168 
MALARIA 
(C) P. malariae—2 strains 
Days to 
rise of 
temperature 
Days to detection of parasites 
(prepatent period) 
Total 
23 32 34 37 
30 
1 . 
1 
35 
1 . 
1 
36 
. 1 
1 
40 
. 1 
1 
49 
. 1 
1 
Total 
1112 
5 
or do not coincide in the data given is as 
follows: 
TABLE Y 
Variations in Incubation Period 
Falciparum 
Vivax 
Malariae 
Parasites & fever 
same day . 
10 
52 
0 
Parasites precede 
129 
180 
5 \ 
Fever precedes ... 
16 
70 
0 
No fever. 
2 
11 
0 
Total. 
157 
313 
5 
From this table it is to be seen that the 
parasites are usually, but not always, de¬ 
tectable on or before the clinical onset. 
Most clinical onsets have occurred between 
the 11th and 14th days, while attention 
should be especially directed to the pro¬ 
longed intrinsic incubation period of quar¬ 
tan. We have never observed the incuba¬ 
tion periods of falciparum to exceed the 
limits shown in Table IV, but occasionally 
we have found longer periods for vivax. 
The number of cases of this type which 
we have observed contemporary to the cases 
with normal limits are listed in Table VI. 
These five cases constitute about 2 per 
cent of the McCoy strain inoculations. 
Other strains of P. vivax propagated 
abroad have shown a much higher propor¬ 
tion of inoculations followed by deferred 
onsets. Thus autochthonous Dutch strains 
employed in Holland have been followed 
by deferred onsets in 38 per cent of those 
inoculated, but when the use of the exotic 
Madagascar strain was begun, the propor- 
TABLE YI 
Protracted Incubation in Simple Vivax 
Inoculations 
Case 
num¬ 
ber 
Date 
inoc. 
Mosquitoes 
Days to 
Lot No. pos. 
First 
parasites 
First 
fever 
106 
12/15/32 
122 
3 
97 
104 
108 
12/15/32 
122 
3 
85 
86 
285 
6/21/37 
570 
4 
304 
302 
305 
11/19/37 
690 
4 
280 
302 
340 
8/ 1/38 
747 
7 
30 
56 
tion dropped to six per cent (Swellengrebel 
and de Buck 1938). 
Autochthonous vivax malaria in Holland 
shows a high vernal incidence. Schuffner, 
Korteweg and Swellengrebel (1929) dem¬ 
onstrated that autumnal vivax inoculations 
by lightly infected mosquitoes gave rise to 
clinical attacks in the following summer, 
while heavier autumnal inoculations re¬ 
sulted in autumnal attacks after “normal” 
incubations, which were followed by numer¬ 
ous vernal relapses in the spring (Swellen¬ 
grebel 1933). James, Nicol and Shute 
(1936) also express the opinion that light 
inoculations with sporozoites are commonly 
followed by protracted incubation periods, 
and state that they also occur in persons 
who are under the influence of atebrine at 
the time they are inoculated. Our own 
experience does not suggest that small dos¬ 
age is a universal explanation for deferred 
onsets. 
The duration of the intrinsic incubation 
period might conceivably show a variation 
with the strain of parasite. Thus our 
Coker and Long strains have given the re¬ 
sults listed in Table VII. 
Although the Long strain has shown 
greater uniformity than the Coker in the 
days elapsing to the first detection of para¬ 
sites (Boyd and Kitchen 1937a), the mean 
duration of their clinical incubation periods 
is practically identical. 
The clinical incubation periods of two 
vivax strains are shown in Table VIII. 
The mean durations of incubation for 
these two strains of widely different prove- 
