Answer to Case 288

Answer:  Plasmodium ovale

Marco Ligozzi and Dr. Rajani nicely described the features of P. ovale:

"The parasitized erythrocytes are oval in shape and fimbriated at the end," "Schuffner's stippling is present, more evident especially at the edges of the erythrocyte"and malaria "pigment is scattered, coarse."

Of note, Dr. Rajani points out that Schuffner's dots are "relevant to P. ovale infection; yet, these dots also appear in cases of P. vivax infection."  Therefore it is incumbent upon us to look at other features that would allow us to differentiate P. ovale from P. vivax.  The features that I find most useful for differentiating the 2 species are:

• 1/3 or more of P. ovale infected erythrocytes are oval, while P. vivax infected cells are only rarely oval.
• The parasite forms of P. ovale are more compact than those of P. vivax (which are characteristically ameboid)
• Fimbriation is common with P. ovale.

Other minor features that are useful for differentiating the 2 species:

• Plasmodium ovale has coarser pigment than P. vivax.
• Mature schizonts of P. ovale have 6-14 merozoites while those of P. vivax have 12-24 (obviously this feature is only helpful when you see a schizont with >14 merozoites - indicating that the species is P. vivax; immature schizonts of both species can have less than 12 merozoites, so this is less helpful).

   Here is an image from this case, demonstrating some of these important features: