THE TAXONOMY OF THE HUMAN MALARIA 
PARASITES WITH NOTES ON THE PRIN¬ 
CIPAL AMERICAN STRAINS 
By G. ROBERT COATNEY and MARTIN D. YOUNG 
U. S. PUBLIC HEALTH SERVICE, COLUMBIA, S. C. 
The plasmodia as single-celled animals 
belong to a large and heterogeneous group, 
the phylum Protozoa. The presence of syn- 
gamy places them in the subphylum Plas- 
modroma. As spore producers, they belong 
to the class Sporozoa and as the spores are 
formed at the end of the cycle and have 
neither polar capsules nor polar filaments 
they fall in the subclass Telosporidia. Hav¬ 
ing motile zygotes and sporozoites without 
envelopes, they belong to the order Haemo- 
sporidia. They are in the family Plas- 
modiidae, which is characterized by schi¬ 
zogony in the peripheral blood, pigment 
production, and the mosquito as an inverte¬ 
brate host. This family has a single genus, 
Plasmodium, which has the same character¬ 
istics as the family. 
Valid Species 
Until comparatively recently certain 
groups of malaria students were reluctant 
to accept the idea of there being more than 
one species of human malaria, even though 
Golgi (1886a) long ago carefully disin- 
guished between tertian and quartan ma¬ 
laria fevers. Part of the confusion resulted 
from observations on mixed infections 
where the different morphological types 
encountered were erroneously thought to 
comprise an entity. 
Specific status of a species, in line with 
the accepted zoological practice, is assessed 
on the basis of certain morphological char¬ 
acters which must remain constant during 
the life of the parasite in the human host 
and in the insect vector. Consequently 
with the advent of malaria therapy oppor¬ 
tunities were afforded for controlled studies 
of both the asexual and sexual cycle under 
laboratory conditions. The results of these 
studies have confirmed that there are sev¬ 
eral species of human plasmodia and that 
these separate species are made up of races 
or strains, which, although as yet indistin¬ 
guishable on morphological grounds, can be 
separated on the basis of antigenic proper¬ 
ties, effect of drugs, virulence, and infec- 
tivity. 
On the basis of available knowledge the 
accepted species of human plasmodia are: 
P. vivax, P. malariae, P. falciparum, and 
P. ovale. 
There was a fair quantity of data in sup¬ 
port of the acceptance of the validity of the 
three classical species prior to 1918, but 
these data had been obtained from the study 
of natural infections or from limited obser¬ 
vations on induced cases in volunteers. We 
now have extensive experimental support¬ 
ing evidence from the Horton Malaria 
Therapy Center in England, Socola Malaria 
Therapy Center in Roumania, Station for 
Malaria Research, Tallahassee, Florida, the 
Williams Malaria Research Laboratory, Co¬ 
lumbia, S. C., and many others. For P. 
ovale, however, the experimental data rest 
for the most part on the observations of 
James and his associates at the Horton Ma¬ 
laria Therapy Center in England, and they 
have presented ample evidence to justify 
ranking P. ovale as a separate species; it is 
specifically different in morphology from 
the other three species, and these differences 
are constant in all infections resulting from 
direct blood inoculation. In the mosquito, 
the character and arrangement of the pig¬ 
ment in the oocysts is decidedly character¬ 
istic and the sporozoites found in the sali¬ 
vary glands are much smaller than those of 
P. vivax, its nearest relative. In the infec¬ 
tions resulting from mosquito bites the 
