Plasmodium ovale: Morphology

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Plasmodium ovale

P.ovale has a relatively restricted distribution – Mainly believed to affect West Africa, Phillipines, Indonesia and Papua New Guinea, although a wider occurrence is likely. Like P.vivax, the resting hynozoite stage supports late recrudescence and allows the parasite to exist in more temperate environment that P.facliparum.

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The early trophozoite

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During this later growth stage the rings enlarge and begin to modify the erythrocyte, forming typical features of the species.

• Generally large rings forms, red cells not usually multiply infected
• Red cells begin to enlarge and may have ovoid or fimbriated apearance
• James' dots (morphologically indistinguishable from Schüffner's dots) begin to appear
• Pigment will not generally be detected at the early trophozoite stage

GALLERY: P.ovale early trophozoites

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The late trophozoite

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As parasites grow but generally retain their general ring shape, maturation is accompanied by clear modification of red cells to become ovoid and/or fimbriated; metabolism of haemoglobin causes malaria pigment to form.

• Parasites become larger and thickened, although the ring form is generally retained
• Red cell enlargement may lead to the ovoid fimbriated form often described as "comet forms"
• James' dots will now be prominent in appropriately stained specimens
• Pigment will now be seen over the surface of the parasite

GALLERY: P.ovale late trophozoites

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The schizont

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The asexual schizont stage of development will often be identified in the blood of this species, while smaller than P.vivax, species-specific features may not be clearly seen.

• A range of "schizont forms" will generally be present within moderately enlarged red cells
• When mature schizonts may contain 16-24 separate merozoites
• James' dots may be detected in any residual cytoplasm of the erythrocyte
• Pigment will be visible in irregularly distributed clumps

GALLERY: P.ovale schizonts

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The gametocyte

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The macro and micro gametocytes in this species resemble those of P.vivax although generally smaller. Some species-specific features may be seen.

• Red cells will generally enlarged and may have ovoid or fimbriated form
• Macrogametocytes (female form) will often entirely fill the erythrocyte
• Microgametocytes (male form) have a cytoplasmic rim with visible James' dots
• Malaria pigment is often circumferential around the gametocyte

GALLERY: P.ovale gametocytes

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