MALARIA AND THE COMMUNITY 
By L. W. HACKETT 
INTERNATIONAL HEALTH DIVISION, ROCKEFELLER FOUNDATION, BUENOS AIRES, ARGENTINA 
The most important aspect of malaria as 
it affects the human community is its en- 
demicity, or degree of prevalence. This 
covers a wide range of manifestations, from 
sporadic cases to the hyperendemic situa¬ 
tion in which few persons escape infection. 
But the more intensely malarious communi¬ 
ties differ from the less malarious ones in 
more than the mere number of cases. With 
increasing intensity new factors intervene 
such as changes in the vitality and relative 
prevalence of the different species of ma¬ 
laria parasite, the accumulation of mixed 
infections and chronic cases in the popula¬ 
tion, and most important of all, a develop¬ 
ing group tolerance to the effects of the 
disease which at a certain height begins to 
alter the picture of malaria in a radical 
way. 
Since the nature and growth of this re¬ 
sistance is affected considerably by the 
racial composition of the population, the 
nature of the climate and such an incon¬ 
stant factor as the relative prevalence of 
the plasmodial species, malaria assumes a 
particular expression in each community 
which is the function of a great many vari¬ 
ables. Nevertheless, it takes its essential 
character from the rate of transmission, 
which is the frequency with which its in¬ 
habitants are bitten by infective anopheles 
—technically, the number of infective bites 
per 100 persons per unit of time. The fre¬ 
quency of inoculation will evidently depend 
on the numbers of the anopheline vectors, 
and of their sources of infection, the game- 
tocyte carriers in the population. 
The Anopheline Factor 
The efficiency of the local anopheles as 
vectors of malaria depends not only on 
their quantity, but on other factors of great 
importance, such as their susceptibility to 
infection, their longevity, their season of 
prevalence in relation to the periodicity of 
the local plasmodia, the accessibility of 
their hosts, the rapidity of passage and 
hence the vitality of the malarial organism, 
environmental conditions such as extremes 
of temperature and humidity, and many 
secondary circumstances which influence 
the host-parasite relationship but which 
fall outside the scope of this paper. 
More important than any of these, to the 
human community, is the degree to which 
the local anopheles resort to man for food. 
Anopheles in general obtain their blood 
meals from a wide variety of sources, but 
it has been observed that any one species 
appears to have a range of preferred hosts, 
not necessarily the same from place to 
place, in the presence of which it ignores 
other accessible sources of blood. Man may 
or may not be included in this preferred 
group. If not, the anopheles are harmless, 
regardless of their numbers, as shown by 
the numerous malaria-free areas which 
produce anopheles in variety and abun¬ 
dance. If on the other hand, man is the 
host of predilection, malaria will be extra¬ 
ordinarily severe and spare no one in the 
community, as seen throughout the range 
of A. gambiae in Africa and in the recently 
invaded area of northeastern Brazil. Be¬ 
tween these two extremes fall the other 200 
odd species of anopheles, and the extent to 
which each acts as a vector is closely bound 
up with the degree of its host relationship 
to man. 
Our knowledge of the degree of contact 
of the various species with man was based, 
before 1930, on the place of capture and 
the percentage of females found infected 
in nature. Later the precipitin test as 
modified by Rice and Barber (1935) per¬ 
mitted the determination of the proportion 
of insects which give a positive reaction for 
human blood. This has been termed the 
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