Immunity and Resistance 



633 



by a decrease in litre of the protective antibody, while a rise in titre 

 occurs after recovery from the relapse (47). Furthermore, superinfection 

 with the homologous strain (St. Elizabeth strain of P. vivax) is possible 

 after prolonged periods of latency (49). This indicates a gradual relaxa- 

 tion of the defensive mechanism during latency. On such grounds, it has 

 been suggested (172) that antigenic stimulation during primary attacks 

 with P. vivax and P. malariae usually induces a temporary low-grade im- 

 munity. This immunity wears off after the disappearance of erythrocytic 

 stages. The result is a relapse. Relapses bring further antigenic stimula- 

 tion, producing an immunity which may eventually become potent 

 enough to eliminate the parasites. 



Homologous immunities, similar to those in the human malarias, are 

 developed against parasites of monkeys (160) and of birds (171). In 

 avian and simian, as well as in human malaria, the immunity is believed 

 to be primarily a resistance to superinfection in animals carrying a low- 

 grade infection with the homologous strain. Such an immunity may 

 persist for some time, in diminishing degree, after elimination of the 

 infection. The immunity of canaries to P. cathemerium decreases gradu- 

 ally from the first to the sixth month after cure and is no longer detect- 

 able after eight months (80). Man also develops an apparently sterile 

 immunity to P. knowlesi (115). A similar temporary immunity to P. 

 knowlesi has been obtained by chemotherapeutic elimination of latent 

 infections in monkeys (43, 107) and also against P. vivax in man by 

 dosage with pentaquine (48, 197a). This residual immunity against P. 

 vivax varies in intensity with the number of relapses rather than dura- 

 tion of the infection, and is lost rather rapidly after elimination of the 

 parasites. 



In a few instances, resistance to Plasmodium has been induced with 

 killed vaccines. Resistance of canaries to P. cathemerium has been in- 

 creased by vaccination with formalin-killed parasites (79), and similar 

 vaccines have immunized ducks to P. cathemerium and P. lophnrae (187). 

 Striking results have been obtained in rhesus monkeys vaccinated with 

 killed P. knowlesi, emulsified in paraffin oil containing killed Mycobac- 

 terium tuberculosis (76). Although P. knowlesi is usually lethal, inocula- 

 tion of the vaccinated animals resulted in mild infections of short duration. 

 However, vaccination of man against P. vivax has produced no significant 

 protection (87). 



Different stages in the life-cycle may vary in their susceptibility to 

 antibodies. Apparently normal pre-erythrocytic stages, but few or no 

 erythrocytic stages, have appeared after heavy inoculation of immunized 

 chickens with sporozoites of P. gallinaceum (90). In addition, chickens 

 vaccinated with inactivated sporozoites of P. gallinaceum are partially 

 immune to sporozoites but not to erythrocytic stages (143). 



Active immujiization: Coccidia. Andrews (3), with his observations 



