154 
MALARIA 
representative assortment of all the species 
and many of the strains of plasmodia with 
which the local anopheles are infected. 
But if every chronic case of malaria is 
harboring many different strains of para¬ 
sites and usually more than one species, the 
parasite formula, or relative prevalence of 
parasite species found in a blood survey of 
a community, is not an indication of the 
actual distribution of the parasites but only 
of the frequency with which each happens 
to be dominating the blood picture in in¬ 
fected individuals at the moment of taking 
the blood samples. We must look then for 
other factors besides the chance distribu¬ 
tion of infective bites, which might account 
for the notable variations in the relative 
prevalence of the different species in the 
blood of populations at different times and 
under different conditions. They are in¬ 
ternal factors which govern the host-para¬ 
site relationship and have to do with the 
peculiar biology of each plasmodial species 
as well as with the differential reaction of 
the human organism to the specific infec¬ 
tions. P. falciparum causes an acute dis¬ 
ease and builds up its numbers very rapidly. 
It is greatly favored by a high transmission 
rate and acquires a long lead over the other 
forms in epidemics. It loses ground as 
rapidly when transmission begins to fail, 
possibly because it requires more game- 
tocytes than P. vivax to infect anopheles. 
The more persistent P. vivax takes its place, 
characterized by latent periods and long 
term relapses. P. malariae is masked by 
either of the other species in the acute 
stage: but it long outlasts them both and 
clings indefinitely to the human organism. 
Miihlens reports one case which relapsed, 
with parasites in the blood, 19 years after 
infection. Much of thje so-called mystery 
of quartan fever is explained by the per¬ 
sistence of the parasite and its tendency to 
be recessive. 
The waxing and waning of human im¬ 
munity also plays a part in the appearance 
and disappearance of parasites from the 
circulation, and hence in the parasite for¬ 
mula. From all these considerations, it is 
clear that the parasite formula determined 
from groups containing febrile cases will 
differ from that based on apparently 
healthy carriers. 
The mixed character of apparently simple 
infections is shown by the fact that with low 
transmission one parasite increases at the 
expense of the others. Thus in Sierra 
Leone, Gordon and Davey found the total 
malaria relatively constant, but the pro¬ 
portion of positives for P. malariae varied 
all the way from 0 to 22 per cent in 15 years. 
In tropical areas where the transmission 
rate is continuously high P. falciparum 
dominates the blood picture at all seasons 
and in all ages. Other species are present 
in due proportion during the first years of 
life, but are encountered more and more 
rarely later on. 
There is finally the differential effect 
exerted on the relative prevalence of the 
various organisms by their specific incuba¬ 
tion periods in the mosquito. That of P. 
vivax may be several days shorter than that 
of P. falciparum, and only half that of P. 
malariae. Since under some climatic con¬ 
ditions, these periods may approximate the 
life span of the insect, they may notably 
influence the parasite formula of the com¬ 
munity. 
Thus the multiform character of the 
plasmodium and the specificity of the im¬ 
munity produce in the malarious com¬ 
munity, as in the constantly exposed indi¬ 
vidual, a continuous immunological activity 
which becomes more and more complex as 
the rate of inoculation increases. The fre¬ 
quency of successive heterologous infec¬ 
tions, the waxing and waning of specific 
tolerances, and the behavior peculiar to 
each species of the parasite, together de¬ 
termine the aspect of malaria, which is re¬ 
flected in a way we are not able fully to 
analyze in the blood picture of the indi¬ 
vidual and the parasite formula of the 
community. 
Endemic and Epidemic Malaria 
The transmission rate gives malaria its 
volume and intensity, but the diversity of 
organisms creates that fluctuating and 
polyvalent immunity which characterizes 
