236 
MALARIA 
will often cause a latent infection to become 
active in monkeys after all other attempts 
have failed. 
If a latent infection is necessary for a 
lasting immunity, the question arises as to 
which strain or species is latent, or whether 
there is a latent infection of each strain to 
which the individual is immune. Since one 
strain produces only a partial immunity 
to other strains and none to other species 
the latter alternative must be considered 
further. In highly endemic areas where 
the infection rate in the adult population 
is high, but the febrile attacks mild and 
few, the individual is constantly exposed 
to reinoculation. This cannot be considered 
to be a condition of latency, but one of 
subnormal infection without clinical mani¬ 
festation. In areas where the transmission 
of malaria is seasonal the immunity is much 
less effective in preventing infection. These 
latter conditions would be optimum for the 
establishment of a latent infection, but the 
immunity is found to fluctuate, the spleen 
rate to decrease and the degree of suscepti¬ 
bility to increase. Following epidemics of 
malaria the spleen rate remains high for 
two or three years, then gradually, then 
more rapidly begins to decrease. During 
this time individuals are more or less 
susceptible. 
If the immunity is dependent on a 
latent infection it would appear that a 
single infection would be sufficient to estab¬ 
lish the latent condition and the subsequent 
immunity. In only about 35 per cent of 
cases is there a resistance to reinfection fol¬ 
lowing a single infection. This could be 
explained by the supposition that more 
than one infection is required to establish 
the latent infection in man, or that only 
35 per cent carried a latent infection fol¬ 
lowing the first infection. Although a few 
isolated cases of long standing immunity 
have been reported in the apparent absence 
of infection the indication is that the im¬ 
munity gradually loses its power of pre¬ 
venting reinfection when the tissues are 
free of all parasites. Boyd, Stratman- 
Thomas and Kitchen (1936c) were able to 
demonstrate an effective immunity in one 
case three and one half years after recovery 
from a previous infection. Subinoculations 
of massive doses of blood failed to show 
the presence of a latent infection prior to 
the reinoeulations. James and Ciuca (1938) 
found that one of their patients had re¬ 
mained immune apparently for 5 years. 
The effectiveness of the immunity in 
preventing further attacks, however, does 
not depend on a complete sterilization of 
the blood. In cases of* constant reinocula¬ 
tion where the individual has a low grade 
infection the immunity is maintained at a 
high level by the parasites present in the 
tissues. Under hyperendemic conditions in 
India Christophers found that the rate of 
infection in adults was about 50 per cent, 
but the parasite count was low and attacks 
occurred on an average of once in six 
months. These attacks were relatively mild 
and of short duration. 
Factors Influencing Development 
of Immunity 
Children are generally thought to be 
more susceptible to most diseases than are 
adults. However, where there have been 
specific studies made on this point the 
adult resistance appears to have been ac¬ 
quired by either an acute or latent infec¬ 
tion. Kligler (1930) and Kligler and Mer 
(1933) measured the difference between 
children and adults in their powers to re¬ 
sist malarial infection following treatment. 
Up to 4 years of age they found the re¬ 
duction of the infection to be less than 4 
per cent, while for a group between the 
ages of 4 and 9 years the reduction in in¬ 
fections was 23 per cent after two weeks 
of treatment. Nothing is said, however, of 
the great degree of immunity developed by 
the older group in comparison with that of 
the younger group as a result of twice as 
many years of malaria infection. The 
hyperendemic conditions make it prac¬ 
tically impossible for the older group to 
have escaped infection. Since children of 
this age are seldom found in the hospitals 
for the insane, little controlled experi¬ 
mentation has been possible. It is apparent 
that further facts are necessary before a 
