INFECTION OF THE INTERMEDIATE HOST 
165 
lows natural inoculations, even when the 
mosquitoes are proved to be infected. Some 
of our results are shown in Table I. 
TABLE I 
Proportion op Successful Infections Subse¬ 
quent to the Application of Varying 
Numbers of Demonstrably Infected 
Mosquitoes 
Parasite 
Number 
of mos¬ 
quitoes 
Persons 
inocu¬ 
lated 
Takes 
Per 
cent 
takes 
P. falciparum 
1 
11 
7 
63.6 
2-5 
42 
22 
52.3 
6-10 
39 
24 
61.4 
11-15 
11 
7 
63.6 
Total 
103 
60 
58.3 
P. vivax 
1 
54 
43 
79.6 
2-5 
150 
123 
81.8 
6-10 
155 
130 
84.2 
Ill- 
35 
24 
68.6 
Total 
394 
320 
81.3 
P. malariae 
2-5 
8 
5 
62.5 
These data suggest that the presumptive 
dosage of sporozoites has had little bearing 
on a successful natural inoculation. 
The Intrinsic Incubation Period 
As in the case with inoculation by any 
infecting agent, a period of clinical quies¬ 
cence or incubation ordinarily intervenes 
between inoculation with malarial para¬ 
sites and the onset of symptoms. Practical 
criteria to mark its termination are either 
(a) the first detection of parasites in the 
peripheral blood, or (b) the first elevation 
of temperature to 100° F. In most infec¬ 
tions this period can be explained as the 
interval necessary for a scanty number of 
invading organisms to multiply until their 
numbers are sufficient to produce an effect 
on the host. In malarial infections artifi¬ 
cially induced this interpretation appears 
adequate, but subsequent to natural inocu¬ 
lation the operation of another factor 
appears likely. The incubation period 
subsequent to natural inoculation of the 
intermediate host is called the intrinsic 
incubation period, to distinguish it from 
the period of non-infectiousness following 
infection of the definitive host, which is 
known as the extrinsic incubation period, 
q.v. 
Although the duration of the incubation 
period following artificially induced infec¬ 
tions may vary with the tissues into which 
the parasites are introduced, yet if the 
intravenous route of transfer is followed, 
the duration to a great degree will vary 
inversely with the dosage of trophozoites 
administered (Table II). Sufficient para¬ 
sites may even be given to make them im¬ 
mediately microscopically detectable in the 
TABLE II 
Showing the Inverse Relation Between the Dosage of Trophozoites and the Length of the 
Incubation Pfriod in Artificially Induced Malaria (Intravenous) 
Parasite 
Dose* 
Days from intravenous inoculation to: 
First parasites 
First temperature 100° 
C 
„ .. r „ 3 8 13 18 23 28 
7 12 17 22 27 + 
Total 
„ . , „ 3 8 13 18 23 28 
7 12 17 22 27 + 
Total 
P. vivax 
0-1 
. 1 24 7 . 
32 
. 22 10 . 
32 
1-100 
1 3 1 . 
5 
. 3 2 . 
5 
101 + 
2 . 
2 
1 1 .. 
2 
Total 
3 3 1 1 24 7 . 
39 
1 1 3 2 22 10 . 
39 
P. malariae 
0-1 
1 
1 
1 
1 
1-100 
... 5 2 6 4 1 2 . 
20 
. 7 5 . 6 11 
20 
101 + 
1 2 1 . 1 . 
5 
... 1 ... 3 . 1 
5 
Total 
1 727 42 2 1 ... 
26 
... 1 ... 10 5 . 6 13 
26 
* Doses are in millions of trophozoites. 
