a1 From the Molteno Institute, University of Cambridge
In Plasmodium gallinaceum, parasitic in domestic fowls, in addition to the schizogony in red blood corpuscles, schizogony also occurs in monocytes and cells of the reticulo-endothelial system. This schizogony in cells other than erythrocytes may be termed, for convenience, exo-erythrocytic schizogony.
Exo-erythrocytic schizonts are characterized by never having malarial pigment (as they live in cells devoid of haemoglobin), by growing to a large size and forming numerous merozoites, up to 50 or 60 or even more.
Exo-erythrocytic schizonts are not confined to leucocytes of the blood stream, but can also develop in fixed endothelial cells of organs such as the brain, lung, liver and spleen.
The brain is an especially important focus of development and, after sporozoite infection, it is the first organ in which exo-erythrocytic schizonts develop. These schizonts may be found in capillaries of the brain as soon as parasitized erythrocytes are present in the peripheral circulation, but as yet they have not been found before parasites-are present in the peripheral blood.
In the brain rows of schizonts may develop and ultimately occlude most of the capillaries in the brain. This blockage of capillaries results in symptoms of general paralysis in the infected birds and death follows in a few days; and it frequently occurs in birds which apparently have been cured of the peripheral infection by treatment with quinine.
The exo-erythrocytic stages of P. gallinaceum occur in birds which have been inoculated with blood or with sporozoites.
During growth, the body of the schizont breaks up into a number of masses or cytomeres, on the periphery of which the merozoites are developed on regularly arranged rows of digitiform processes. A mass of chromatin migrates to the distal end of each process and is cut off to form a merozoite.
The mature schizont consists of a mass of irregular merozoites enclosed in a sac-like membrane which may be the remains of the host cell. The merozoites are composed almost entirely of chromatin and when they are fully developed practically no cytoplasm of the schizont remains.