132 
MALARIA 
were very unreliable until recently. The 
writer believes, with Faust, that the mor¬ 
tality statistics of malaria, as kept by the 
various Boards of Health of the southern 
states, are of such accuracy that the de¬ 
ductions based upon them are warranted, 
and that two cyclical variations of long 
amplitude have occurred in the incidence 
of malaria in these states during the past 
15 years. 
Causative Factors 
The factors causing cyclical variations 
of short and long amplitude are many and 
have to do both with the mosquitoes trans¬ 
mitting these infections and with the re¬ 
cipient of the infection, i.e., man. We are 
familiar with many of the factors causing 
cyclical variations of short amplitude but 
we know much less of those having to do 
with cyclical variations of long amplitude. 
Factors operating on the vectors and 
parasites. Of the many factors operative 
in causing cyclical variations in the in¬ 
cidence of malaria of short amplitude, 
climate and season are probably the most 
important, not only in influencing the 
breeding and development of the transmit¬ 
ting mosquitoes but also in affecting the 
growth and development of the malaria 
plasmodia within the transmitting mos¬ 
quitoes. It has already been stated that in 
temperate regions, as the United States, 
malaria is most prevalent during the sum¬ 
mer and fall months and least prevalent in 
the northern portions of the United States, 
because of the colder climate which does 
not favor the breeding of anopheline mos¬ 
quitoes; or, if it does not interfere with 
the breeding of mosquitoes, does prevent 
the development of the malaria plasmodia 
within the mosquitoes. 
The influence of temperature upon the 
development of the various species of 
malaria plasmodia in mosquitoes has long 
been known and each species of plasmodium 
has an optimum temperature in which de¬ 
velopment occurs most rapidly and com¬ 
pletely. Thus P. vivax develops best in the 
mosquito at a temperature of about 25° C 
and tPe mosquito becomes infective in about 
eleven days; P. malariae develops best in 
the mosquito at a temperature of 22° C, 
and the mosquito becomes infective in from 
18 to 21 days; while P. falciparum com¬ 
pletes its development in the mosquito best 
at a temperature of 30° C, and the mos¬ 
quito becomes infective in from 10 to 12 
days. This relationship of temperature to 
the development of the plasmodia in the 
transmitting mosquitoes readily explains 
why infections with the latter species of 
plasmodium occurs in temperate regions in 
the summer and early fall, when the high¬ 
est temperatures are present, and are never 
observed, as initial infections, in the spring, 
when temperatures are too low for success¬ 
ful development in the mosquito. 
Humidity is also of great importance in 
cyclical variations in malarial incidence. 
If there is not sufficient humidity present 
the malaria plasmodia will not develop in 
mosquitoes, and the nearer it is to satura¬ 
tion the more rapid is the development in 
these insects. Humidity also has a pro¬ 
found effect upon the breeding and develop¬ 
ment of the transmitting mosquitoes, and 
the amount and character of the rainfall 
very largely governs the number of 
anopheline mosquitoes in any locality. 
Thus, although malaria is present through¬ 
out the year in the tropics, it is always 
greatest in incidence during the latter por¬ 
tion of the rainy season and immediately 
following that season, and least prevalent 
during the dry season; while in temperate 
regions, as the United States, these infec¬ 
tions are most common in the summer and 
early fall when conditions as regards rain¬ 
fall and temperature are most favorable. 
The species of anopheline mosquitoes 
present in any locality has much to do with 
cyclical variations. Many species of anoph- 
elines do not transmit malaria while others 
are either good or poor transmitters. If 
species which are good transmitters are 
present, i.e., those in which the malaria 
plasmodia develop best, and non-immunes 
are present in large numbers, malaria will 
occur in epidemic form, while if smaller 
numbers of infective mosquitoes are pres¬ 
ent and few non-immunes, malaria will be 
rarely observed. Cyclical variations in the 
