CYCLICAL VAFIATION IN INCIDENCE OF MALARIA 
133 
prevalence of these infections are caused 
by cyclical variations in the number of 
efficient mosquito transmitters, depending 
upon conditions favorable to the breeding 
and development of these insects. In the 
United States, A. quadrimaculatus and A. 
maculipennis are the mosquitoes most fre¬ 
quently concerned in the transmission of 
malaria, especially the former, and wher¬ 
ever these species are present, provided 
human ‘ ‘ carriers ’ ’ are also present, malaria 
is more or less frequently encountered. It 
is probable that cyclical variations in the 
incidence of malaria of both short and long 
amplitude are due to variations in the 
species of anopheline mosquitoes present. 
The influence of the species of anophe- 
lines upon the incidence of malaria is strik¬ 
ingly illustrated by the introduction of A. 
gambiae into Brazil from Africa, as re¬ 
corded by Barber (1940). This species is 
especially active as a transmitter of malaria 
and its breeding habits are such as to favor 
its infection from man. Since its introduc¬ 
tion into Brazil it has spread rapidly and 
malaria has greatly increased in amount 
and has become epidemic in many localities. 
Its spread into other countries of South 
America, and even into the United States, 
is possible and would undoubtedly be ac¬ 
companied by a great rise in malarial in¬ 
cidence and Barber well says: 
There is no doubt that this invasion of gambiae 
threatens the Americas with a catastrophe in com¬ 
parison with which ordinary pestilence, conflagra¬ 
tion, or even war are but small and temporary 
calamities. Gambiae literally enters into the very 
veins of a country and may remain to plague it 
for centuries. 
The number of infective mosquitoes 
present in any locality, other things being 
equal, will determine the amount of ma¬ 
laria there present. It is not essential that 
a large percentage of anopheline mos¬ 
quitoes be infective, for even in regions 
where malaria is common the percentage 
of infective mosquitoes, as determined by 
dissection, has been found to vary between 
1.5 to 3 per cent, but in hyperendemic areas 
the percentage is higher, usually varying 
between 6 and 10 per cent. In the ease of 
A. gambiae Barber (1940) found a sporo¬ 
zoite rate of 30.5 per cent in a town in 
West Africa, but the average for the sur¬ 
rounding districts was only 6.5 per cent. 
In South America, Barber found that the 
sporozoite rate of this species of mosquito 
varied from 2.7 per cent to 10 per cent, 
according to the length of time the species 
had been transmitting malaria in a par¬ 
ticular locality. 
The apparently low percentage of in¬ 
fected mosquitoes in regions where there 
are many human carriers of the plasmodia 
is largely due to the fact that only a small 
percentage of mosquitoes biting a carrier 
becomes infected and because repeated 
feedings are usually necessary before the 
insect becomes infected. Again, it has been 
shown that certain individuals are good 
infectors of mosquitoes while others seldom 
transmit the infection to mosquitoes, so 
that the number of efficient human carriers 
in any locality has much to do with the 
incidence of malaria. 
Migration. The migration of non- 
immune individuals into a malarial region 
is always followed by a rise in the inci¬ 
dence, and if such migrations occur at 
regular intervals, we may have cyclical 
variations in incidence of both short and 
long amplitude, depending upon the length 
of time between such migrations. If effi¬ 
cient human carriers of malaria are pres¬ 
ent, the mosquitoes of the region will be 
infected and it follows that the arrival of 
large numbers of non-immune individuals 
will be followed by the infection of many 
of them and a marked rise in the incidence 
of the malarial fevers in that locality. 
After infancy and childhood, the native 
population of such regions will have ac¬ 
quired a considerable amount of premuni¬ 
tion 1 and immunity, so that symptoms are 
not frequently observed in the adult popu¬ 
lation, although infants and children suffer 
markedly from these infections. If a large 
number of individuals from regions free 
from malaria migrate into such a commu- 
1 In the present instance ‘ ‘ premunition ’ ’ is em¬ 
ployed to designate a state of resistance conferred 
by a substance or substances formed while a para¬ 
site is residing in the host and the resistance 
terminates with the disappearance of the germ. 
