Exoerythrocytic Stages in the Asexual Cycle of Plasmodium Circumflexum

Abstract
Summary A total of thirty-six birds infected with Plasmodium circumflexum have been examined for exoerythrocytic stages. Fifteen of these carried chronic infections and twenty-one acute. Sixteen of the twenty-one acute infections were found to have exoerythrocytic stages. The strains with which these birds were infected came from a song sparrow caught in Syracuse (five cases), from a white-throated sparrow also caught in Syracuse (2 cases), from a red-winged black bird caught on Cape Cod (six cases), and from a thrush caught in Germany (three cases). There was evidence of exoerythrocytic infection in only one of the chronic cases which was a relapse terminating in the death of the bird. The following conclusions seem justified: 1. Schizogony in infections with Plasmodium circumflexum may occur in endothelial and lymphoid-macrophage (reticuloendothelial) cells as well as in the erythrocytes. 2. The organs most commonly infected seem to be the lungs, spleen, liver, bone marrow, brain, heart muscle, and ovaries. Parasites have been most frequently found in the lungs, and have also been present in greater numbers there than elsewhere. 3. In the brain the cells infected are the endothelial cells, but in other organs the monocytes are most commonly parasitized. But parasites are also to be found in lymphocytes (both large and small), the fixed phagocytic cells, endothelial cells, and even occasionally the granulocytes (heterophils). The phagocytic cells of the blood have so far never been found so parasitized. 4. It is believed that there is no essential difference between the parasites occurring in the erythrocytes and those found in other types of cells, and that it may be a matter of chance as to which type of cell a parasite enters. The character of the cell may thereafter determine how many merozoites will be eventually produced. 5. The exoerythrocytic stages found in Plasmodium circumflexum appear to be very similar to those seen in the other four species in which a similar type of schizogony has been demonstrated. The mechanism of schizogony also appears to be like that in other species. 6. There is probably a definite relationship between the pathogenicity of an infection and the presence of these stages in the internal organs. This relationship is perhaps a function of the immune mechanism in the part of the bird against the parasite. 7. The demonstration of exoerythrocytic schizogony in the avian malarias should result in the union of the two families Hemoproteidae and Plasmodiidae, if this type of schizogony is found to occur generally, or in the transfer of those species in which it does occur to the former group, with, of course, appropriate changes in which the Hemoproteidae are defined.

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