Morphologic Identification of Malaria Species in Peripheral Blood Smears

 

Steps:

(1) make sure malaria is present, excluding other parasites (such as Babesia species) and artifacts (especially platelets or stain debris; platelets are especially a problem for the inexperienced)

(2) try to identify all of the forms and features present after an extensive examination

(3) consider the possibility of a mixed infection with more than one species

 

Technical issues:

(1) The proper pH of the Giemsa stain is very important, especially to show stippling.

(2) A thick smear should be used to exclude low level parasitemia but may not be adequate for speciation.

(3) A thin smear should be used to speciate.

Features in Blood	P. falciparum	P. vivax	P. malariae	P. ovale
affected RBC	normal size	enlarged, a few ovalocytes	normal size	normal to increased; ovalocytosis
forms in peripheral blood	ring trophozoites and gametocytes only	all forms	all forms	all forms
immature trophozoites (ring forms)	2 chromatin dots; 2+ rings per RBC; applique forms; rings delicate	rarely may have 2 chromatin dots or multiple rings per RBC
mature trophozoites	not seen	ameboid	band or bayonet forms
schizonts	not seen	12-24 merozoites in mature schizonts	6-12 merozoites in mature schizont, may form a circle ("daisy")	8-12 merozoites in mature schizont
gametocytes	banana shaped
stippling	Mauer's dots or clefts (occasional)	Schuffner's dots (usual)	Ziemann's dots (rare; if overstain)	James's stippling

 

where:

• applique forms = ring trophozoites at the edge of the erythrocyte, up against the red cell membrane, seen in P. falciparum

• stippling may or may not be present; best seen with Giemsa stain at pH 7.0 to 7.2

 

Plasmodium falciparum:

(1) features if present: banana-shaped gametocytes, 2+ rings per RBC, 2 chromatin dots, applique ring forms

(2) features suggesting mixed with another species (not pure): all forms present, enlarged RBCs affected, band/bayonet trophozoites, daisy schizont, ameboid trophozoites

 

Plasmodium malariae:

(1) features if present: all forms present, band/bayonet trophozoites, daisy schizont, <= 12 schizonts (may have 6 or 7)

(2) features suggesting mixed with another species (not pure): enlarged RBCs affected, banana gametocytes, > 14 schizonts, 2+ ring forms per RBC, 2 chromatin dots, applique ring forms

 

Plasmodium ovale:

(1) features if present: ovalocytosis, all forms present, 8-12 schizonts

(2) features suggesting mixed with another species (not pure): 6 schizonts, > 14 schizonts, banana gametocytes, band/bayonet trophozoites, ameboid trophozoites, daisy schizonts, 2+ ring trophozoites per RBC, 2 chromatin dots per ring, applique ring forms

 

Plasmodium vivax:

(1) features if present: ameboid trophozoites, all forms present, >= 14 schizonts; may rarely show 2+ rings per RBC or 2 chromatin dots

(2) features suggesting mixed with another species (not pure): normal RBCs infected, banana gametocytes, band/bayonet trophozoites, daisy schizont, < 12 schizonts, applique ring forms