Answer to Case 330

Answer:  Plasmodium falciparum, heavy parasitemia

This case shows several of the characteristic morphologic features of P. falciparum infection, including normal size of the infected RBCs, the presence of only delicate ring forms (early trophozoites), and specialized "applique" and "head phone" forms.  There are also multiply-infected RBCs, which is characteristic (but not definitive) for P. falciparum. No stippling is present.  As noted by several readers, this is a heavy infection.  Calculation of percent parasitemia is important for guiding therapy and predicting patient prognosis. Note the presence of malaria pigment (hemozoin) inside of the neutrophils in the second image):

An important feature pointed out by Florida Fan is that a few of the RBCs are crenated, which should not be mistaken for the "fimbriations" of P. ovale.  Some clues that can help in this regard are that the infected RBCs are not enlarged (as would be expected for P. ovale - particularly in later stages) and that crenation is seen both infected and non-infected cells.  The latter is probably the most important point, since this indicates that crenation effect is due to either host factors or, more likely, an artifact of smear preparation, and not due to the presence of the parasite inside the RBCs.