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  P. Falciparum P. Vivax P. Ovale P. Malariae P. Knowlesi

size infected RBC normal normal to enlarged
(up to 1 1/2× to 2×)
normal or slightly enlarged
(up to 1 1/4×)
round to oval
sometimes fimbriated
normal or smaller than normal
(3/4×)
normal

ring/trophozoite stage
  – ring forms <1/3 of RBC
– multiply-infected RBCs
– classic “head phone” form
– appliqué/accolé forms
– ameboid ring form
– Schüffner’s dots
– multiply-infected RBCs
– compact trophozoites
– coarse Schüffner’s dots
– band-form trophozoites
-“basket-form” trophozoites
– often coarse chromatin dot
– 1/3 to 1/2 of the RBC
– band-form trophozoites
– multiply-infected RBCs
– slightly amoeboid and irregular
– appliqué forms (sometimes)
– fine dark brown pigment

schizonts
  – seldom seen in PB
– 8 to 24 small merozoites
– dark clumped pigment
– 12 to 24 merozoites
– yellowish-brown coagulated pigment
– 6 to 14 merozoites
– large nuclei
– mass of dark-brown pigment
– 6 to 12 merozoites
– sometimes “rosette” appearance
– coarse, dark-brown pigment
– maximum of 16 merozoites
– pigment in many small granules or clumps of brownish black round mass

gametocytes
  – crescent or sausage shaped
– diffuse, scattered pigment
– enlarged RBC
– scattered pigment
– large nucleus
– macrogametocyt course pigment
– microgametocyt diffuse pigment
– discrete red nucleus
– Schüffner’s dots
– compact, fills RBC
– scattered brown pigment
– compact chromatin
– irregularly distributed pigment grains

note:
P. Knowlesi and P. Falciparum can be hard to differentiate when the infecting parasites are predominantly at the early trophozoite or ring form developmental stage, such as double chromatin dots, multiply-infected erythrocytes and appliqué forms.
P. Knowlesi and P. Malariae can not be differentiated by morphological features from mature trophozoites, schizonts and gametocytes.
– severe malaria and a parasitaemia greater than 5,000/μl blood, together with a history of time spent in the forest and forest fringe areas of Southeast Asia, should strongly implicate a P. Knowlesi rather than a P. Malariae infection.