CYCLICAL VARIATION IN THE INCIDENCE 
OF MALARIA 
By CHARLES F. CRAIG 
SAN ANTONIO, TEXAS 
Cyclical variations in the incidence of 
the malarial fevers have been noted by 
practically all investigators since the etiol¬ 
ogy of these fevers has been understood, 
and long before we were acquainted with 
the malaria plasmodia writers upon the 
subject called attention to the seasonal 
variations and the occurrence of epidemics 
of these fevers in regions usually free or in 
endemic centers. These cyclical variations 
were noted to be of two types, i.e., of short 
amplitude, or seasonal, and of long ampli¬ 
tude. 
Cyclical Variations of Short 
Amplitude 
Variations in incidence of short ampli¬ 
tude are usually called seasonal variations 
and occur to a greater or lesser extent in 
all malarial regions, whether endemic or 
hyperendemic. In the tropics these varia¬ 
tions are connected with the rainy and dry 
seasons, but in temperate regions, as the 
United States, seasonal variations are con¬ 
fined to certain periods in the year and 
depend upon the presence of temperatures 
favorable or unfavorable to the breeding 
and development of the mosquitoes trans¬ 
mitting the malarial infections or to the 
development of the malaria plasmodia 
within the transmitting mosquitoes. Thus, 
infections with P. vivax, the benign tertian 
plasmodium, occur in the spring, summer 
and autumn months in temperate regions, 
while infections with P. falciparum, the 
malignant tertian or estivo-autumnal plas¬ 
modium, occur almost entirely in the sum¬ 
mer and autumn. One never observes an 
initial infection with P. falciparum in tem¬ 
perate regions much before the middle of 
June, while the vast majority of such infec¬ 
tions occur in the late summer and early 
fall. The name “estivo-autumnal” is, 
therefore, truly descriptive of the occur¬ 
rence of this type of malaria in the southern 
parts of the United States, where it is 
prevalent. Cyclical variations in the in¬ 
cidence of malaria of short amplitude occur 
yearly and in some localities are of short 
duration, due to seasonal conditions en¬ 
tirely. 
Cyclical Variations of Long 
Amplitude 
Many malariologists in other countries 
have recorded cyclical variations in the in¬ 
cidence of malaria of long amplitude, in 
which increases in the amount of malaria 
present have occurred at intervals of years 
but with fair regularity. In the United 
States the only observer who has investi¬ 
gated this subject is Faust (1933, 1939b), 
who has been able to show that such varia¬ 
tions occur in the mortality curve of ma¬ 
laria in the southern states, and, pre¬ 
sumably, in the morbidity curve, and that 
these long-interval variations in the in¬ 
cidence have been more or less regular in 
type. 
Since 1930, Faust has reported yearly 
upon the malaria mortality in the southern 
states and his observations have demon¬ 
strated that, beginning in 1927, the malarial 
death rate rose slowly to a peak in 1928- 
1929, and then receded to a low point in 
1931-1932. In 1933 the mortality rate 
again rose quite sharply and then slowly 
receded until a low point was reached in 
1939. 
While it is admitted that mortality rates 
in malaria may not be an absolutely correct 
index of the incidence of these infections, 
owing to the possibility of many mistaken 
diagnoses, it is the only index that Faust 
could employ, as the morbidity statistics of 
the malarial fevers in the southern states 
