112 
MALARIA 
tinely kept at 4° C, at which tempera¬ 
ture vivax sporozoites remain viable only 
40 to 50 days while falciparum sporozoites 
do not live longer than 30 days. James 
and Shute (1926) record one example of 
A. maculipennis which retained viable 
sporozoites for 92 days. Barber (1936b) 
could find no single factor which seemed 
to be responsible for sporozoite degenera¬ 
tion. He saw none in West Africa (1200 
specimens) but found it very commonly 
in Greek Macedonia. It was not associated 
with any degree of humidity nor with the 
food of the mosquitoes. He did not be¬ 
lieve that age or low temperatures were 
necessary factors in the cause of degen¬ 
eration of sporozoites in any of the species 
he studied. It is obvious that the problem 
of analyzing the causes of loss of viability 
of sporozoites is extremely complex. 
It has been acknowledged for a long time 
that the relative humidity of the air sur¬ 
rounding the mosquito has little or no 
direct effect on the development of the 
parasites. Mayne (1930a) found no direct 
effect of humidity upon the numbers of 
mosquitoes of the species Culex fatigans 
and Anopheles stephensi infected with bird 
malaria and P. vivax respectively. In my 
studies with Culex pipiens and avian 
malaria (1941) there was also no measur¬ 
able effect of different degrees of humidity 
upon the numbers or size of oocysts which 
grew in susceptible individuals. 
No extensive investigations have been 
made of the effect of various food sub¬ 
stances upon infection in the mosquito. It 
has been conjectured that alfalfa had an 
antagonistic effect, but Stratman-Thomas 
(1931) has brought epidemiological evi¬ 
dence against this hypothesis, and Mayne 
(1930a) has shown that coumarin from 
alfalfa in a wide range of dilutions did 
not prevent infection in C. fatigans. Rus¬ 
sell and Mohan (1939a, 1939b) have shown 
that larvae of A. stephensi grown in con¬ 
trasting environments grew into adults 
which showed little or no difference in sus¬ 
ceptibility to P. falciparum. Some of the 
larvae were grown in tap water and others 
in water to which cow dung had been 
added. 
Since temperature has such a marked 
influence on the rate of growth of malarial 
oocysts, the question arises as to whether 
the mean temperatures of different indi¬ 
vidual mosquitoes may differ enough to 
account for the large differences in mean 
size of the oocysts in different mosquitoes. 
Although these temperatures have not been 
measured directly, it has been shown (Huff 
1941) that the rates of growth are not 
affected by activity of the infected mos¬ 
quitoes. Dealated mosquitoes and mos¬ 
quitoes kept in light were infected and 
compared with their respective controls, 
winged mosquitoes and those kept in the 
dark. In neither case were there signifi¬ 
cant differences in mean sizes of oocysts 
in the different lots of mosquitoes. 
By way of summary, it can probably be 
safely conjectured that of the factors influ¬ 
encing infection of Anopheles with ma¬ 
larial parasites none seems to be more im¬ 
portant than those present in the inherent 
make-up of the individual mosquito, and 
it can be stated that, of the environmental 
factors so far studied, none seems to play 
any appreciable role except temperature 
which, indeed, plays a very important 
part in determining whether or not a mos¬ 
quito may be a good vector of malaria. 
Hardly any other phase of malariology 
has been so badly neglected as the study 
of all factors influencing infection in the 
mosquito. 
