IMMUNITY TO HUMAN MALARIA 
233 
malaria in the hospitals it is possible to 
obtain some indication of the rate of de¬ 
velopment of the immunity following 
various types of infections. 
It has been indicated in the previous sec¬ 
tion that an immunity can be established to 
a certain strain of malarial parasite in a 
period of approximately one year by allow¬ 
ing the infection to relapse and recrudesce 
with no treatment except when the febrile 
attacks reach a condition endangering the 
life of the individuals. 
Ciuca, Ballif, and Chelarescu-Vieru 
(1934) found that only 34 per cent of 
their patients were resistent to P. vivax 
after the first infection. Following the 
second infection the number was increased 
to 72 per cent, and to 87 per cent after 
the third. The remaining 13 per cent of 
susceptibles developed a solid immunity 
only after the fourth infection. All were 
resistant to the fifth inoculation. Using P. 
malariae on 322 patients they found that 
35 per cent resisted a second inoculation, 
but this had increased to only 81 per cent 
following the fifth inoculation. In order 
to produce immunity in all cases it was 
necessary to inoculate some for the sixth 
time. After the tenth inoculation with P. 
falciparum 3 per cent of 544 persons still 
were susceptible. 
Sinton (1940a) found that 48 per cent of 
P. ovale infections produced immunity to 
a second inoculation. A third inoculation 
showed that 41 per cent were still sus¬ 
ceptible, but the intensity and the duration 
of the attacks had considerably decreased. 
During the third infection only 40 per cent 
of those susceptible developed fever and 
the attacks lasted on an average less than 
one day. 
An apparent contradiction to these ex¬ 
perimental findings occurs in hyperendemic 
areas where a constant high infection is 
found up to the tenth year. The febrile 
attacks fall off but may continue inter¬ 
mittently for about 5 years more after 
which the individual usually remains free 
from attacks though parasites may be found 
in the blood. The two different pictures 
presented can now be readily explained on 
the basis of heterologous strains of each 
species of plasmodium. Even during the 
period following the cessation of febrile 
attacks while there are still numerous para¬ 
sites to be found in the peripheral blood 
a heterologous strain will provoke a new 
attack. The implication here is that one 
is at least partially susceptible until he has 
experienced attacks and recovered from all 
the strains within the area of infection. 
Garnham (1935) found the population of 
certain endemic centers immune to sub¬ 
tertian and quartan malaria, but very 
susceptible to P. vivax which was not 
endemic to the area. In areas where little 
protection against infection can be pro¬ 
vided children under two years of age have 
the highest parasite count. Christophers 
reports that in some areas in India the 
average is 10,000 parasites per cubic milli¬ 
meter of blood. The spleen is palpable in 
88 per cent and attacks are almost con¬ 
tinuous. Garnham (1935) finds that the 
children of Kenya are highly infected, but 
suffer little from febrile attacks. There are 
very few deaths among the natives, but out¬ 
side children are much more severely af¬ 
fected and deaths occur frequently. Gill 
(1914) has found that a constant spleen 
rate over a period of years is correlated 
with the immunity, but likewise is depend¬ 
ent on almost unbroken infections of para¬ 
sites. Even a seasonal fluctuation of trans¬ 
mission interferes with the development of 
the immunity. 
Specificity 
As shown by Boyd and Stratman-Thomas 
(1933c) and Ciuca, Ballif and Chelarescu- 
Vieru (1933) the immunity produced to 
one strain of parasites is much less effec¬ 
tive against all other strains of the same 
species. Some resistance is shown to a 
heterologous strain, the resulting infection 
producing fewer febrile attacks and end¬ 
ing spontaneously. There seems to be no 
limit to the number of strains, however, 
as individual infections found within a 
radius of a few miles differ enough to pro¬ 
vide only partial protection to each other. 
This fact may explain the length of time 
