(1973) n°3 - Royal Academy for Overseas Sciences

— 518 —
accept criteria based on exoerythrocytic schizogony; if they paid
sufficient attention to these two requirements they would probably
change their minds. Another argument that is sometimes
advanced against their value in the identification of malaria
parasites of animals, is that the methods necessary for eliciting
the diagnostic characters cannot be applied in the field or are
too difficult. This is of course an entirely irrelevant consideration
for the taxonomist. It is true that the blood stages are
much easier to observe, but the details of sporogony and exoerythrocytic
schizogony are invaluable. W ith full knowledge of
all the data, it should be possible to identify the species (and
frequently the subspecies) of a primate malaria parasite by its
liver stages alone.
One is sometimes asked what is the practical value of knowledge
about the tissue stages. The simplest answer is that these
are the forms which are the target of drug prophylaxis of the
disease. Until one had learnt where they grew and their appearance,
it was impossible to observe directly the effect of new
compounds on this stage of the parasite. The perfect prophylactic
drug however has yet to be discovered.
Unlike the stages in the blood and in the mosquito, exoerythrocytic
schizonts are most unlikely ever to be found in natural
infections in man. Their appearance is ephemeral and their
numbers minute in the extreme. The tissue stages of the primate
species only occur after sporozoite infection; they cannot arise
from blood forms. As such they cause no harm to the host.
Almost certainly the exoerythrocytic schizonts are the most primitive
or original stages in the life history of the parasite and
represent the survival of a remote coccidian ancestor.
REFERENCES
A r a g â o , H. d e B. (1908) : Archiv. Protistenk., 12, 154.
B r a y , R.S. (1957): Studies on the Exoerythrocytic Cycle of Plasmodium
(H.K. Lewis, London).
— (I960): Amer. J. trop. M ed. Hyg., 9, 455.
— & G a r n h a m , P.C.C. (1962) : Indian J. Malariol., 16, 153.
C o a t n e y , G.R., C o l l in s , W.E., W a r r e n , M. and C o n t a c o s , P.G.
(1971): The Primate Malarias (U.S. Dept. Hlth. Educ. Welf.
Bethesda).