ACQUIRED IMMUNITY 147 



ated strains, such as trypanosomes which, as a result of exposure to heat 

 or other adverse conditions, have lost their power of producing actual 

 infection, have been injected into animals. In a similar manner killed 

 trypanosomes, trypanosomes which have been broken up by immersion 

 in fluids which bring about cytolysis, dried trypanosomes, as well as 

 cultural forms of trypanosomes, which often have ceased to be infective 

 to animals, have been tried, but in none of these cases was satisfactory 

 evidence obtained that the animals inoculated with these altered trypano- 

 somes had acquired any immunity to inoculation with a virulent strain, 

 though the application of certain serological tests, such as that of the 

 complement fixation, has demonstrated that a specific change may have 

 taken place in the serum of the animals. The response as regards pro- 

 duction of immunity cannot be compared with that which occurs in the 

 case of bacteria. Ponselle (1923a) has found that by keeping the heart- 

 blood of a mouse containing Trypanosoma brucei for twenty-four hours in 

 a medium of dihydrogen potassium phosphate and hydrogen disodium 

 phosphate it loses its power of infecting mice, but if injected will render 

 mice immune to infection with unaltered Trypanosoma brucei (see p. 454). 

 The bulk of work in connection with the production of immunity in 

 Protozoal infections has been carried out with trypanosomes, but certain 

 investigations have been made with other Protozoa. Thus, the Sergents, 

 Et. and Ed. (19216), have produced a certain degree of immunity in the 

 case of the parasite of bird malaria, Plasmodium, prcecox. Normal canaries 

 were very easily infected with this parasite, only 0-72 per cent, resisting 

 infection out of 965 birds inoculated. If canaries are inoculated with 

 the sporozoites of the parasite which have been rendered non-infective by 

 keeping them for twelve to forty-eight hours after removal from the 

 mosquito, a certain degree of immunity results. It was found that 

 29*5 per cent, of twenty-four canaries thus treated resisted subsequent 

 inoculation with the parasite. Similarly, it was found that if the blood 

 of a canary was drawn off after it had been inoculated with the parasite, 

 and before the infection had established itself by the appearance of 

 parasites in the blood, this blood, if injected into healthy birds, produced 

 an immunity which protected from subsequent inoculation 21 "3 per cent, 

 of sixty-one canaries. 



Many observers have attempted to produce immunity in cattle against 

 infection with Babesia bigemina and Theileria parva. From both these 

 infections animals may recover naturally, and possess an absolute immunity 

 to further infection, but the death-rate is ahvays high, especially in the 

 case of East Coast fever. No means of producing an immunity apart from 

 actual infection are known, though in the case of piroplasmosis it is 

 possible to inoculate the animals at a time when they are best able to 



