DRUGS IN PROTOZOAL INFECTIONS. 



It is possible that the disappearance of parasites as a result of the 

 administration of drugs is, in many cases at least, not the result of a 

 direct poisonous action of the drug upon the parasite. It would seem 

 natural to suppose that the good effects observed in amoebic infections 

 which result from the use of emetine and those following the ingestion 

 of quinine in malaria are due to the direct effect of the drugs upon the 

 parasites concerned. It appears that the action may be a much more 

 complicated one, and that drugs may act indirectly by stimulating the 

 tissues of the host to produce substances which may be regarded as anti- 

 bodies which are directly responsible for the suppression of the infection. 

 In support of this contention may be urged the fact that drugs such as 

 emetine, which are therapeutically active, are not more toxic to the 

 organisms when tested in vitro than other drugs which have no thera- 

 peutic properties. The investigations of Dale and Dobell (1917) on the 



