DIFFERENTIAL DIAGNOSIS OF MALARIAL PARASITES 
61 
(particularly the less experienced) for 
study and for classification of species and 
determination of mixed infections. Schiiff- 
ner’s dots frequently show quite plainly 
here, whereas in the thicker portion they 
may be lacking entirely or may show only 
as a delicate pink area around the parasite. 
Parasites in the Thick Film 
Young trophozoites. In the thick film the 
young parasites of all species often appear, 
not as complete ring forms, hut as chro¬ 
matin dots associated with only a portion 
of the cytoplasmic circle. These forms have 
been described aptly, according to their ap¬ 
pearance, as “interrupted rings, exclama¬ 
tion marks, and swallow forms” (Field and 
LeFleming 1939). When only this stage of 
the parasite is present in the blood, diffi¬ 
culty is encountered in differentiating the 
species. However, if the rings are very 
small and delicate, or if there are many ring 
forms and no older stages of the parasite, 
the species is very likely P. falciparum. 
Half grown and old trophozoites. The 
older trophozoites of P. vivax exhibit a de¬ 
cided tendency for the cytoplasm to be 
fragmented and arranged in a cluster of 
varying-sized particles often with no visible 
connection. This cytoplasm is associated 
with a round or irregularly shaped red or 
magenta mass of chromatin. The pigment 
appears as a yellowish haze or as small light 
brown grains on the cytoplasm. Some of 
the older trophozoites, with one chromatin 
mass, are rather solid and regular in out¬ 
line. Those of P. malariae exhibit a heavy 
dark pigment in the compact cytoplasm 
which gives a dense appearance to many of 
the parasites of this stage. The chromatin 
is often not conspicuous, though it may be 
found in an elongated mass so character¬ 
istic of this stage in the thin film. The older 
trophozoite of P. falciparum is very small, 
often non-vacuolated, the cytoplasm is a 
lighter color than that of P. malariae, and 
the pigment, even at this stage, is usually 
clumped in one or two very small masses. 
Presegmenting schizont. In the preseg¬ 
menting stage of P. vivax the cytoplasm is 
less tenuous than in the trophozoite and the 
parasite is generally more compact in ap¬ 
pearance. The chromatin is divided into 
several more or less irregular masses, and 
as the segmenting stage approaches these 
appear more rounded, and are individually 
associated with smaller amounts of the cyto¬ 
plasm. The pigment meanwhile is gradu¬ 
ally collected into one or more definite 
clumps. This stage of P. malariae is very 
similar to the corresponding one of P. vivax 
and often is difficult to classify in a thick 
film unless more readily recognized stages 
are present also. When this stage of P. 
falciparum is found in the peripheral blood, 
it is small, compact, irregularly shaped, 
with deeply staining cytoplasm in which are 
embedded the vague magenta chromatin di¬ 
visions, and a small dense mass of dark 
pigment. 
Segmenting schizont. The segmenting 
schizonts of the three species resemble very 
closely the same stages in the thin film, 
practically the only difference being the 
absence of the cell outline and a possible 
shrinkage of the parasite. Species may be 
differentiated by the comparative size of 
the parasites and by the number of daugh¬ 
ter cells. It is unusual to find this stage of 
P. falciparum except in severe cases. 
. Gametocytes. In the thick film it is im¬ 
possible to distinguish the macrogameto- 
cytes of either P. vivax or P. malariae 
from the compact, rounded or oval mature 
trophozoites. Microgametecytes of these 
two species are more easily determined be¬ 
cause of their large, usually rounded nu¬ 
cleus surrounded by a small amount of very 
light staining or colorless cytoplasm con¬ 
taining numerous grains of prominent pig¬ 
ment. The nucleus stains more deeply, 
comparatively speaking, than in the thin 
film. There is no other stage that resembles 
this closely. In thick films which have 
dried slowly, exflagellation of fully matured 
microgametoeytes may occasionally be ob¬ 
served. 
The gametocytes of P. falciparum .are 
easily determined by their characteristic 
elongate or sausage-like shape, though it is 
often difficult to differentiate the sexes in 
