180 
MALARIA 
(a) The detection of parasites. The de¬ 
tection of the parasites in a blood smear 
is incontestable proof of the existence of a 
malarial infection. Excepting for a period 
of a few days to a week subsequent to the 
clinical onset of an attack in highly sus¬ 
ceptible untreated patients, they are more 
or less readily detectable on microscopical 
examination of blood smears taken during 
an attack. As clinical activity diminishes 
and the infection becomes latent their num¬ 
bers gradually diminish to submicroscop- 
ical levels. The importance of the subject 
justifies its separate consideration in an 
independent section ( q.v .). It is deplored 
that the method is not more widely used 
by medical practitioners. 
In order to facilitate the detection of 
parasites when their density is too low to 
be revealed in ordinary thick smears, 
various procedures have been devised which 
can only be briefly mentioned. Bass (1915) 
developed a technique by which the para¬ 
sites in 10 cc of blood were concentrated by 
centrifugation at the top of the blood 
column, from which thick smears were 
made. The culture technique of Bass 
(1912), as modified by Thomson and 
Thomson (1913), is at times used by Indian 
clinicians to aid in the detection of para¬ 
sites (Knowles and Senior-White 1927). 
Reliable means to reactivate latent infec¬ 
tions, either to facilitate diagnosis or to 
prolong therapeutic infections, are not 
available. The administration of different 
drugs for this purpose shows that the re¬ 
sponses evoked in general fall into two 
classes. Some, such as epinephrine hydro¬ 
chloride and amyl nitrite, may produce an 
increase in the number of parasites in the 
peripheral circulation within a few hours 
of their administration. Since the time 
elapsing until the increase in density is 
observed is too short to attribute this result 
to their multiplication,'they must have been 
expelled from some viscus. The adminis¬ 
tration of others, such as tuberculin and 
typhoid vaccine, may bring about a gradual 
rise extending over a period of several days, 
indicative of multiplication. The circum¬ 
stances suggest that in the latter case the 
immune mechanism has been depressed. 
The most reliable means of excluding 
the existence of an otherwise undetectable 
latent infection is to subinoculate a sus¬ 
ceptible patient with 10 or more cubic 
centimeters of the blood of the patient 
suspected of being infected. The oppor¬ 
tunities to apply this procedure are limited 
to institutions where malarial therapy is 
practiced. 
(b) Splenic enlargement or spleno¬ 
megaly. Enlargement of the spleen may 
be detected by palpation within a few days 
after the clinical onset. The rate of in¬ 
crease in size is more rapid in persons who 
have some degree of immunity, while the 
degree of enlargement is roughly propor¬ 
tional to the duration of the clinical attack. 
On cessation of the clinical attack, particu¬ 
larly if interrupted by treatment, the en¬ 
largement rapidly diminishes and may al¬ 
together subside. The persistence of en¬ 
largement implies the continuance of a 
latent infection and the probability of 
relapse. Further enlargement will follow 
a renewal of clinical activity (Boyd 1930b). 
(c) Hematological changes. The hema¬ 
tological changes are significant from the 
standpoint of the symptoms exhibited, but 
at the best only supply information of 
indirect or inferential diagnostic value. 
As may be expected in an infection the 
parasites of which prey on the erythrocytes, 
malarial infections are characterized by 
marked destruction of red blood cells and 
of hemoglobin. Consequently anemia is a 
prominent symptom. Rate of destruction 
varies with the species of parasite, and is 
often more rapid than can be explained by 
the assumption that each parasite in the 
course of its growth exhausts only a single 
host cell. Some of this discrepancy may 
be explained by the phagocytosis of both 
infected and uninfected erythrocytes in the 
splenic pulp, while hemolysis also may 
occur. 
The different parasites usually vary 
materially in the maximum density which 
they ordinarily attain and hence differently 
influence the rate of destruction. The 
densities in quartan are usually the lowest, 
rarely reaching 20,000 per cmm; in vivax 
malaria, 50,000 per cmm is but rarely ex- 
