TRANSMISSION OP MALARIA BY ANOPHELES IN NORTH AMERICA 
117 
maintained for 7 years in an insectary and 
another strain collected out of doors. In¬ 
fection occurred in 63 per cent of the 
insectary-bred mosquitoes and in 73.4 per 
cent of the wild ones. Boyd (1940c) ob¬ 
served that with large numbers of mos¬ 
quitoes the incidence of P. vivax infections 
revealed by stomach or gland dissections 
will be in substantial agreement. 
Boyd and Kitchen observed that in 180 
successful inoculations of P. vivax malaria 
out of 182 from 8 to 23 days elapsed before 
the first microscopic detection of parasites. 
Apparently the incubation period was not 
influenced by the period of intrinsic incuba¬ 
tion in the source of infection, or the period 
of extrinsic incubation in the mosquitoes. 
It was influenced inversely by the number 
of mosquitoes used. They concluded that 
the varying susceptibility of human hosts 
was more important in determining the 
length of the incubation period than any 
discernible factors modifying the sporo¬ 
zoites. 
P. falciparum. Thayer (1900) was also 
the first to report infection of A. quadri- 
maculatus with P. falciparum. Boyd 
(1934) observed that the incubation period 
in A. quadrimaculatus, at 20° C, is 23 days. 
The minimal parasite density required to 
infect this mosquito is about 11 males and 
11 females per 100 leucocytes (Boyd, Strat- 
man-Thomas and Kitchen 1935). Boyd, 
Kitchen and Kupper (1937) reported the 
infection of 923, or 46.4 per cent, of 1,992 
insectary-bred A. quadrimaculatus. They 
concluded that “While the results ob¬ 
tained appeared to indicate that A. quadri¬ 
maculatus is a less efficient host to P. falci¬ 
parum than to P. vivax, and while a 
definite proportion of a large series of 
mosquitoes appear to be refractory to each 
species, yet in view of the fact that given 
a sufficiently high gametocyte density A. 
quadrimaculatus can be infected in ap¬ 
proximately the same degree with P. falci¬ 
parum as with P. vivax, we incline to the 
opinion that the differences noted may more 
properly be attributed to characteristics of 
the parasites themselves than to a differ¬ 
ential susceptibility on the part of A. 
quadrimaculatus.” Boyd and Kitchen 
(1938b), from a study of 230 lots of A. 
quadrimaculatus infected with P. vivax 
and 166 lots infected with P. falciparum, 
noted that there was a higher percentage 
of P. vivax infections in mosquitoes that 
fed when exflagellation was demonstrable, 
but that no such difference occurred with 
P. falciparum. Boyd and Kitchen (1937b) 
determined that in the experimental infec¬ 
tion of A. quadrimaculatus, P. vivax game- 
tocytes are about 10 times as effective as 
are P. falciparum gametocytes. After the 
inoculation of patients with P. vivax, fully 
matured infectious gametocytes are pres¬ 
ent within 5 days of the appearance of 
parasites and they are present for some 
time after the end of the clinical attack. 
Some of the mosquitoes were infected 
when the gametocyte density was less than 
10 per cmm. P. falciparum gametocytes 
were not found until 10 days after the ap¬ 
pearance of parasites and in some instances 
not before the end of the primary attack. 
Submicroscopic densities of gametocytes 
did not cause infection, and densities less 
than 100 per cmm generally failed to 
infect. Boyd, Kitchen and Kupper (1937) 
have shown experimentally that A. quad¬ 
rimaculatus can be infected simultaneously 
with two different species of malarial para¬ 
sites and that within the mosquito neither 
species appears to injure or inhibit the 
other. Such mosquitoes can transmit 
both parasites to man and produce infec¬ 
tion. Boyd and Jobbins (1940) compared 
the susceptibility of A. quadrimaculatus 
from Florida and A. albimanus from Pan¬ 
ama to strains of P. falciparum from 
Florida, Mexico and Panama. The infec¬ 
tion rates were (a) A. albimanus, to Flor¬ 
ida parasites 7.5 per cent, to Mexican 
parasites 13.5 per cent and to Panaman 
strain 13.6 per cent; (b) A. quadrimacu- 
latus, to Florida strain 48.9 per cent, to 
Mexican strain 57.1 per cent, to Panaman 
strain 17.6 per cent. 
P. malariae. Beyer, Pothier, Couret and 
Lemann (1902b) reported the experimen¬ 
tal infection of 2 of 5 A. quadrimaculatus 
with P. malariae. Boyd and Stratman- 
