134 
MALARIA 
nity, there will be a great increase in 
symptomatic malaria in both pre-adult and 
adult new-comers, and severe epidemics of 
malaria may thus arise. The bearing of 
immunity to the prevalence of the malarial 
fevers and its relation to malaria in the 
community is fully considered in the con¬ 
tribution by Hackett (1937). 
Importation of exotic strains. It has 
been conclusively demonstrated by numer¬ 
ous observers that the premunition and 
immunity acquired after repeated attacks 
of malaria are limited to the species of 
malaria plasmodia infecting the individual 
and to certain strains of these species. 
Thus, premunition or immunity acquired 
to P. vivax is largely inoperative against 
P. malariae, P. falciparum or P. ovale and 
vice versa. Not only is this true, but both 
are limited to certain strains of the par¬ 
ticular species of the plasmodium, i.e., to 
the strain with which the individual has 
been infected. It follows, therefore, that 
if new species or strains of species of plas¬ 
modia be introduced into a malarial region 
the incidence of malaria will be raised be¬ 
cause the inhabitants have no premunition 
or immunity to such species or strains of 
species. Such an occurrence might explain 
cyclical variations of both short and long 
amplitude. 
As an illustration of the effect of the 
introduction of new species of malarial 
plasmodia into a locality, the author may 
mention a personal observation demon¬ 
strating its importance. In the state of 
Connecticut, in 1898, the only species of 
malarial plasmodium present in a certain 
region was P. vivax and benign tertian 
malaria was endemic in the locality for 
many years. After the end of the Spanish- 
American War, a company of the National 
Guard from this locality, the men of which 
had been on duty in a southern camp where 
infections with P. falciparum were numer¬ 
ous, returned to their homes. Within a 
few weeks cases of infection with this plas¬ 
modium began to appear, and before the 
end of autumn an epidemic of estivo- 
autumnal malaria had occurred, with a few 
fatal infections. Thus, in this locality 
where this type of malaria had never before 
occurred, the importation of carriers of P. 
falciparum in the Guard company men¬ 
tioned, during favorable climatic condi¬ 
tions, resulted in an epidemic of this for¬ 
eign type of malaria among a non-immune 
population. 
