632 Immunity and Resistance 



inoculation with a virulent strain, but absolute immunity was rarely 

 produced (138). 



Active immunization: Babesia. Smith and Kilbourne (162) noted that 

 cattle surviving an attack of Texas fever possessed an immunity asso- 

 ciated with a persisting low-grade infection. Such infections, with con- 

 comitant immunity, may last as long as 12 years (149). Resistance of the 

 host is not absolute, since relapses may follow environmental or other 

 disturbances to the equilibrium. 



Active immunization: Plasmodium. In areas where malaria is endemic, 

 many natives have the disease as children and the survivors seem to de- 

 velop a resistance to malaria. This resistance is believed to accompany 

 low-grade infections and to disappear gradually after the infections are 

 terminated. 



Active immunization to P. vivax has been produced repeatedly under 

 experimental conditions (30, 31, 92, 198). Recovery from attacks is accom- 

 panied by an immunity which often prevents clinical attacks upon rein- 

 oculation with the homologous strain (27). However, there is little or no 

 protection against other strains (heterologous strains). Infection with 

 homologous trophozoites is frequently inhibited, although the introduc- 

 tion of homologous sporozoites may lead to a subclinical infection (26). 

 The exact duration of immunity to P. vivax is unknown. However, im- 

 munity may persist for six or seven years, as indicated by light infections 

 with mild symptoms or none at all following reinoculation with the 

 homologous strain (29). A comparable immunity, developed against P. 

 falciparum, often aborts a second clinical attack, although some multipli- 

 cation of the parasites may follow reinoculation (33). Inoculation of a 

 heterologous strain may induce a new clinical attack almost as severe as 

 the first but an increased tolerance is sometimes indicated by a shorter 

 attack and a lower parasitemia (28). In the case of P. ovale, the immu- 

 nity is usually more effective against heterologous strains than it is in 

 P. falciparum or P. vivax (161). 



The phenomenon of relapse, although apparently less common in P. 

 ovale infections (109), is characteristic of other human malarias. Relapses 

 in falciparum malaria are usually of the "short-term" type, occurring 

 within a few weeks after apparent recovery from the original attack. Re- 

 lapses in vivax malaria, and especially in quartan malaria, are often 

 "long-term." Cases of the former many relapse after 6-12 months, and 

 quartan after even longer periods. The occurrence of relapse is sometimes 

 attributed to factors which lower resistance of the host — adverse climatic 

 changes, prolonged fatigue, surgical shock, pregnancy, inadequate diet, 

 and the like. However, an adequate immunological explanation for this 

 recurrence of symptoms, after a specific immunity presumably has been 

 developed, is highly desirable. Certain experimental data are suggestive. 

 For example, a relapse in monkeys infected with P. knowlesi is preceded 



